Sperm-specific electroneutral NHE proteins (sNHE or SLC9C1) feature a unique tripartite structure including an exchanger homology domain, a prototypical channel-like voltage-sensing domain (VSD), and a cyclic nucleotide-binding domain (CNBD). Because SLC9C1 proteins have not been functionally expressed, their physiological properties are largely unknown. Here, we characterize the activation properties of SLC9C1 from Strongylocentrotus purpuratus (SpSLC9C1). SpSLC9C1 activity is gated by the VSD during membrane hyperpolarization and modulated by direct binding of cAMP. We recorded SpSLC9C1 activity and the underlying gating currents by single cell electrophysiology and voltage clamp fluorimetry. Both, the voltage dependence of activation and that of the corresponding gating currents are shifted towards more permissive membrane voltages by binding of cAMP to the CNBD. Our results indicate that the VSD is the pivot that gates the exchanger domain and itself is regulated by cAMP binding to the CNBD. Thus, SpSLC9C1 represents a phylogenetic hybrid between ion antiporters, voltage-gated channels, and cyclic nucleotide-gated channels.

Sperm guidance is controlled by chemical and physical cues. In many species, Ca2+ bursts in the flagellum govern navigation to the egg. In Arbacia punctulata, a model system of sperm chemotaxis, a cGMP signaling pathway controls these Ca2+ bursts. The underlying Ca2+ channel and its mechanisms of activation are unknown. Here, we identify CatSper Ca2+ channels in the flagellum of A. punctulata sperm. We show that CatSper mediates the chemoattractant‐evoked Ca2+ influx and controls chemotactic steering; a concomitant alkalization serves as a highly cooperative mechanism that enables CatSper to transduce periodic voltage changes into Ca2+ bursts. Our results reveal intriguing phylogenetic commonalities but also variations between marine invertebrates and mammals regarding the function and control of CatSper. The variations probably reflect functional and mechanistic adaptations that evolved during the transition from external to internal fertilization.