Rem GTPase interacts with the proximal CaV1.2 C-terminus and modulates calcium-dependent channel inactivation

The Rem, Rem2, Rad and Gem/Kir (RGK) GTpases, comprise a subfamily of small Ras-related GTp-binding proteins, and have been shown to potently inhibit high voltage-activated Ca2+ channel current following overexpression. although the molecular mechanisms underlying RGK-mediated Ca2+ channel regulation remain controversial, recent studies suggest that RGK proteins inhibit Ca2+ channel currents at the plasma membrane in part by interactions with accessory channel beta subunits. In this paper, we extend our understanding of the molecular determinants required for RGK-mediated channel regulation by demonstrating a direct interaction between Rem and the proximal C-terminus of Ca(V)1.2 (pCT), including the CB/IQ domain known to contribute to Ca2+/calmodulin (CaM)-mediated channel regulation. The Rem2 and Rad GTpases display similar patterns of pCT binding, suggesting that the Ca(V)1.2 C-terminus represents a common binding partner for all RGK proteins. In vitro Rem: pCT binding is disrupted by Ca2+/CaM, and this effect is not due to Ca2+/CaM binding to the Rem C-terminus. In addition, co-overexpression of CaM partially relieves Rem-mediated L-type Ca2+ channel inhibition and slows the kinetics of Ca2+-dependent channel inactivation. Taken together, these results suggest that the association of Rem with the pCT represents a crucial molecular determinant in RGK-mediated Ca2+ channel regulation and that the physiological function of the RGK GTpases must be re-evaluated. Rather than serving as endogenous inhibitors of Ca2+ channel activity, these studies indicate that RGK proteins may play a more nuanced role, regulating Ca2+ currents via modulation of Ca2+/CaM-mediated channel inactivation kinetics.