Auxiliary component of the CatSper complex, a complex involved in sperm cell hyperactivation (PubMed:34225353, PubMed:19516020). Sperm cell hyperactivation is needed for sperm motility which is essential late in the preparation of sperm for fertilization (PubMed:19516020). 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, PubMed:21224844, PubMed:19516020). HSPA1 may be an additional auxiliary complex member (PubMed:19516020). 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). Testis-specific. Specifically expressed in the principal piece of the sperm tail (at protein level). Expressed in spermatocytes and spermatids within the seminiferous tubule but not in interstitial cells. Catsperg2 is absent in sperm from mice lacking Catsper1, suggesting that stable expression requires Catsper1. Belongs to the CATSPERG family.