The beta(1) subunit but not the beta(2) subunit colocalizes with the human heart Na+ channel (hH1) already within the endoplasmic reticulum

Voltage-dependent Na+ channels are heteromultimers consisting of a pore-forming alpha subunit and accessory beta subunits. In order to provide more insight into the trafficking and assembly of the cardiac Na+ channel complex, we investigated the subcellular localization of the Na+ channel beta(1) and beta(2) subunits, both in the absence and presence of the human heart Na+ channel (hH1). We fused spectrally distinct variants of the green fluorescent protein (GFP) to hH1 and to the beta(1) and beta(2) subunit, and expressed the optically labeled P subunits separately or in combination with hH1 in HEK293 cells. In contrast to the predominant localization of hH1 channels within the endoplasmic reticulum. (ER), both beta subunits were clearly targeted to the plasma membrane when expressing their cDNAs alone. Upon coexpression of the a subunit, the beta(1) subunit was efficiently retained within the ER and found to be colocalized with hH1. In contrast to this, hH1 and the beta(2) subunit were not colocalized, i.e., they were detected mainly within the ER and the plasma membrane, respectively. These results indicate that hH1 and the beta(2) subunit are transported separately to the plasma membrane whereas the hH1/beta(1) complex occurs already within the ER, which possibly facilitates trafficking of the channel complex to the plasma membrane.