Direct inhibition of P/Q-type voltage-gated Ca2+ channels by Gem does not require a direct Gem/Cavβ interaction

摘要

The Rem, Rem2, Rad, and Gem/Kir (RGK) family of small GTP-binding proteins potently inhibits high voltage-activated (HVA) Ca²⁺ channels, providing a powerful means of modulating neural, endocrine, and muscle functions. The molecular mechanisms of this inhibition are controversial and remain largely unclear. RGK proteins associate directly with Ca²⁺ channel β subunits (Cavβ), and this interaction is widely thought to be essential for their inhibitory action. In this study, we investigate the molecular underpinnings of Gem inhibition of P/Q-type Ca²⁺ channels. We find that a purified Gem protein markedly and acutely suppresses P/Q channel activity in inside-out membrane patches, that this action requires Cavβ but not the Gem/Cavβ interaction, and that Gem coimmunoprecipitates with the P/Q channel α1 subunit (Cavα1) in a Cavβ-independent manner. By constructing chimeras between P/Q channels and Gem-insensitive low voltage-activated T-type channels, we identify a region encompassing transmembrane segments S1, S2, and S3 in the second homologous repeat of Cavα1 critical for Gem inhibition. Exchanging this region between P/Q and T channel Cavα1 abolishes Gem inhibition of P/Q channels and confers Cavβ-dependent Gem inhibition to a chimeric T channel that also carries the P/Q I-II loop (a cytoplasmic region of Cavα1 that binds Cavβ). Our results challenge the prevailing view regarding the role of Cavβ in RGK inhibition of high voltage-activated Ca²⁺ channels and prompt a paradigm in which Gem directly binds and inhibits Ca_vβ-primed Ca_vα₁ on the plasma membrane.