Apocalmodulin and Ca2+Calmodulin-Binding Sites on the CaV1.2 Channel

The cardiac L-type voltage-dependent calcium channel is responsible for initiating excitation-contraction coupling. Three sequences (amino acids 1609–1628, 1627–1652, and 1665–1685, designated A, C, and IQ, respectively) of its α1 subunit contribute to calmodulin (CaM) binding and Ca2+-dependent inactivation. Peptides matching the A, C, and IQ sequences all bind Ca2+CaM. Longer peptides representing A plus C (A-C) or C plus IQ (C-IQ) bind only a single molecule of Ca2+CaM. Apocalmodulin (ApoCaM) binds with low affinity to the IQ peptide and with higher affinity to the C-IQ peptide. Binding to the IQ and C peptides increases the Ca2+ affinity of the C-lobe of CaM, but only the IQ peptide alters the Ca2+ affinity of the N-lobe. Conversion of the isoleucine and glutamine residues of the IQ motif to alanines in the channel destroys inactivation (Zuhlke et al., 2000). The double mutation in the peptide reduces the interaction with apoCaM. A mutant CaM unable to bind Ca2+ at sites 3 and 4 (which abolishes the ability of CaM to inactivate the channel) binds to the IQ, but not to the C or A peptide. Our data are consistent with a model in which apoCaM binding to the region around the IQ motif is necessary for the rapid binding of Ca2+ to the C-lobe of CaM. Upon Ca2+ binding, this lobe is likely to engage the A-C region.