Rad Phosphorylation Via β‐Adrenergic Signaling Modulates Its Association with the Calcium Channel

Abstract

Rem and Rad, small GTPases of the RGK (Rem, Rad, Gem/Kir) subfamily, potently block L‐type Ca2+ channel (LTCC) current when overexpressed. Phosphorylation of Rem downstream of α‐adrenergic (AR) signaling has been found to relieve LTCC inhibition, suggesting that Rem may be regulated by phosphorylation rather than the classic GTP/GDP guanine nucleotide cycle. Phosphoproteomic data mining indicated that Rad is subject to phosphorylation as well, but the lack of sequence conservation surrounding the Rem and Rad phosphorylation sites suggested that different regulatory kinases may be involved. Our recent study demonstrating that Rad‐/‐ mouse hearts exhibit a basal phenotype mirroring chronic β‐AR activation of LTCCs prompted us to investigate whether β‐AR signaling could regulate Rad‐mediated LTCC inhibition. This study tests the hypothesis that LTCC inhibition by Rad, like Rem, is reversed by phosphorylation but that different upstream kinase cascades afford regulatory specificity. We have observed that α‐AR activation triggers phosphorylation of Rem but not Rad. In contrast, robust phosphorylation of Rad occurs following β‐AR stimulation and is blocked by PKA inhibition. Co‐immunoprecipitations indicate that Rad phosphorylation weakens LTCC association while strengthening 14‐3‐3 binding, suggesting a mechanism for modulating Rad function. Thus, phosphorylation may be a general mechanism for controlling RGK actions, and the unique phosphorylation of Rem and Rad downstream of α‐ and β‐AR signaling suggests that these closely related GTPases are not functionally redundant but rather may allow for control of LTCC current in response to different upstream stimuli.