Serine/threonine kinase which is involved in male germ cell development and in mature sperm function (PubMed:17927909, PubMed:23599433, PubMed:23054012, PubMed:25361759, PubMed:26940607). May be involved in the Cre/Creb signaling pathway (PubMed:26940607). Phosphorylates CREB1 on 'Ser-133' in vitro and can stimulate Cre/Creb pathway in cells (By similarity). Phosphorylates CREM on 'Ser-116' in vitro (PubMed:26940607). Phosphorylates ODF2 on 'Ser-95' (PubMed:26961893). Catalytically inactive. Catalytically inactive. Catalytically inactive. 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 autophosphorylation on Thr-197. Homodimer (PubMed:17927909, PubMed:23054012). Interacts with HSP90; this interaction stabilizes and activates TSSK4 (PubMed:23599433). Interacts with ODF2 (via C-terminus); this interaction promotes ODF2 phosphorylation on 'Ser-95' (PubMed:25361759, PubMed:26961893). May interact with CREM (PubMed:26940607). Interacts with CREB1; this interaction facilitates CREB1 phosphorylation on 'Ser-133' (By similarity). Interacts with QRICH2 (By similarity). In spermatozoa, present in the sperm head and in the flagellum. Expressed in spermatocytes and mature sperm (at protein level) (PubMed:20729278, PubMed:23054012, PubMed:25361759, PubMed:26940607). Highly expressed in the spleen, heart and testis (PubMed:17927909, PubMed:20729278, PubMed:23054012). Isoform 2, isoform 3, and isoform 4: Expressed at highest level in testis and heart and at low levels in the liver, spleen, kidney, brain and thymus (PubMed:17927909). Detected throughout spermiogenesis, in round spermatids, elongated spermatids, and mature spermatozoa (PubMed:23054012, PubMed:20729278, PubMed:26940607). Detected at low levels on postnatal day 14, with significantly increased expression on postnatal day 16. Expression levels are unchanged after postnatal day 16 (PubMed:17927909, PubMed:20729278). Activated by autophosphorylation on Thr-197. ODF2 potentiates the autophosphorylation activity of TSSK4 at Thr-197. Ubiquitinated; HSP90 activity negatively regulates ubiquitination and degradation. Male mice are subfertile due to reduced sperm motility. The reduced motility is due to morphological defects in the sperm flagellum at the midpiece-principal piece junction caused by the disordered arrangement of doublet microtubules and outer dense fibers. Belongs to the protein kinase superfamily. CAMK Ser/Thr protein kinase family. Intron retention.