Second immunoglobulin domain of Cluster of Differentiation (CD) 33 and related Siglecs; member of the C2-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig) domain of Cluster of Differentiation (CD) 33 (also known as sialic-acid binding immunoglobulin type-lectin 3 (Siglec-3)) and related Siglecs. CD33, a Siglec family member, is a well-known immunotherapeutic target in acute myeloid leukemia (AML). It is an inhibitory sialoadhesin expressed in human leukocytes of the myeloid lineage and some lymphoid subsets, including natural killer (NK) cells. Siglecs are primarily expressed on immune cells and recognize sialic acid-containing glycan ligands. Siglecs are organized as an extracellular module composed of Ig-like domains (an N-terminal variable set of Ig-like carbohydrate recognition domains, and 1 to 16 constant Ig-like domains), followed by transmembrane and short cytoplasmic domains. Human Siglecs are classified into two subgroups, one subgroup is comprised of sialoadhesin (Siglec-1), CD22 (Siglec-2), and MAG (Siglec-4, myelin-associated glycoprotein), the other subgroup is comprised of CD33-related Siglecs which include CD33 (Siglec-3) and human Siglecs 5-11. CD33 (Siglec-3) is the smallest Siglec member. It preferentially binds to alpha2-6- and alpha2-3-sialylated glycans and strongly binds to sialylated ligands on leukemic cell lines. Ig Superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. This group includes CD33-related Siglecs which belong to the C2-set of IgSF domains. Unlike the C1-set, the C2-set structures do not have a D strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 141 PNIVVPPEVVAGTEVEVSCMVPDNCPELRPELSWLGHEGLGEPTVLGRlREDEGTWVQVSLLHFVPTREANGHRLGCQAA 220
Cdd:cd20987     1 PNIHVPEELEAGQEVTLTCSVPDNCPALSPEFSWLGAALTPLPPVLGR-LEEEGTTTHSSVLTFTPRPEDHGTNLTCQVK 79

                          90
                  ....*....|....*
gi 1907177917 221 FPNTTLQFEGYASLD 235
Cdd:cd20987    80 FPGATVTTERTIQLN 94

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917  333 GTVVAVEGETVSILCSTQSNPDPILTIFKEKQIL------ATVIYES-QLQLELPAVTPEDDGEYWCVAENQYGQRATAF 405
Cdd:smart00410   2 PSVTVKEGESVTLSCEASGSPPPEVTWYKQGGKLlaesgrFSVSRSGsTSTLTISNVTPEDSGTYTCAATNSSGSASSGT 81

                   ....
gi 1907177917  406 NLSV 409
Cdd:smart00410  82 TLTV 85

Second immunoglobulin domain of Cluster of Differentiation (CD) 33 and related Siglecs; member of the C2-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig) domain of Cluster of Differentiation (CD) 33 (also known as sialic-acid binding immunoglobulin type-lectin 3 (Siglec-3)) and related Siglecs. CD33, a Siglec family member, is a well-known immunotherapeutic target in acute myeloid leukemia (AML). It is an inhibitory sialoadhesin expressed in human leukocytes of the myeloid lineage and some lymphoid subsets, including natural killer (NK) cells. Siglecs are primarily expressed on immune cells and recognize sialic acid-containing glycan ligands. Siglecs are organized as an extracellular module composed of Ig-like domains (an N-terminal variable set of Ig-like carbohydrate recognition domains, and 1 to 16 constant Ig-like domains), followed by transmembrane and short cytoplasmic domains. Human Siglecs are classified into two subgroups, one subgroup is comprised of sialoadhesin (Siglec-1), CD22 (Siglec-2), and MAG (Siglec-4, myelin-associated glycoprotein), the other subgroup is comprised of CD33-related Siglecs which include CD33 (Siglec-3) and human Siglecs 5-11. CD33 (Siglec-3) is the smallest Siglec member. It preferentially binds to alpha2-6- and alpha2-3-sialylated glycans and strongly binds to sialylated ligands on leukemic cell lines. Ig Superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. This group includes CD33-related Siglecs which belong to the C2-set of IgSF domains. Unlike the C1-set, the C2-set structures do not have a D strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 141 PNIVVPPEVVAGTEVEVSCMVPDNCPELRPELSWLGHEGLGEPTVLGRlREDEGTWVQVSLLHFVPTREANGHRLGCQAA 220
Cdd:cd20987     1 PNIHVPEELEAGQEVTLTCSVPDNCPALSPEFSWLGAALTPLPPVLGR-LEEEGTTTHSSVLTFTPRPEDHGTNLTCQVK 79

                          90
                  ....*....|....*
gi 1907177917 221 FPNTTLQFEGYASLD 235
Cdd:cd20987    80 FPGATVTTERTIQLN 94

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917  333 GTVVAVEGETVSILCSTQSNPDPILTIFKEKQIL------ATVIYES-QLQLELPAVTPEDDGEYWCVAENQYGQRATAF 405
Cdd:smart00410   2 PSVTVKEGESVTLSCEASGSPPPEVTWYKQGGKLlaesgrFSVSRSGsTSTLTISNVTPEDSGTYTCAATNSSGSASSGT 81

