cAMP-dependent regulation of cardiac L-type Ca²⁺ channels requires membrane targeting of PKA and phosphorylation of channel subunits

The cardiac L-type Ca²⁺ channel is a textbook example of an ion channel regulated by protein phosphorylation; however, the molecular events that underlie its regulation remain unknown. Here, we report that in transiently transfected HEK293 cells expressing L-type channels, elevations in cAMP resulted in phosphorylation of the α(1c) and β(2a) channel subunits and increases in channel activity. Channel phosphorylation and regulation were facilitated by submembrane targeting of protein kinase A (PKA), through association with an A-kinase anchoring protein called AKAP79. In transfected cells expressing a mutant AKAP79 that is unable to bind PKA, phosphorylation of the α(1c) subunit and regulation of channel activity were not observed. Furthermore, we have demonstrated that the association of an AKAP with PKA was required for β-adrenergic receptor-mediated regulation of L-type channels in native cardiac myocytes, illustrating that the events observed in the heterologous expression system reflect those occurring in the native system. Mutation of Ser1928 to alanine in the C-terminus of the α(1c) subunit resulted in a complete loss of cAMP-mediated phosphorylation and a loss of channel regulation. Thus, the PKA-mediated regulation of L-type Ca²⁺ channels is critically dependent on a functional AKAP and phosphorylation of the α(1c) subunit at Ser1928.