Pore-forming subunit of the CatSper complex, a sperm-specific voltage-gated calcium channel, that plays a central role in calcium- dependent physiological responses essential for successful fertilization, such as sperm hyperactivation, acrosome reaction and chemotaxis towards the oocyte. {ECO:0000269|PubMed:21412338, ECO:0000269|PubMed:21412339}. This is the function of CATSPER2 (cation channel sperm associated 2, Ensembl gene identifier ENSG00000166762).