A GTP-DEPENDENT STEP IN THE ACTIVATION MECHANISM OF CAPACITATIVE CALCIUM INFLUX

Calcium influx in electrically non-excitable cells is regulated by the firing state of intracellular calcium stores. Depletion of stores act ivates plasma membrane channels that are voltage-independent and highl y selective for Ca2+ ions. We report here that the activation of plasm a membrane Ca2+ currents induced by depletion of Ca2+ stores requires a diffusible cytosolic factor that washes out with time when dialyzing cells in the whole-cell configuration of the patch-clamp technique. T he activation of calcium release-activated calcium current (I(CRAC)) b y ionomycin- or inositol 1,4,5-trisphosphate-induced store depletion i s blocked by guanosine 5'-3-O(thio)triphosphate (GTPgammaS) and guanyl -5'-yl imidodiphosphate, non-hydrolyzable analogs of GTP, suggesting t he involvement of a GTP-binding protein. The inhibition by GTPgammaS o ccurs at a step prior to the activation of I(CRAC) and is prevented by the addition of GTP. We conclude that the activation mechanism of dep letion-induced Ca2+ influx encompasses a GTP-dependent step, possibly involving an as yet unidentified small GTP-binding protein.