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Individual members of the RGK family of Ras-related GTPases, which comprise Rad, Gem/Kir, Rem and Rem2, have been implicated in important functions such as the regulation of voltage-gated calcium channel activity and remodeling of cell shape. The GTPase Kir/Gem inhibits the activity of calcium channels by interacting with the β-subunit and also regulates cytoskeleton dynamics by inhibiting the Rho-Rho kinase pathway. In addition, Kir/Gem interacts with 14-3-3 and calmodulin, but the significance of this interaction on Kir/Gem function is poorly understood. Here, we present a comprehensive analysis of the binding of 14-3-3 and calmodulin to Kir/Gem. We show that 14-3-3, in conjunction with calmodulin, regulates the subcellular distribution of Kir/Gem between the cytoplasm and the nucleus. In addition, 14-3-3 and calmodulin binding modulate Kir/Gem-mediated cell shape remodeling and downregulation of calcium channel activity. Competition experiments show that binding of 14-3-3, calmodulin and calcium channel β-subunits to Kir/Gem is mutually exclusive, providing a rationale for the observed regulatory effects of 14-3-3 and calmodulin on Kir/Gem localization and function.

kir/Gem, Rad, Rem and Rem2 comprise the RGK (Rad/Gem/kir) family of Ras-related small G-proteins. Two important functions of RGK proteins are the regulation of the VDCC (voltage-dependent Ca2+ channel) activity and cell-shape remodelling. RGK proteins interact with 14-3-3 and CaM (calmodulin), but their role on RGK protein function is poorly understood. In contrast with the other RGK family members, Rem2 has been reported to bind neither 14-3-3 nor induce membrane extensions. Furthermore, although Rem2 inhibits VDCC activity, it does not prevent cell-surface transport of Ca2+ channels as has been shown for kir/Gem. In the present study, we re-examined the functions of Rem2 and its interaction with 14-3-3 and CaM. We show that Rem2 in fact does interact with 14-3-3 and CaM and induces dendrite-like extensions in COS cells. 14-3-3, together with CaM, regulates the subcellular distribution of Rem2 between the cytoplasm and the nucleus. Rem2 also interacts with the beta-subunits of VDCCs in a GTP-dependent fashion and inhibits Ca2+ channel activity by blocking the alpha-subunit expression at the cell surface. Thus Rem2 shares many previously unrecognized features with the other RGK family members.

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The members of the RGK small GTP‐binding protein family, Kir/Gem, Rad, Rem and Rem2, are multifunctional proteins that regulate voltage‐gated calcium channel activity and cell shape remodeling. Calmodulin (CaM) or CaM 14‐3‐3 are regulators of RGK functions and their association defines the subcellular localization of RGK proteins. Abolition of CaM association results in the accumulation of RGK proteins in the nucleus, whereas 14‐3‐3 binding maintains them in the cytoplasm. Kir/Gem possesses nuclear localization signals (NLS) that mediate nuclear accumulation through an importin α5‐dependent pathway (see Mahalakshmi RN, Nagashima K, Ng MY, Inagaki N, Hunziker W, Béguin P. Nuclear transport of Kir/Gem requires specific signals and importin α5 and is regulated by Calmodulin and predicted service phosphorylations. Traffic 2007; doi: 10.1111/j.1600‐0854.2007.00598.x). Because the extent of nuclear localization depends on the RGK protein and the cell type, the mechanism and regulation of nuclear transport may differ. Here, we extend our analysis to the other RGK members and show that Rem also binds importin α5, whereas Rad associates with importins α3, α5 and β through three conserved NLS. Predicted phosphorylation of a serine residue within the bipartite NLS affects, as observed for Kir/Gem, nuclear accumulation of Rem, but not that of Rad or Rem2. We also identify an additional regulatory phosphorylation for all RGK proteins that prevents binding of 14‐3‐3 and thereby interferes with their cytosolic relocalization by 14‐3‐3. Functionally, nuclear localization of RGK proteins contributes to the suppression of RGK‐mediated cell shape remodeling. Importantly, we show that endogenous RGK proteins are localized predominantly in the nucleus of individual cells of the brain cortex ‘in situ’ as well as in primary hippocampal cells, indicating that transport between the nucleus and their site of action in the cytoplasm (i.e., cytoskeleton, endoplasmic reticulum or plasma membrane) is of physiological relevance for the regulation of RGK protein function.