Auxiliary component of the CatSper complex, a complex involved in sperm cell hyperactivation (PubMed:34225353, PubMed:31056283). pH-dependent Ca(2+) sensor required to activate the CatSper channel (PubMed:31056283). Sperm cell hyperactivation is needed for sperm motility which is essential late in the preparation of sperm for fertilization (PubMed:31056283). Associates with the CatSper complex via direct interaction with CATSPERZ, and senses intracellular Ca(2+) (PubMed:31056283). Together with CATSPERZ, associates with the CatSper channel pore and is required for the two-row structure of each single CatSper channel (PubMed:31056283). Component of the CatSper complex or CatSpermasome composed of the core pore-forming members CATSPER1, CATSPER2, CATSPER3 and CATSPER4 as well as auxiliary members CATSPERB, CATSPERG2, CATSPERD, CATSPERE, CATSPERZ, C2CD6/CATSPERT, SLCO6C1, TMEM249, TMEM262 and EFCAB9 (PubMed:34225353). HSPA1 may be an additional auxiliary complex member (By similarity). The core complex members CATSPER1, CATSPER2, CATSPER3 and CATSPER4 form a heterotetrameric channel (PubMed:34225353). The auxiliary CATSPERB, CATSPERG2, CATSPERD and CATSPERE subunits form a pavilion-like structure over the pore which stabilizes the complex through interactions with CATSPER4, CATSPER3, CATSPER1 and CATSPER2 respectively (PubMed:34225353). SLCO6C1 interacts with CATSPERE and TMEM262/CATSPERH interacts with CATSPERB, further stabilizing the complex (PubMed:34225353). C2CD6/CATSPERT interacts at least with CATSPERD and is required for targeting the CatSper complex in the flagellar membrane (Probable). Interacts with CATSPERZ; the interaction is direct, Ca(2+)-dependent and connects EFCAB9 with the CatSper complex (PubMed:31056283). Dissociates from CATSPERZ at elevated pH (PubMed:31056283). Localizes to the principal piece of the sperm tail. Testis-specific. Only expressed after meiosis in testis. Mice are normal but males show severe male subfertility despite normal sperm morphology (PubMed:31056283). Subfertility is caused by defects in sperm motility: spermatozoa have a lower basal level of intracellular calcium and aberrant Ca(2+) homeostasis (PubMed:31056283).