                   ....
gi 1907177917  406 NLSV 409
Cdd:smart00410  82 TLTV 85

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1907177917 328 KPTV---NGTVVAVEGETVSILCSTQSNPDPILTIFKEKQILATV------IYESQLQLELPAVTPEDDGEYWCVAEN 396
Cdd:pfam13927   1 KPVItvsPSSVTVREGETVTLTCEATGSPPPTITWYKNGEPISSGstrsrsLSGSNSTLTISNVTRSDAGTYTCVASN 78

Immunoglobulin domain; The members here are composed of the immunoglobulin (Ig) domain found in the Ig superfamily. The Ig superfamily is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. Members of this group are components of immunoglobulin, neuroglia, cell surface glycoproteins, including T-cell receptors, CD2, CD4, CD8, and membrane glycoproteins, including butyrophilin and chondroitin sulfate proteoglycan core protein. A predominant feature of most Ig domains is a disulfide bridge connecting the two beta-sheets with a tryptophan residue packed against the disulfide bond. Ig superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Typically, the V-set domains have A, B, E, and D strands in one sheet and A', G, F, C, C' and C" in the other. The structures in C1-set are smaller than those in the V-set; they have one beta sheet that is formed by strands A, B, E, and D and the other by strands G, F, C, and C'. Moreover, a C1-set Ig domain contains a short C' strand (three residues) and lacks A' and C" strand. Unlike other Ig domain sets, C2-set structures do not have a D strand. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1907177917 343 VSILCSTQSNPDPILTIFKEKQILAT------VIYESQLQLELPAVTPEDDGEYWCVAENQYGQRATA 404
Cdd:cd00096     1 VTLTCSASGNPPPTITWYKNGKPLPPssrdsrRSELGNGTLTISNVTLEDSGTYTCVASNSAGGSASA 68

Second immunoglobulin domain of Cluster of Differentiation (CD) 33 and related Siglecs; member of the C2-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig) domain of Cluster of Differentiation (CD) 33 (also known as sialic-acid binding immunoglobulin type-lectin 3 (Siglec-3)) and related Siglecs. CD33, a Siglec family member, is a well-known immunotherapeutic target in acute myeloid leukemia (AML). It is an inhibitory sialoadhesin expressed in human leukocytes of the myeloid lineage and some lymphoid subsets, including natural killer (NK) cells. Siglecs are primarily expressed on immune cells and recognize sialic acid-containing glycan ligands. Siglecs are organized as an extracellular module composed of Ig-like domains (an N-terminal variable set of Ig-like carbohydrate recognition domains, and 1 to 16 constant Ig-like domains), followed by transmembrane and short cytoplasmic domains. Human Siglecs are classified into two subgroups, one subgroup is comprised of sialoadhesin (Siglec-1), CD22 (Siglec-2), and MAG (Siglec-4, myelin-associated glycoprotein), the other subgroup is comprised of CD33-related Siglecs which include CD33 (Siglec-3) and human Siglecs 5-11. CD33 (Siglec-3) is the smallest Siglec member. It preferentially binds to alpha2-6- and alpha2-3-sialylated glycans and strongly binds to sialylated ligands on leukemic cell lines. Ig Superfamily (IgSF) domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. This group includes CD33-related Siglecs which belong to the C2-set of IgSF domains. Unlike the C1-set, the C2-set structures do not have a D strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 141 PNIVVPPEVVAGTEVEVSCMVPDNCPELRPELSWLGHEGLGEPTVLGRlREDEGTWVQVSLLHFVPTREANGHRLGCQAA 220
Cdd:cd20987     1 PNIHVPEELEAGQEVTLTCSVPDNCPALSPEFSWLGAALTPLPPVLGR-LEEEGTTTHSSVLTFTPRPEDHGTNLTCQVK 79

                          90
                  ....*....|....*
gi 1907177917 221 FPNTTLQFEGYASLD 235
Cdd:cd20987    80 FPGATVTTERTIQLN 94

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917  333 GTVVAVEGETVSILCSTQSNPDPILTIFKEKQIL------ATVIYES-QLQLELPAVTPEDDGEYWCVAENQYGQRATAF 405
Cdd:smart00410   2 PSVTVKEGESVTLSCEASGSPPPEVTWYKQGGKLlaesgrFSVSRSGsTSTLTISNVTPEDSGTYTCAATNSSGSASSGT 81

                   ....
gi 1907177917  406 NLSV 409
Cdd:smart00410  82 TLTV 85

Immunoglobulin domain; This family contains immunoglobulin-like domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*...
gi 1907177917 328 KPTV---NGTVVAVEGETVSILCSTQSNPDPILTIFKEKQILATV------IYESQLQLELPAVTPEDDGEYWCVAEN 396
Cdd:pfam13927   1 KPVItvsPSSVTVREGETVTLTCEATGSPPPTITWYKNGEPISSGstrsrsLSGSNSTLTISNVTRSDAGTYTCVASN 78

Immunoglobulin like; IG domains that cannot be classified into one of IGv1, IGc1, IGc2, IG.

