Practice question

Chemotactic calcium signaling in sea urchin sperm relies on specific ion channels opened downstream of resact binding. Resact activates receptor guanylate cyclase raising cGMP, opening tetraKCNG channels causing hyperpolarization, then depleting cGMP leading to depolarization that activates voltage-dependent calcium entry. Electrophysiology and molecular cloning identified entry path as CatSper channel complex encoded by four homologous CatSper1-4 genes forming heterotetramer in principal piece of flagellum, associated with auxiliary subunits. CatSper null sperm lack calcium pulses and chemotaxis. Bindin, resact, or hyalin genes encode structural adhesion proteins or chemoattractants, not calcium channel subunits responsible for chemosensation.