Sites on calmodulin that interact with the C-terminal tail of Ca v1.2 channel

Abstract

Two fragments of the C-terminal tail of the α1 subunit (CT1, amino acids 1538-1692 and CT2, amino acids 1596-1692) of human cardiac L-type calcium channel (Cav1.2) have been expressed, refolded, and purified. A single Ca2+-calmodulin binds to each fragment, and this interaction with Ca2+-calmodulin is required for proper folding of the fragment. Ca2+-calmodulin, bound to these fragments, is in a more extended conformation than calmodulin bound to a synthetic peptide representing the IQ motif, suggesting that either the conformation of the IQ sequence is different in the context of the longer fragment, or other sequences within CT2 contribute to the binding of calmodulin. NMR amide chemical shift perturbation mapping shows the backbone conformation of calmodulin is nearly identical when bound to CT1 and CT2, suggesting that amino acids 1538-1595 do not contribute to or alter calmodulin binding to amino acids 1596-1692 of Cav1.2. The interaction with CT2 produces the greatest changes in the backbone amides of hydrophobic residues in the N-lobe and hydrophilic residues in the C-lobe of calmodulin and has a greater effect on residues located in Ca2+ binding loops I and II in the N-lobe relative to loops III and IV in the C-lobe. In conclusion, Ca2+-calmodulin assumes a novel conformation when part of a complex with the C-terminal tail of the Cav1.2 α1 subunit that is not duplicated by synthetic peptides corresponding to the putative binding motifs. © 2005 by The American Society for Biochemistry and Molecular Biology, Inc.