Binding characteristics of calpastatin domain L to NaV1.5 sodium channel and its IQ motif mutants.

NaV1.5 channel is an integral membrane protein involved in the initiation and conduction of action potentials. IQ motif is located in the C-terminal domain of NaV1.5 sodium channel, which is highly conserved in human sodium channel subtypes. IQ motif is involved in the Ca2+-dependent regulation through interaction with the regulatory proteins such as calpastatin domain L (CSL). Mutations in SCN5A, the gene encoding NaV1.5 channel, have been linked to many cardiac arrhythmias, such as Long QT syndrome type 3 (LQT3) and Brugada syndrome (BRS). LQT3-associated mutations in NaV1.5 IQ motif, IQQ1909R and IQR1913H, have been reported to affect the late INa. A BRS-associated mutation in NaV1.5 IQ motif, IQA1924T, has been reported to affect the peak INa. But the detailed pathogenic mechanisms of LQT3 and BRS remains unclear. To explore the binding properties of CSL to IQ motif and its muants associated with LQT3/BRS, molecular docking and GST pull down assay were performed in this study. As a result, S58 and E59 in CSL activating channel effect region L54-64 were involved in the conformation of the CSL/IQWT complex by protein-protein docking. IQ motif could bind to CSL in a [CSL]-dependent and [Ca2+]-dependent manner by pull down assay. However, the binding affinities of IQQ1909R and IQR1913H to CSL were decreased and its reaction rates with CSL were slower. The binding characteristics of IQA1924T to CSL was opposite in a [Ca2+]-dependent manner and its binding efficacy became smaller. The changes of the binding characteristics of IQmutants to CSL would affect the regulation of NaV1.5 channel, which may be related to LQT3 and BRS.