RING finger (Really Interesting New Gene) domain and U-box domain superfamily; The RING finger is a specialized type of Zn-finger of 40 to 60 residues that binds two atoms of zinc. It is defined by the "cross-brace" motif that chelates zinc atoms by eight amino acid residues, typically Cys or His, arranged in a characteristic spacing. Canonical RING motifs have been categorized into two major subclasses, RING-HC (C3HC4-type) and RING-H2 (C3H2C3-type), according to their Cys/His content. There are also many variants of RING fingers: some have different Cys/His patterns while some lack a single Cys or His residue at typical Zn ligand positions (the fourth or eighth zinc ligand is prevalently exchanged for an Asp, which can indeed chelate Zn in a RING finger as well). C4C4-, C3HC3D-, C2H2C4-, and C3HC5-type RING fingers are closely related to RING-HC fingers. In contrast, C4HC3- (RING-CH alias RINGv), C3H3C2-, C3H2C2D-, C3DHC3-, and C4HC2H-type RING fingers are more closely related to RING-H2 fingers. However, not all RING finger-containing proteins display regular RING finger features, and the RING finger family has turned out to be multifarious. The degenerate RING fingers of the Siz/PIAS RING (SP-RING) family proteins and sporulation protein RMD5, are characterized by lacking the second, fifth, and sixth Zn2+ ion-coordinating residues. They bind only one Zn2+ ion. On the other hand, the RING fingers of the human APC11 and RBX1 proteins can bind a third Zn atom since they harbor four additional Zn ligands. U-box is a modified form of the RING finger domain that lacks metal chelating Cys and His residues. It resembles the cross-brace RING structure consisting of three beta-sheets and a single alpha-helix, which would be stabilized by salt bridges instead of chelated metal ions. U-box proteins are widely distributed among eukaryotic organisms and show a higher prevalence in plants than in other organisms. RING finger/U-box-containing proteins are a group of diverse proteins with a variety of cellular functions, including oncogenesis, development, viral replication, signal transduction, the cell cycle and apoptosis. Many of them are ubiquitin-protein ligases (E3s) that serve as scaffolds for binding to ubiquitin-conjugating enzymes (E2s, also referred to as ubiquitin carrier proteins or UBCs) in close proximity to substrate proteins, which enable efficient transfer of ubiquitin from E2 to the substrates.

                          10        20
                  ....*....|....*....|..
gi 1829010880   3 WRLPTGHQLCRLCLLRKPRPAL 24
Cdd:cd16537    20 YRLPCGHLLCRPCLAEKQKSLA 41

RING finger, HC subclass, found in RING finger protein 37 (RNF37); RNF37, also known as KIAA0860, U-box domain-containing protein 5 (UBOX5), UbcM4-interacting protein 5 (UIP5), or ubiquitin-conjugating enzyme 7-interacting protein 5, is an E3 ubiquitin-protein ligase found exclusively in the nucleus as part of a nuclear dot-like structure. It interacts with the molecular chaperone VCP/p97 protein. RNF37 contains a U-box domain followed by a potential nuclear location signal (NLS) and a C-terminal C3HC4-type RING-HC finger. The U-box domain is a modified RING finger domain that lacks the hallmark metal-chelating cysteines and histidines of the latter, and is likely to adopt a RING finger-like conformation. The presence of the U-box, but not of the RING finger, is required for the E3 activity. The U-box domain can directly interact with several E2 enzymes, including UbcM2, UbcM3, UbcM4, UbcH5, and UbcH8, suggesting a similar function as the RING finger in the ubiquitination pathway. This model corresponds to the RING-HC finger.

                          10        20
                  ....*....|....*....|..
gi 1829010880   3 WRLPTGHQLCRLCLLRKPRPAL 24
Cdd:cd16537    20 YRLPCGHLLCRPCLAEKQKSLA 41

RING finger, HC subclass, found in RING finger protein 37 (RNF37); RNF37, also known as KIAA0860, U-box domain-containing protein 5 (UBOX5), UbcM4-interacting protein 5 (UIP5), or ubiquitin-conjugating enzyme 7-interacting protein 5, is an E3 ubiquitin-protein ligase found exclusively in the nucleus as part of a nuclear dot-like structure. It interacts with the molecular chaperone VCP/p97 protein. RNF37 contains a U-box domain followed by a potential nuclear location signal (NLS) and a C-terminal C3HC4-type RING-HC finger. The U-box domain is a modified RING finger domain that lacks the hallmark metal-chelating cysteines and histidines of the latter, and is likely to adopt a RING finger-like conformation. The presence of the U-box, but not of the RING finger, is required for the E3 activity. The U-box domain can directly interact with several E2 enzymes, including UbcM2, UbcM3, UbcM4, UbcH5, and UbcH8, suggesting a similar function as the RING finger in the ubiquitination pathway. This model corresponds to the RING-HC finger.

                          10        20
                  ....*....|....*....|..
gi 1829010880   3 WRLPTGHQLCRLCLLRKPRPAL 24
Cdd:cd16537    20 YRLPCGHLLCRPCLAEKQKSLA 41