Regulation of Ca2+ channel expression at the cell surface by the small G-protein kir/Gem.

Voltage-dependent calcium (Ca2+) channels are involved in many specialized cellular functions1,2,3, and are controlled by intracellular signals such as heterotrimeric G-proteins4, protein kinases5,6 and calmodulin (CaM)7,8. However, the direct role of small G-proteins in the regulation of Ca2+ channels is unclear. We report here that the GTP-bound form of kir/Gem, identified originally as a Ras-related small G-protein that binds CaM9,10,11, inhibits high-voltage-activated Ca2+ channel activities by interacting directly with the β-subunit. The reduced channel activities are due to a decrease in α1-subunit expression at the plasma membrane. The binding of Ca2+/CaM to kir/Gem is required for this inhibitory effect by promoting the cytoplasmic localization of kir/Gem. Inhibition of L-type Ca2+ channels by kir/Gem prevents Ca2+-triggered exocytosis in hormone-secreting cells. We propose that the small G-protein kir/Gem, interacting with β-subunits, regulates Ca2+ channel expression at the cell surface.