Glycosylation increases potassium channel stability and surface expression in mammalian cells

N-linked glycosylation is not required for the cell surface expression of functional Shaker potassium channels in Xenopus oocytes (Santacruz-Toloza, L., Huang, Y., John, S. A., and Papazian, D. M. (1994) Biochemistry 33, 5607-5613). We have now investigated whether glycosylation increases the stability, cell surface expression, and proper folding of Shaker protein expressed in mammalian cells. The turnover rates of wild-type protein and an unglycosylated mutant (N259Q,N263Q) were compared in pulse-chase experiments. The wild-type protein was stable, showing little degradation after 48 h. In contrast, the unglycosylated mutant was rapidly degraded (t(1/2) = approximately 18 h). Lactacystin slowed the degradation of the mutant protein, implicating cytoplasmic proteasomes in its turnover. Rapid lactacystin-sensitive degradation could be conferred on wild-type Shaker by a glycosylation inhibitor. Expression of the unglycosylated mutant on the cell surface, assessed using immunofluorescence microscopy and biotinylation, was dramatically reduced compared with wild type. Folding and assembly were analyzed by oxidizing intersubunit disulfide bonds, which provides a fortuitous hallmark of the native structure. Surprisingly, formation of disulfide-bonded adducts was quantitatively similar in the wild-type and unglycosylated mutant proteins. Our results indicate that glycosylation increases the stability and cell surface expression of Shaker protein but has little effect on acquisition of the native structure.