                           10        20        30        40        50        60        70        80
                   ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917  247 SSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVL--------REAVAKSLYLDLEEVTPGEDGVYACLAENAYGQDNRTV 318
Cdd:smart00410   2 PSVTVKEGESVTLSCEASGSPPPEVTWYKQGGKLlaesgrfsVSRSGSTSTLTISNVTPEDSGTYTCAATNSSGSASSGT 81

                   ....
gi 1907177917  319 ELSV 322
Cdd:smart00410  82 TLTV 85

Immunoglobulin I-set domain;

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 335 VVAVEGETVSILCSTQSNPDPILTIFKEKQILAT-----VIYESQLQ-LELPAVTPEDDGEYWCVAENQYGQRATAFNLS 408
Cdd:pfam07679  10 VEVQEGESARFTCTVTGTPDPEVSWFKDGQPLRSsdrfkVTYEGGTYtLTISNVQPDDSGKYTCVATNSAGEAEASAELT 89

                  .
gi 1907177917 409 V 409
Cdd:pfam07679  90 V 90

Second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor, and similar domains; member of the I-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor. FGF receptors bind FGF signaling polypeptides. FGFs participate in multiple processes such as morphogenesis, development, and angiogenesis. FGFs bind to four FGF receptor tyrosine kinases (FGFR1, FGFR2, FGFR3, FGFR4). Receptor diversity is controlled by alternative splicing producing splice variants with different ligand binding characteristics and different expression patterns. FGFRs have an extracellular region comprised of three Ig-like domains, a single transmembrane helix, and an intracellular tyrosine kinase domain. Ligand binding and specificity reside in the Ig-like domains 2 and 3, and the linker region that connects these two. FGFR activation and signaling depend on FGF-induced dimerization, a process involving cell surface heparin or heparin sulfate proteoglycans. This group also contains fibroblast growth factor (FGF) receptor like-1(FGFRL1). FGFRL1 does not have a protein tyrosine kinase domain at its C-terminus; neither does its cytoplasmic domain appear to interact with a signaling partner. It has been suggested that FGFRL1 may not have any direct signaling function, but instead acts as a decoy receptor trapping FGFs and preventing them from binding other receptors.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1907177917 254 GSHVSLLCGADSNPPPLLTWMRDGMVLR--------EAVAKSLYLDLEEVTPGEDGVYACLAENAYGQDNRTVELSV 322
Cdd:cd05729    19 ANKVRLECGAGGNPMPNITWLKDGKEFKkehriggtKVEEKGWSLIIERAIPRDKGKYTCIVENEYGSINHTYDVDV 95

Second Ig-like domain of the giant muscle protein titin Z1z2 in the sarcomeric Z-disk, and similar domains; a member of the I-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig)-like domain of the giant muscle protein titin Z1z2 in the sarcomeric Z-disk and similar proteins. Titin is a key component in the assembly and functioning of vertebrate striated muscles. By providing connections at the level of individual microfilaments, it contributes to the fine balance of forces between the two halves of the sarcomere. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Unlike the V-set, one of the distinctive features of I-set domains is the lack of a C" strand. The structure of the titin Z1z2 lacks this strand and thus it belongs to the I-set of the IgSF. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins titin, telokin, and twitchin, the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D that is involved in axonal guidance, immune function and angiogenesis.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 239 PPVIVEMNSSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVLREAV------AKSLY-LDLEEVTPGEDGVYACLAENAY 311
Cdd:cd20972     1 PPQFIQKLRSQEVAEGSKVRLECRVTGNPTPVVRWFCEGKELQNSPdiqihqEGDLHsLIIAEAFEEDTGRYSCLATNSV 80

                          90
                  ....*....|.
gi 1907177917 312 GQDNRTVELSV 322
Cdd:cd20972    81 GSDTTSAEIFV 91

Third immunoglobulin (Ig)-like domain of Neural Cell Adhesion Molecule 1 (NCAM-1); member of the I-set of IgSF domains; The members here are composed of the third immunoglobulin (Ig)-like domain of Neural Cell Adhesion Molecule (NCAM-1). NCAM plays important roles in the development and regeneration of the central nervous system, in synaptogenesis and neural migration. NCAM mediates cell-cell and cell-substratum recognition and adhesion via homophilic (NCAM-NCAM), and heterophilic (NCAM-non-NCAM), interactions. NCAM is expressed as three major isoforms having different intracellular extensions. The extracellular portion of NCAM has five N-terminal Ig-like domains and two fibronectin type III domains. The double zipper adhesion complex model for NCAM homophilic binding involves Ig1, Ig2, and Ig3. By this model, Ig1 and Ig2 mediate dimerization of NCAM molecules situated on the same cell surface (cis interactions), and Ig3 domains mediate interactions between NCAM molecules expressed on the surface of opposing cells (trans interactions) through binding to the Ig1 and Ig2 domains. The adhesive ability of NCAM is modulated by the addition of polysialic acid chains to the fifth Ig-like domain.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 239 PPVIVEMNSSVEAIE--GSHVSLLCGADSNPPPLLTWMRDGMVLREAVAKSLY------LDLEEVTPGEDGVYACLAENA 310
Cdd:cd05730     1 PPTIRARQSEVNATAnlGQSVTLACDADGFPEPTMTWTKDGEPIESGEEKYSFnedgseMTILDVDKLDEAEYTCIAENK 80

