Rem, Rem2, Rad, and Gem/Kir (RGK) represent a distinct GTPase family with largely unknown physiological functions. We report here that both Rem and Rad bind directly to Ca2+ channel beta-subunits (CaV beta) in vivo. No calcium currents are recorded from human embryonic kidney 293 cells coexpressing the L type Ca2+ channel subunits CaV1.2, CaV beta 2a, and Rem or Rad, but CaV1.2 and CaV beta 2a transfected cells elicit Ca2+ channel currents in the absence of these small G proteins. Importantly, CaV3 (T type) Ca2+ channels, which do not require accessory subunits for ionic current expression, are not inhibited by expression of Rem. Rem is expressed in primary skeletal myoblasts and, when overexpressed in C2C12 myoblasts, wild-type Rem inhibits L type Ca2+ channel activity. Deletion analysis demonstrates a critical role for the Rem C terminus in both regulation of functional Ca2+ channel expression and beta-subunit association. These results suggest that all members of the RGK GTPase family, via direct interaction with auxiliary beta-subunits, serve as regulators of L type Ca2+ channel activity. Thus, the RGK GTPase family may provide a mechanism for achieving cross talk between Ras-related GTPases and electrical signaling pathways.