Catalytic domain of testis-specific serine/threonine kinases and similar proteins
STKs catalyze the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. TSSK proteins are almost exclusively expressed postmeiotically in the testis and play important roles in spermatogenesis and/or spermiogenesis. There are five mammalian TSSK proteins which show differences in their localization and timing of expression. TSSK1 and TSSK2 are expressed specifically in meiotic and postmeiotic spermatogenic cells, respectively. TSSK3 has been reported to be expressed in the interstitial Leydig cells of adult testis. TSSK4, also called TSSK5, is expressed in testis from haploid round spermatids to mature spermatozoa. TSSK6, also called SSTK, is expressed at the head of elongated sperm. TSSK1/TSSK2 double knock-out and TSSK6 null mice are sterile without manifesting other defects, making these kinases viable targets for male contraception. The TSSK-like subfamily is part of a larger superfamily that includes the catalytic domains of other STKs, protein tyrosine kinases, RIO kinases, aminoglycoside phosphotransferase, choline kinase, and phosphoinositide 3-kinase.
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