                          90
                  ....*....|..
gi 1907177917 311 YGQDNRTVELSV 322
Cdd:cd05730    81 AGEQEAEIHLKV 92

Fourth immunoglobulin domain of the Drosophila melanogaster Dscam protein, and similar domains; a member of the I-set of IgSF domains; The members here are composed of the fourth immunoglobulin domain of the Drosophila melanogaster Down syndrome cell adhesion molecule (DSCAM) protein and similar proteins. Down syndrome cell adhesion molecule (DSCAM) is a cell adhesion molecule that plays critical roles in neural development, including axon guidance and branching, axon target recognition, self-avoidance and synaptic formation. DSCAM belongs to the immunoglobulin superfamily and contributes to defects in the central nervous system in Down syndrome patients. Vertebrate DSCAMs differ from Drosophila Dscam1 in that they lack the extensive alternative splicing that occurs in the insect gene. Drosophila melanogaster Dscam has 38,016 isoforms generated by the alternative splicing of four variable exon clusters, which allows every neuron in the fly to display a distinctive set of Dscam proteins on its cell surface. Drosophila Dscam1 is a cell-surface protein that plays important roles in neural development and axon tiling of neurons. It is shown that thousands of isoforms bind themselves through specific homophilic (self-binding) interactions, a process which mediates cellular self-recognition. Drosophila Dscam2 is also alternatively spliced and plays a key role in the development of two visual system neurons, monopolar cells L1 and L2. This group is a member of the I-set Ig domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 239 PPVIVEMNSSVEAIEGSHVSLLCGADSNPPPLLTWMRDGM-------------VLREAVAKSlYLDLEEVTPGEDGVYAC 305
Cdd:cd20956     1 APVLLETFSEQTLQPGPSVSLKCVASGNPLPQITWTLDGFpipesprfrvgdyVTSDGDVVS-YVNISSVRVEDGGEYTC 79

                  ....*...
gi 1907177917 306 LAENAYGQ 313
Cdd:cd20956    80 TATNDVGS 87

Third immunoglobulin (Ig) domain of contactin; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the third immunoglobulin (Ig) domain of contactins. Contactins are neural cell adhesion molecules and are comprised of six Ig domains followed by four fibronectin type III (FnIII) domains anchored to the membrane by glycosylphosphatidylinositol. The first four Ig domains form the intermolecular binding fragment, which arranges as a compact U-shaped module via contacts between Ig domains 1 and 4, and between Ig domains 2 and 3. Contactin-2 (TAG-1, axonin-1) may play a part in the neuronal processes of neurite outgrowth, axon guidance and fasciculation, and neuronal migration. This group also includes contactin-1 and contactin-5. The different contactins show different expression patterns in the central nervous system. During development and in adulthood, contactin-2 is transiently expressed in subsets of central and peripheral neurons. Contactin-5 is expressed specifically in the rat postnatal nervous system, peaking at about 3 weeks postnatal, and a lack of contactin-5 (NB-2) results in an impairment of neuronal activity in the rat auditory system. Contactin-5 is highly expressed in the adult human brain in the occipital lobe and in the amygdala. Contactin-1 is differentially expressed in tumor tissues and may, through a RhoA mechanism, facilitate invasion and metastasis of human lung adenocarcinoma. This group belongs to the I-set of IgSF domains.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....
gi 1907177917 239 PPVIVEMNSSVEAIEGSHVSLLCGADSNPPPLLTWMR-DGMVLREAVAKSL--YLDLEEVTPGEDGVYACLAENAYGQD 314
Cdd:cd04968     1 PSIKVRFPADTYALKGQTVTLECFALGNPVPQIKWRKvDGSPSSQWEITTSepVLEIPNVQFEDEGTYECEAENSRGKD 79

Third immunoglobulin (Ig)-like domain in Robo (roundabout) receptors; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the third immunoglobulin (Ig)-like domain in Robo (roundabout) receptors. Robo receptors play a role in the development of the central nervous system (CNS), and are receptors of Slit protein. Slit is a repellant secreted by the neural cells in the midline. Slit acts through Robo to prevent most neurons from crossing the midline from either side. Three mammalian Robo homologs (Robo1, Robo2, Robo3), and three mammalian Slit homologs (Slit-1,Slit-2, Slit-3), have been identified. Commissural axons, which cross the midline, express low levels of Robo; longitudinal axons, which avoid the midline, express high levels of Robo. Robo1, Robo2, and Robo3 are expressed by commissural neurons in the vertebrate spinal cord and Slit-1, Slit-2, and Slit-3 are expressed at the ventral midline. Robo-3 is a divergent member of the Robo family which instead of being a positive regulator of Slit responsiveness, antagonizes Slit responsiveness in precrossing axons. The Slit-Robo interaction is mediated by the second leucine-rich repeat (LRR) domain of Slit and the two N-terminal Ig domains of Robo, Ig1 and Ig2. The primary Robo binding site for Slit2 has been shown by surface plasmon resonance experiments and mutational analysis to be the Ig1 domain, while the Ig2 domain has been proposed to harbor a weak secondary binding site. This group belongs to the I-set of IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....
gi 1907177917 247 SSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVL---REAVAKSLYLDLEEVTPGEDGVYACLAENAYGQDNRTVELSV 322
Cdd:cd05725     5 QNQVVLVDDSAEFQCEVGGDPVPTVRWRKEDGELpkgRYEILDDHSLKIRKVTAGDMGSYTCVAENMVGKIEASATLTV 83

