Membrane targeting of ATP-sensitive potassium channel. Effects of glycosylation on surface expression

Oligosaccharides play significant roles in trafficking, folding, and sorting of membrane proteins. Sulfonylurea receptors (SURx), members of the ATP binding cassette family of proteins, associate with the inward rectifier Kir6.x to form ATP-sensitive potassium channels (K(ATP)). These channels are found on the plasma membrane in many tissues and play a pivotal role in synchronizing electrical excitability with cell metabolic state. Trafficking defects resulting from three independent SUR1 mutations involved in the disease persistent hyperinsulinemic hypoglycemia of infancy have been described. Two of these mutations displayed notable decreases in glycosylation. Here we have investigated the relationship between the two N-linked glycosylation sites (Asn(10) and Asn(1050)) and SUR1 trafficking. Using patch clamp analysis, surface biotinylation, and immunofluorescence microscopy, we demonstrate a significant decrease in surface expression of SUR1 single or double glycosylation site mutants (N10Q,N1050Q) when co-expressed with Kir6.2. Additionally, we show prominent retention within the ER of the SUR1 double glycosylation mutant under the same conditions. Further investigation revealed that mutation of the ER retention signal was able to partially restore surface expression of the SUR1 double glycosylation mutant. These studies suggest that SUR1 glycosylation is a key element for the proper trafficking and surface expression of K(ATP) channels.