ABSTRACT
The voltage-gated sodium channel is critical for cardiomyocyte function and consists of a protein complex comprising a pore-forming α subunit and two associated ß subunits. It has been shown previously that the associated ß2 subunits promote cell surface expression of the α subunit. The major α isoform in the adult human heart is NaV1.5, and germline mutations in the NaV1.5-encoding gene, sodium voltage-gated channel α subunit 5 (SCN5A), often cause inherited arrhythmias. Here, we investigated the mechanisms that regulate ß2 trafficking and how they may determine proper NaV1.5 cell surface localization. Using heterologous expression in polarized Madin-Darby canine kidney cells, we show that ß2 is N-glycosylated in vivo and in vitro at residues 42, 66, and 74, becoming sialylated only at Asn-42. We found that fully nonglycosylated ß2 was mostly retained in the endoplasmic reticulum, indicating that N-linked glycosylation is required for efficient ß2 trafficking to the apical plasma membrane. The nonglycosylated variant reached the cell surface by bypassing the Golgi compartment at a rate of only approximately one-third of that of WT ß2. YFP-tagged, nonglycosylated ß2 displayed mobility kinetics in the plane of the membrane similar to that of WT ß2. However, it was defective in promoting surface localization of NaV1.5. Interestingly, ß2 with a single intact glycosylation site was as effective as the WT in promoting NaV1.5 surface localization. In conclusion, our results indicate that N-linked glycosylation of ß2 is required for surface localization of NaV1.5, a property that is often defective in inherited cardiac arrhythmias.