Second immunoglobulin (Ig)-like domain of protein tyrosine kinase (PTK) 7; The members here are composed of the second immunoglobulin (Ig)-like domain in protein tyrosine kinase (PTK) 7, also known as CCK4. PTK7 is a subfamily of the receptor protein tyrosine kinase family, and is referred to as an RPTK-like molecule. RPTKs transduce extracellular signals across the cell membrane and play important roles in regulating cell proliferation, migration, and differentiation. PTK7 is organized as an extracellular portion having seven Ig-like domains, a single transmembrane region, and a cytoplasmic tyrosine kinase-like domain. PTK7 is considered a pseudokinase as it has several unusual residues in some of the highly conserved tyrosine kinase (TK) motifs; it is predicted to lack TK activity. PTK7 may function as a cell-adhesion molecule. PTK7 mRNA is expressed at high levels in placenta, melanocytes, liver, lung, pancreas, and kidney. PTK7 is overexpressed in several cancers, including melanoma and colon cancer lines.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 240 PVIVEMNSSVEAIE-GSHVSLLCGADSNPPPLLTWMRDGMVLREA------VAKSLYLDLEEVTPGEDGVYACLAENAYG 312
Cdd:cd05760     1 PVVLKHPASAAEIQpSSRVTLRCHIDGHPRPTYQWFRDGTPLSDGqgnysvSSKERTLTLRSAGPDDSGLYYCCAHNAFG 80

                          90
                  ....*....|..
gi 1907177917 313 Q--DNRTVELSV 322
Cdd:cd05760    81 SvcSSQNFTLSI 92

Immunoglobulin domain I1 of the titin I-band and similar proteins; a member of the I-set of IgSF domains; The members here are composed of the immunoglobulin domain I1 of the titin I-band and similar proteins. Titin is a key component in the assembly and functioning of vertebrate striated muscles. By providing connections at the level of individual microfilaments, it contributes to the fine balance of forces between the two halves of the sarcomere. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. The two sheets are linked together by a conserved disulfide bond between B strand and F strand. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. The Ig I1 domain of the titin I-band is a member of the I-set Ig domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand but lack a C" strand. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins titin, telokin, and twitchin, the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D that is involved in axonal guidance, immune function and angiogenesis.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 339 EGETVSILCSTQSNPDPILTIFKEKQILATVIYESQLQ---------LELPAVTPEDDGEYWCVAENQYGQRATAFNLSV 409
Cdd:cd20951    14 EKSDAKLRVEVQGKPDPEVKWYKNGVPIDPSSIPGKYKieseygvhvLHIRRVTVEDSAVYSAVAKNIHGEASSSASVVV 93

                  .
gi 1907177917 410 E 410
Cdd:cd20951    94 E 94

Fourth immunoglobulin (Ig)-like domain of L1, Ng-CAM (Neuron-glia CAM cell adhesion molecule), and NrCAM (Ng-CAM-related); The members here are composed of the fourth immunoglobulin (Ig)-like domain of L1, Ng-CAM (Neuron-glia CAM cell adhesion molecule), and NrCAM (Ng-CAM-related). These proteins belong to the L1 subfamily of cell adhesion molecules (CAMs) and are comprised of an extracellular region having six Ig-like domains and five fibronectin type III domains, a transmembrane region and an intracellular domain. These molecules are primarily expressed in the nervous system. L1 is associated with an X-linked recessive disorder, X-linked hydrocephalus, MASA syndrome, or spastic paraplegia type 1, that involves abnormalities of axonal growth.

