Catalytic domain of testis-specific serine/threonine kinase 1, TSSK2, 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. TSSK2 is localized in the sperm neck, equatorial segment, and mid-piece of the sperm tail. Both TSSK1 and TSSK2 phosphorylate their common substrate TSKS (testis-specific-kinase-substrate). TSSK1/TSSK2 double knock-out mice are sterile without manifesting other defects, making these kinases viable targets for male contraception. The TSSK1/2-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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