Four CatSper ion channel subunit genes (CatSpers 1-4) are required for sperm cell hyperactivation and male fertility. The four proteins assemble (presumably as a tetramer) to form a sperm-specific, alkalinization-activated Ca(2+)-selective channel. We set out to identify proteins associating with CatSper that might help explain its unique role in spermatozoa. Using a transgenic approach, a CatSper1 complex was purified from mouse testis that contained heat shock protein 70-2, a testis-specific chaperone, and CatSperbeta, a novel protein with two putative transmembrane-spanning domains. Like the CatSper ion channel subunits, CatSperbeta was restricted to testis and localized to the principal piece of the sperm tail. CatSperbeta protein is absent in CatSper1(-/-) sperm, suggesting that it is required for trafficking or formation of a stable channel complex. CatSperbeta is the first identified auxiliary protein to the CatSper channel.
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