Abstract

Inactivation of voltage-gated calcium channels is crucial for the spatiotemporal coordination of calcium signals and prevention of toxic calcium buildup. Only one member of the highly conserved family of calcium channel β-subunits - CaVβ - inhibits inactivation. This unique property has been attributed to short variable regions of the protein; however, here we report that this inhibition actually is conferred by a conserved guanylate kinase (GK) domain and, moreover, that this domain alone recapitulates CaVβ-mediated modulation of channel activation. We expressed and refolded the GK domain of CaVβ2a, the unique variant that inhibits inactivation, and of CaVβ1b, an isoform that facilitates it. The refolded domains of both CaVβ variants were found to inhibit inactivation of CaV2.3 channels expressed in Xenopus laevis oocytes. These findings suggest that the GK domain endows calcium channels with a brake restraining voltage-dependent inactivation, and thus facilitation of inactivation by full-length CaVβ requires additional structural determinants to antagonize the GK effect. We found that CaV can switch the inactivation phenotype conferred to CaV2.3 from slow to fast after posttranslational modifications during channel biogenesis. Our findings provide a framework within which to understand the modulation of inactivation and a new functional map of CaVβ in which the GK domain regulates channel gating and the other conserved domain (Src homology 3) may couple calcium channels to other signaling pathways. © 2008 by The National Academy of Sciences of the USA.