STKs (serine/threonine-protein kinases) catalyse the transfer of the gamma-phosphoryl group from ATP to serine/threonine residues on protein substrates. TSSK (testis-specific serine kinase) 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 [, ].