                          10        20        30        40        50        60
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*.
gi 1907177917 253 EGSHVSLLCGADSNPPPLLTWMRDGMVLREAVAKSLYLD------LEEVTPGEDGVYACLAENAYG 312
Cdd:cd04978    13 PGETGELICEAEGNPQPTITWRLNGVPIEPAPEDMRRTVdgrtliFSNLQPNDTAVYQCNASNVHG 78

C-terminal immunoglobulin-like domain of the myosin-associated giant protein kinase Twitchin, and similar domains; member of the I-set IgSF domains; The members here are composed of the C-terminal immunoglobulin-like domain of the myosin-associated giant protein kinase Twitchin and similar proteins, including Caenorhabditis elegans and Aplysia californica Twitchin, Drosophila melanogaster Projectin, and similar proteins. These are very large muscle proteins containing multiple immunoglobulin (Ig)-like and fibronectin type III (FN3) domains and a single kinase domain near the C-terminus. In humans these proteins are called Titin. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. The Ig-like domain of the Twitchin is a member of the I-set IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand but lack a C" strand. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins (titin, telokin, and twitchin), the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 243 VEMNSSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVLREAVAK-SLY------LDLEEVTPGEDGVYACLAENAYGQDN 315
Cdd:cd20949     3 TENAYVTTVKEGQSATILCEVKGEPQPNVTWHFNGQPISASVADmSKYriladgLLINKVTQDDTGEYTCRAYQVNSIAS 82

                  ....*..
gi 1907177917 316 RTVELSV 322
Cdd:cd20949    83 DMQERTV 89

C-terminal immunoglobulin-like domain of the myosin-associated giant protein kinase Twitchin, and similar domains; member of the I-set IgSF domains; The members here are composed of the C-terminal immunoglobulin-like domain of the myosin-associated giant protein kinase Twitchin and similar proteins, including Caenorhabditis elegans and Aplysia californica Twitchin, Drosophila melanogaster Projectin, and similar proteins. These are very large muscle proteins containing multiple immunoglobulin (Ig)-like and fibronectin type III (FN3) domains and a single kinase domain near the C-terminus. In humans these proteins are called Titin. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. The Ig-like domain of the Twitchin is a member of the I-set IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand but lack a C" strand. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins (titin, telokin, and twitchin), the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1907177917 330 TVNGTVVAV-EGETVSILCSTQSNPDPILTI-FKEKQILATVIYESQLQ-----LELPAVTPEDDGEYWCVAENQYG 399
Cdd:cd20949     3 TENAYVTTVkEGQSATILCEVKGEPQPNVTWhFNGQPISASVADMSKYRiladgLLINKVTQDDTGEYTCRAYQVNS 79

Immunoglobulin (Ig)-like domain of human titin C terminus and similar proteins; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fifth immunoglobulin (Ig)-like domain from the C-terminus of human titin x and similar proteins. Titin (also called connectin) is a fibrous sarcomeric protein specifically found in vertebrate striated muscle. Titin is gigantic; depending on isoform composition it ranges from 2970 to 3700 kDa, and is of a length that spans half a sarcomere. Titin largely consists of multiple repeats of Ig-like and fibronectin type 3 (FN-III)-like domains. Titin connects the ends of myosin thick filaments to Z disks and extends along the thick filament to the H zone and appears to function similar to an elastic band, keeping the myosin filaments centered in the sarcomere during muscle contraction or stretching. This group belongs to the I-set of IgSF domains, having A-B-E-D strands in one beta-sheet and A'-G-F-C-C' in the other. Like the V-set Ig domains, members of the I-set have a discontinuous A strand, but lack a C" strand.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 317 TVELSVMYAPWKPTVNgtvvavEGETVSILCSTQSNPDPILTIFKEKQILAT----VIYESQLQ--LELPAVTPEDDGEY 390
Cdd:cd05747     1 TLPATILTKPRSLTVS------EGESARFSCDVDGEPAPTVTWMREGQIIVSsqrhQITSTEYKstFEISKVQMSDEGNY 74

                          90
                  ....*....|....*...
gi 1907177917 391 WCVAENQYGQRATAFNLS 408
Cdd:cd05747    75 TVVVENSEGKQEAQFTLT 92

Second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor; member of the I-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor. FGF receptors bind FGF signaling polypeptides. FGFs participate in multiple processes such as morphogenesis, development, and angiogenesis. FGFs bind to four FGF receptor tyrosine kinases (FGFR1, FGFR2, FGFR3, FGFR4). Receptor diversity is controlled by alternative splicing producing splice variants with different ligand binding characteristics and different expression patterns. FGFRs have an extracellular region comprised of three IG-like domains, a single transmembrane helix, and an intracellular tyrosine kinase domain. Ligand binding and specificity reside in the Ig-like domains 2 and 3, and the linker region that connects these two. FGFR activation and signaling depend on FGF-induced dimerization, a process involving cell surface heparin or heparin sulfate proteoglycans.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1907177917 340 GETVSILCSTQSNPDPILTIFK-------EKQILATVIYESQLQLELPAVTPEDDGEYWCVAENQYGQRATAFNLSV 409
Cdd:cd05857    19 ANTVKFRCPAAGNPTPTMRWLKngkefkqEHRIGGYKVRNQHWSLIMESVVPSDKGNYTCVVENEYGSINHTYHLDV 95

