Stargazin Modulates Neuronal Voltage-dependent Ca2+ Channel Cav2.2 by a Gβγ-dependent Mechanism

Loss of neuronal protein stargazin (gamma(2)) is associated with recurrent epileptic seizures and ataxia in mice. Initially, due to homology to the skeletal muscle calcium channel gamma(1) subunit, stargazin and other family members (gamma(3-8)) were classified as gamma subunits of neuronal voltage-gated calcium channels (such as Ca(V)2.1-Ca(V)2.3). Here, we report that stargazin interferes with G protein modulation of Ca(V)2.2 (N-type) channels expressed in Xenopus oocytes. Stargazin counteracted the G beta gamma-induced inhibition of Ca(V)2.2 channel currents, caused either by coexpression of the G beta gamma dimer or by activation of a G protein-coupled receptor. Expression of high doses of G beta gamma overcame the effects of stargazin. High affinity G beta gamma scavenger proteins m-c beta ARK and m-phosducin produced effects similar to stargazin. The effects of stargazin and m-c beta ARK were not additive, suggesting a common mechanism of action, and generally independent of the presence of the Ca-V beta(3) subunit. However, in some cases, coexpression of Ca-V beta(3) blunted the modulation by stargazin. Finally, the G beta gamma-opposing action of stargazin was not unique to Ca(V)2.2, as stargazin also inhibited the G beta gamma-mediated activation of the G protein-activated K+ channel. Purified cytosolic C-terminal part of stargazin bound G beta gamma in vitro. Our results suggest that the regulation by stargazin of biophysical properties of Ca(V)2.2 are not exerted by direct modulation of the channel but via a G beta gamma-dependent mechanism.