Small GTPases SAR1A and SAR1B regulate the trafficking of the cardiac sodium channel Na_v1.5

The cardiac sodium channel Na_v1.5 is essential for the physiological function of the heart and causes cardiac arrhythmias and sudden death when mutated. Many disease-causing mutations in Na_v1.5 cause defects in protein trafficking, a cellular process critical to the targeting of Na_v1.5 to cell surface. However, the molecular mechanisms underlying the trafficking of Na_v1.5, in particular, the exit from the endoplasmic reticulum (ER) for cell surface trafficking, remain poorly understood.

Here we investigated the role of the SAR1 GTPases in trafficking of Na_v1.5. Overexpression of dominant-negative mutant SAR1A (T39N or H79G) or SAR1B (T39N or H79G) significantly reduces the expression level of Na_v1.5 on cell surface, and decreases the peak sodium current density (I_Na) in HEK/Na_v1.5 cells and neonatal rat cardiomyocytes. Simultaneous knockdown of SAR1A and SAR1B expression by siRNAs significantly reduces the I_Na density, whereas single knockdown of either SAR1A or SAR1B has minimal effect. Computer modeling showed that the three-dimensional structure of SAR1 is similar to RAN. RAN was reported to interact with MOG1, a small protein involved in regulation of the ER exit of Na_v1.5. Co-immunoprecipitation showed that SAR1A or SAR1B interacted with MOG1. Interestingly, knockdown of SAR1A and SAR1B expression abolished the MOG1-mediated increases in both cell surface trafficking of Na_v1.5 and the density of I_Na.

These data suggest that SAR1A and SAR1B are the critical regulators of trafficking of Na_v1.5. Moreover, SAR1A and SAR1B interact with MOG1, and are required for MOG1-mediated cell surface expression and function of Na_v1.5.