Catalytic domain of testis-specific serine/threonine kinase 4 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. TSSK4, also called TSSK5, is expressed in testis from haploid round spermatids to mature spermatozoa. It phosphorylates Cre-Responsive Element Binding protein (CREB), facilitating the binding of CREB to the specific cis cAMP responsive element (CRE), which is important in activating genes related to germ cell differentiation. Mutations in the human TSSK4 gene is associated with infertile Chinese men with impaired spermatogenesis. The TSSK4-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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