Structural differences in Apo-, D132E- and Holo-Calmodulin bound to the IQ domain of the cardiac L-type calcium...

Proper regulation of the cardiac L-type calcium channel (Ca_V1.2) involves calcium-dependent inactivation (CDI) of the channel. CDI is mediated by the calcium-sensing protein calmodulin (CaM), which interacts with the IQ domain of Ca_V1.2. CaM mutations have been implicated in dangerous cardiac arrhythmias such as long-QT syndrome (LQTS), but it is not clear how structural changes at the CaM-Ca_V1.2 binding interface can lead to LQTS. In this study, we examine structural differences in holo-, apo- and D132E-CaM bound to the IQ domain of Ca_V1.2 using molecular dynamics (MD) simulations and molecular mechanics Poisson-Boltzmann surface area (MMPBSA) binding free energy calculations. Calcium (Ca²⁺) binding to CaM is shown to stabilise key binding interaction triplets in the C-lobe of holo-CaM-Ca_V1.2. We additionally find that D132E-CaM-Ca_V1.2 is able to bind with higher affinity than holo-CaM-Ca_V1.2 due to subtle structural changes taking place at the CaM-Ca_V1.2 binding interface. The differences in the CaM-Ca_V1.2 binding interface are larger when calcium is not bound, as opposed to the differences induced by the mutation, which suggests that the D132E mutation may lead to LQTS via defective binding of Ca²⁺ to CaM as opposed to structural changes occurring at the CaM-Ca_V1.2 binding interface.