agrin-responsive first immunoglobulin-like domains (Ig1) of the MuSK ectodomain; a member of the I-set of IgSF domains; The members here are composed of the first immunoglobulin-like domains (Ig1) of the Muscle-specific kinase (MuSK). MuSK is a receptor tyrosine kinase specifically expressed in skeletal muscle, where it plays a central role in the formation and maintenance of the neuromuscular junction (NMJ). MuSK is activated by agrin, a neuron-derived heparan sulfate proteoglycan. The activation of MUSK in myotubes regulates the formation of NMJs through the regulation of different processes including the specific expression of genes in subsynaptic nuclei, the reorganization of the actin cytoskeleton and the clustering of the acetylcholine receptors (AChR) in the postsynaptic membrane. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Unlike the V-set, one of the distinctive features of I-set domains is the lack of a C" strand. The structure of the MuSK lacks this strand and thus it belongs to the I-set of the IgSF. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins titin, telokin, and twitchin, the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D that is involved in axonal guidance, immune function and angiogenesis.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|...
gi 1907177917 329 PTVNGTVVAVEGETVSILCSTQSNPDPILTIFKEKQILAT-----VIYESQLQLELPAVTPEDDGEYWCVAEN 396
Cdd:cd20970     6 PQPSFTVTAREGENATFMCRAEGSPEPEISWTRNGNLIIEfntryIVRENGTTLTIRNIRRSDMGIYLCIASN 78

agrin-responsive second immunoglobulin-like domains (Ig2) of the Muscle-specific kinase (MuSK) ectodomain; a member of the I-set of Ig superfamily domains; The members here are composed of the second immunoglobulin-like (Ig) domains of the Muscle-specific kinase (MuSK) ectodomain. MuSK is a receptor tyrosine kinase specifically expressed in skeletal muscle, where it plays a central role in the formation and maintenance of the neuromuscular junction (NMJ). MuSK is activated by agrin, a neuron-derived heparan sulfate proteoglycan. The activation of MUSK in myotubes regulates the formation of NMJs through the regulation of different processes including the specific expression of genes in subsynaptic nuclei, the reorganization of the actin cytoskeleton and the clustering of the acetylcholine receptors (AChR) in the postsynaptic membrane. The Ig superfamily (IgSF) is a heterogenous group of proteins, built on a common fold comprised of a sandwich of two beta sheets. IgSF domains can be divided into 4 main classes based on their structures and sequences: the Variable (V), Constant 1 (C1), Constant 2 (C2), and Intermediate (I) sets. Unlike the V-set, one of the distinctive features of I-set domains is the lack of a C" strand. The structure of the MuSK lacks this strand and thus it belongs to the I-set of the IgSF. I-set domains are found in several cell adhesion molecules (such as VCAM, ICAM, and MADCAM), and are also present in numerous other diverse protein families, including several tyrosine-protein kinase receptors, the hemolymph protein hemolin, the muscle proteins titin, telokin, and twitchin, the neuronal adhesion molecule axonin-1, and the signaling molecule semaphorin 4D that is involved in axonal guidance, immune function and angiogenesis.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 236 VKYPPVivemnsSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVLRE----AVAKSLYLDLEEVTPGEDGVYACLAENAY 311
Cdd:cd20968     2 ITRPPT------NVTIIEGLKAVLPCTTMGNPKPSVSWIKGDDLIKEnnriAVLESGSLRIHNVQKEDAGQYRCVAKNSL 75

                  .
gi 1907177917 312 G 312
Cdd:cd20968    76 G 76

Third immunoglobulin (Ig)-like domain of peroxidasin; The members here are composed of the third immunoglobulin (Ig)-like domain in peroxidasin. Peroxidasin has a peroxidase domain and interacting extracellular motifs containing four Ig-like domains. It has been suggested that peroxidasin is secreted and has functions related to the stabilization of the extracellular matrix. It may play a part in various other important processes such as removal and destruction of cells which have undergone programmed cell death and protection of the organism against non-self.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....
gi 1907177917 253 EGSHVSLLCGADSNPPPLLTWMRDGMVL----REAVAKSLYLDLEEVTPGEDGVYACLAENAYGQDNRTVELSV 322
Cdd:cd05745     1 EGQTVDFLCEAQGYPQPVIAWTKGGSQLsvdrRHLVLSSGTLRISRVALHDQGQYECQAVNIVGSQRTVAQLTV 74

Fourth Ig domain of the neural cell adhesion molecule contactin-2, and similar domains; The members here are composed of the fourth Ig domain of the neural cell adhesion molecule contactin-2. Contactins are comprised of six Ig domains followed by four fibronectin type III (FnIII) domains anchored to the membrane by glycosylphosphatidylinositol. Contactin-2 (also called TAG-1, axonin-1) facilitates cell adhesion by homophilic binding between molecules in apposed membranes. The first four Ig domains form the intermolecular binding fragment which arranges as a compact U-shaped module by contacts between Ig domains 1 and 4, and domains 2 and 3. It has been proposed that a linear zipper-like array forms, from contactin-2 molecules alternatively provided by the two apposed membranes.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*
gi 1907177917 327 WKPTVNGTVVAVeGETVSILCSTQSNPDPILTIFKEKQILATV--IYESQLQLELPAVTPEDDGEYWCVAENQYG 399
Cdd:cd05728     2 WLKVISDTEADI-GSSLRWECKASGNPRPAYRWLKNGQPLASEnrIEVEAGDLRITKLSLSDSGMYQCVAENKHG 75

