Catalytic domain of testis-specific serine/threonine kinase 6 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. TSSK6, also called SSTK, is expressed at the head of elongated sperm. It can phosphorylate histones and associate with heat shock protens HSP90 and HSC70. Male mice deficient in TSSK6 are infertile, showing spermatogenic impairment including reduced sperm counts, impaired DNA condensation, abnormal morphology and decreased motility rates. The TSSK6-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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