The mechanism of LQTS related CaM mutation E141G interfering with Ca_V1.2 channels function through its C-lobe

Mutations in the CALM1-3 genes, which encode calmodulin (CaM), have been reported in clinical cases of long QT syndrome (LQTS). Specifically, the CaM mutant E141G (CaM_E141G) in the variant CALM1 gene has been identified as a causative factor in LQTS. This mutation disrupts the normal Ca²⁺-dependent inactivation (CDI) function of Ca_V1.2 channels. However, it is still unclear how CaM_E141G interferes with the regulatory role of wild-type (WT) CaM on Ca_V1.2 channels and leads to abnormal CDI. A CaM molecule contains two lobes with similar structure, the N-lobe and the C-lobe. In this study, a CaM-truncated C-lobe mutant E141G (C-lobe_E141G) was engineered to exclude the impact of the unmutated N-lobe. Our findings revealed that at low Ca²⁺ concentration ([Ca²⁺]), the binding of C-lobe_E141G to the preIQ, IQ and N-terminus (NT) of Ca_V1.2 channels has higher binding capacity (B_max: 0.17, 0.22, 0.13) compared with those of WT C-lobe (B_max: 0.04, 0.14, 0.11) in GST pull-down assay. With an increase in [Ca²⁺], the Ca²⁺-dependency for C-lobe_E141G binding to Ca_V1.2 channels was impaired. Moreover, C-lobe_E141G induced the relative channel activity to 240.58 ± 51.37% at resting [Ca²⁺], but it was unable to diminish the channel activity at high [Ca²⁺] even in the presence of WT N-lobe, which may be responsible for the abnormal CDI of Ca_V1.2 channels affected by the LQTS-related CaM mutation. Our research provides preliminary insights into the mechanism by which the CaM mutation interferes with Ca_V1.2 channels function through its C-lobe.