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

Background: The β-adrenergic augmentation of cardiac contraction, by increasing the conductivity of L-type voltage-gated Ca_V1.2 channels, is of great physiological and pathophysiological importance. Stimulation of β-adrenergic receptors (βAR) activates protein kinase A (PKA) through separation of regulatory (PKAR) from catalytic (PKAC) subunits. Free PKAC phosphorylates the inhibitory protein Rad, leading to increased Ca²⁺ influx. In cardiomyocytes, the core subunit of Ca_V1.2, Ca_V1.2α₁, exists in two forms: full-length or truncated (lacking the distal C-terminus (dCT)). Signaling efficiency is believed to emanate from protein interactions within multimolecular complexes, such as anchoring PKA (via PKAR) to Ca_V1.2α₁ by A-kinase anchoring proteins (AKAPs). However, AKAPs are inessential for βAR regulation of Ca_V1.2 in heterologous models, and their role in cardiomyocytes also remains unclear. Results: We show that PKAC interacts with Ca_V1.2α₁ in heart and a heterologous model, independently of Rad, PKAR, or AKAPs. Studies with peptide array assays and purified recombinant proteins demonstrate direct binding of PKAC to two domains in Ca_V1.2α₁-CT: the proximal and distal C-terminal regulatory domains (PCRD and DCRD), which also interact with each other. Data indicate both partial competition and possible simultaneous interaction of PCRD and DCRD with PKAC. The βAR regulation of Ca_V1.2α₁ lacking dCT (which harbors DCRD) was preserved, but subtly altered, in a heterologous model, the Xenopus oocyte. Conclusions: We discover direct interactions between PKAC and two domains in Ca_V1.2α₁. We propose that these tripartite interactions, if present in vivo, may participate in organizing the multimolecular signaling complex and fine-tuning the βAR effect in cardiomyocytes.