Tripartite interactions of PKA catalytic subunit and C-terminal domains of cardiac Ca ²⁺ channel modulate its β-adrenergic regulation

The adrenergic nervous system augments cardiac contraction by increasing the activity of L-type voltage-gated Ca _V 1.2 channels. Dysregulation of this process is linked to severe cardiac dysfunctions. The signaling cascade involves activation of β-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 Ca ²⁺ influx. In cardiomyocytes, the core subunit of Ca _V 1.2 (α _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 α _1C by A-kinase anchoring proteins (AKAPs). However, most AKAPs do not interact with the truncated α _1C , and their role in βAR regulation of cardiac Ca _V 1.2 remains unclear. Here we show that PKAC, independently of PKAR or AKAPs, directly interacts with α _1C at two domains in α _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 βAR regulation of Ca _V 1.2. We propose that the newly discovered interactions take part in governing colocalization of regulatory proteins within the βAR-Ca _V 1.2 multimolecular signaling complexes in cardiomyocytes.