C-terminal immunoglobulin (Ig)-like domain of myomesin and M-protein; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the C-terminal immunoglobulin (Ig)-like domain of myomesin and M-protein (also known as myomesin-2). Myomesin and M-protein are both structural proteins localized to the M-band, a transverse structure in the center of the sarcomere, and are candidates for M-band bridges. Both proteins are modular, consisting mainly of repetitive Ig-like and fibronectin type III (FnIII) domains. Myomesin is expressed in all types of vertebrate striated muscle; M-protein has a muscle-type specific expression pattern. Myomesin is present in both slow and fast fibers; M-protein is present only in fast fibers. It has been suggested that myomesin acts as a molecular spring with alternative splicing as a means of modifying its elasticity.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 335 VVAVEGETVSILCSTQSNPDPILTIFKEKQILA-------TVIYESQLQLELPAVTPEDDGEYWCVAENQYGQRATAFNL 407
Cdd:cd05737    11 VTIMEGKTLNLTCNVWGDPPPEVSWLKNDQALAfldhcnlKVEAGRTVYFTINGVSSEDSGKYGLVVKNKYGSETSDVTV 90

                  ..
gi 1907177917 408 SV 409
Cdd:cd05737    91 SV 92

Second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor, and similar domains; member of the I-set of IgSF domains; The members here are composed of the second immunoglobulin (Ig)-like domain of fibroblast growth factor (FGF) receptor. FGF receptors bind FGF signaling polypeptides. FGFs participate in multiple processes such as morphogenesis, development, and angiogenesis. FGFs bind to four FGF receptor tyrosine kinases (FGFR1, FGFR2, FGFR3, FGFR4). Receptor diversity is controlled by alternative splicing producing splice variants with different ligand binding characteristics and different expression patterns. FGFRs have an extracellular region comprised of three Ig-like domains, a single transmembrane helix, and an intracellular tyrosine kinase domain. Ligand binding and specificity reside in the Ig-like domains 2 and 3, and the linker region that connects these two. FGFR activation and signaling depend on FGF-induced dimerization, a process involving cell surface heparin or heparin sulfate proteoglycans. This group also contains fibroblast growth factor (FGF) receptor like-1(FGFRL1). FGFRL1 does not have a protein tyrosine kinase domain at its C-terminus; neither does its cytoplasmic domain appear to interact with a signaling partner. It has been suggested that FGFRL1 may not have any direct signaling function, but instead acts as a decoy receptor trapping FGFs and preventing them from binding other receptors.

                          10        20        30        40        50        60        70
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*..
gi 1907177917 340 GETVSILCSTQSNPDPILTIFKE----KQILATVIYESQLQ---LELPAVTPEDDGEYWCVAENQYGQRATAFNLSV 409
Cdd:cd05729    19 ANKVRLECGAGGNPMPNITWLKDgkefKKEHRIGGTKVEEKgwsLIIERAIPRDKGKYTCIVENEYGSINHTYDVDV 95

Fifth domain (immunoglobulin-like) of Trk receptors TrkA, TrkB, and TrkC; member of the I-set of Ig superfamily (IgSF) domains; The members here are composed of the fifth domain of Trk receptors TrkA, TrkB, and TrkC, an immunoglobulin (Ig)-like domain which binds to neurotrophin. The Trk family of receptors are tyrosine kinase receptors. They are activated by dimerization, leading to autophosphorylation of intracellular tyrosine residues, and triggering the signal transduction pathway. TrkA, TrkB, and TrkC share significant sequence homology and domain organization. The first three domains are leucine-rich domains while the fourth and fifth domains are Ig-like domains playing a part in ligand binding. TrkA, TrkB, and TrkC mediate the trophic effects of the neurotrophin Nerve Growth Factor (NGF) family. TrkA is recognized by NGF. TrkB is recognized by brain-derived neurotrophic factor (BDNF) and neurotrophin (NT)-4. TrkC is recognized by NT-3. NT-3 is promiscuous as in some cell systems it activates TrkA and TrkB receptors. TrkA is a receptor found in all major NGF targets, including the sympathetic, trigeminal, and dorsal root ganglia, cholinergic neurons of the basal forebrain, and the striatum. TrKB transcripts are found throughout multiple structures of the central and peripheral nervous systems. The TrkC gene is expressed throughout the mammalian nervous system. This group belongs to the I-set of IgSF domains.

                          10        20        30        40        50        60        70        80
                  ....*....|....*....|....*....|....*....|....*....|....*....|....*....|....*....|
gi 1907177917 239 PPVIVEmnsSVEAIEGSHVSLLCGADSNPPPLLTWMRDGMVLREAvaKSLYLDLEE--VTPGE--------------DGV 302
Cdd:cd04971     1 APVIVR---LEEPELRHHWCIPFTVRGNPKPTLTWYHNGAVLNES--DYIRTEIHYeaATPTEyhgclkfdnpthvnNGN 75

                          90
                  ....*....|....*.
gi 1907177917 303 YACLAENAYGQDNRTV 318
Cdd:cd04971    76 YTLVASNEYGQDSKSI 91