Tripartite interactions of PKA catalytic subunit and C-terminal domains of cardiac Ca2+ channel modulate its β-adrenergic regulation

The adrenergic nervous system augments cardiac contraction by increasing the activity of L-type voltage-gated CaV1.2 channels. Dysregulation of this process is linked to severe cardiac dysfunctions. The signaling cascade involves activation of {beta}-adrenergic receptors, elevation of cAMP levels, separation of protein kinase A (PKA) regulatory subunit (PKAR) from catalytic subunit (PKAC), and phosphorylation of the inhibitory protein Rad leading to increased Ca2+ influx. In cardiomyocytes, the core subunit of CaV1.2 (1C) exists in two forms: full-length (FL) or proteolytically processed (truncated), lacking the distal C-terminus (dCT). Specificity and efficiency in the cascade are believed to emanate from unique protein-protein interactions, such as anchoring PKA (via PKAR) to 1C by A-kinase anchoring proteins (AKAPs). However, most AKAPs do not interact with the truncated 1C, and their role in {beta}AR regulation of cardiac CaV1.2 remains unclear. Here we show that PKAC, independently of PKAR or AKAPs, directly interacts with 1C at two domains in 1C-CT: the proximal and distal C-terminal regulatory domains (PCRD and DCRD), which also interact with each other. Furthermore, we find that DCRD competes with PCRD and reduces its interaction with PKAC. The physiological consequences of these complex interactions are incompletely understood; our data suggest that they may fine-tune the {beta}AR regulation of CaV1.2. We propose that the newly discovered interactions take part in governing colocalization of regulatory proteins within the {beta}AR-CaV1.2 multimolecular signaling complexes in cardiomyocytes.