Testis-specific serine/threonine-protein kinase required during spermatid development (PubMed:20053632, PubMed:23599433). Phosphorylates 'Ser-281' of TSKS (PubMed:20053632). Involved in the late stages of spermatogenesis, during the reconstruction of the cytoplasm (PubMed:20053632). During spermatogenesis, required for the transformation of a ring-shaped structure around the base of the flagellum originating from the chromatoid body (PubMed:20053632). ATP + L-seryl-[protein] = ADP + H(+) + O-phospho-L-seryl-[protein] ATP + L-threonyl-[protein] = ADP + H(+) + O-phospho-L-threonyl-[protein] Mg(2+) and Mn(2+) were both present in the kinase buffer but Mg(2+) is likely to be the in vivo cofactor. Activated by phosphorylation on Thr-174, potentially by autophosphorylation. Interacts with TSSK2 (PubMed:10781952). Interacts with HSP90; this interaction stabilizes TSSK1 (PubMed:23599433). In spermatozoa, present in the sperm head and in the flagellum. Testis-specific. Expressed only in postmeiotic spermatids at the final stages of cytodifferentiation in the seminiferous tubules (at protein level). Not detected in released sperms in the lumen of the seminiferous tubules and the epididymis. Autophosphorylated. Ubiquitinated; HSP90 activity negatively regulates ubiquitination and degradation. Male mice lacking Tssk1b and Tssk2 are sterile due to haploinsufficiency. chimeras show failure to form elongated spermatids, apoptosis of spermatocytes and spermatids, and the appearance of numerous round cells in the epididymal lumen. Elongating spermatids possess a collapsed mitochondrial sheath. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family.