Testis-specific serine/threonine-protein kinase required during spermatid development. Phosphorylates 'Ser-281' of TSKS and SPAG16. Involved in the late stages of spermatogenesis, during the reconstruction of the cytoplasm. During spermatogenesis, required for the transformation of a ring-shaped structure around the base of the flagellum originating from the chromatoid body. 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 TSSK1B (PubMed:10781952, PubMed:18367677). Interacts with HSP90; this interaction stabilizes TSSK2 (PubMed:23599433). Present in the cytoplasm of elongating spermatids. In spermatozoa, localizes in the equatorial segment, neck, the midpiece and in a specific sperm head compartment. In spermatids, concentrates in centrioles during flagellogenesis. Localizes in the tail and acrosomal regions of epididymal sperm. Testis-specific. Expressed only in the spermatids postmeiotically 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. Also present in the epididymal sperm (at protein level). First expression detected at 3.5 to 4 weeks. 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.