Inhibition of protein kinase C (PKC) response of voltage-gated calcium (Cav)2.2 channels expressed in Xenopus oocytes by Cavβ subunits

Abstract

Cav2.2 channels are a substrate for phosphorylation by protein kinase C (PKC) isozymes. The contribution of Cavβ, an auxiliary subunit of these channels, in the PKC modulation was studied. Cav2.2 channels were expressed in Xenopus oocytes in various subunit combinations with or without Cavβ subunits. Currents were recorded using a two-electrode voltage clamp with barium as the charge carrier (IBa). Acetyl-β-methylcholine (MCh), an activator of PKCα, potentiated Cav2.2 currents expressed with Cav2.2α1 alone or Cav2.2α1α2/δ. Similarly PKC isozymes α, βII or ɛ potentiated IBa through Cav2.2α1 subunit channels. In contrast, MCh failed to potentiate currents expressed with Cav2.2α1 and Cavβ1b, β2a, β3 or β4 subunits. Similarly, in the presence of Cavβ1b subunits, PKC isozymes failed to potentiate these currents; contrarily, PKCs α or βII decreased the IBa. MCh failed to potentiate Cav2.2α1 subunit currents in the serine/threonine (Ser/Thr)→alanine mutants, T422A, S1757A or S2132A of Cav2.2α1 subunits. Hence Thr-422, Ser-1757 and Ser-2132 may be PKCα isozyme target sites. The action of PKC on these sites was further substantiated by the increased basal IBa along with the loss of MCh potentiation when Ser/Thr was mutated to aspartate. The observation that MCh or PKC isozymes failed to affect Cav2.2 currents in the presence of Cavβ subunits suggests that these subunits may have interfered with the interaction between PKC and Ser/Thr sites of Cav2.2α1 subunits. In addition to affecting channel expression and current kinetics, Cavβ subunits may also modulate the response of these channels to neurochemicals.

Keywords: Xenopus oocytes; calcium channels; muscarinic receptor 1; protein kinase C; subunits; two-electrode voltage clamp.