The C-terminus of Rad is required for membrane localization and L-type calcium channel regulation

L-type Ca_V1.2 current (I_Ca,L) links electrical excitation to contraction in cardiac myocytes. I_Ca,L is tightly regulated to control cardiac output. Rad is a Ras-related, monomeric protein that binds to L-type calcium channel β subunits (Ca_Vβ) to promote inhibition of I_Ca,L. In addition to Ca_Vβ interaction conferred by the Rad core motif, the highly conserved Rad C-terminus can direct membrane association in vitro and inhibition of I_Ca,L in immortalized cell lines. In this work, we test the hypothesis that in cardiomyocytes the polybasic C-terminus of Rad confers t-tubular localization, and that membrane targeting is required for Rad-dependent I_Ca,L regulation. We introduced a 3xFlag epitope to the N-terminus of the endogenous mouse Rrad gene to facilitate analysis of subcellular localization. Full-length 3xFlag-Rad (Flag-Rad) mice were compared with a second transgenic mouse model, in which the extended polybasic C-termini of 3xFlag-Rad was truncated at alanine 277 (Flag-RadΔCT). Ventricular cardiomyocytes were isolated for anti-Flag-Rad immunocytochemistry and ex vivo electrophysiology. Full-length Flag-Rad showed a repeating t-tubular pattern whereas Flag-RadΔCT failed to display membrane association. I_Ca,L in Flag-RadΔCT cardiomyocytes showed a hyperpolarized activation midpoint and an increase in maximal conductance. Additionally, current decay was faster in Flag-RadΔCT cells. Myocardial I_Ca,L in a Rad C-terminal deletion model phenocopies I_Ca,L modulated in response to β-AR stimulation. Mechanistically, the polybasic Rad C-terminus confers Ca_V1.2 regulation via membrane association. Interfering with Rad membrane association constitutes a specific target for boosting heart function as a treatment for heart failure with reduced ejection fraction.