RGK Small GTP-binding Proteins Interact with the Nucleotide Kinase Domain of Ca super(2+)-channel beta -Subunits via an Uncommon Effector Binding Domain

RGK proteins (Kir/Gem, Rad, Rem, and Rem2) form a small subfamily of the Ras superfamily. Despite a conserved GTP binding core domain, several differences suggest that structure, mechanism of action, and functional regulation differ from Ras. RGK proteins down-regulate voltage-gated calcium channel activity by binding in a GTP-dependent fashion to the Ca sub(v) beta subunits. Mutational analysis combined with homology modeling reveal a novel effector binding mechanism distinct from that of other Ras GTPases. In this model the Switch 1 region acts as an allosteric activator that facilitates electrostatic interactions between Arg-196 in Kir/Gem and Asp-194, -270, and -272 in the nucleotide-kinase (NK) domain of Ca sub(v) beta 3 and wedging Val-223 and His-225 of Kir/Gem into a hydrophobic pocket in the NK domain. Kir/Gem interacts with a surface on the NK domain that is distinct from the groove where the voltage-gated calcium channel Ca sub(v) alpha 1 subunit binds. A complex composed of the RGK protein and the Ca sub(v) beta 3 and Ca sub(v)1.2 subunits could be revealed in vivo using coimmunoprecipitation experiments. Intriguingly, docking of the RGK protein to the NK domain of the Ca sub(v) beta subunit is reminiscent of the binding of GMP to guanylate kinase.