As shown in Figure 4A, co-expression of CNIH-2 reduced the apparent molecular weight of surface GluA2i isolated by surface biotinylation. To investigate whether the shift in mass was due to differences in glycosylation, we tested surface GluA2 receptors assembled in the absence or presence of CNIH-2 for their sensitivities to treatment with endoglycosidase H (Endo H) and PNGase F. While Endo H selectively cleaves high-mannose oligosaccharides, PNGase F removes all glycosylations [40]. As shown in Figure 4B, surface GluA2 receptors formed upon co-expression with CNIH-2 ran at smaller apparent MW and retained sensitivity to Endo H, while the surface population of homomeric GluA2 was only sensitive to treatment by PNGase F. This suggested that in HeLa cells CNIH-2 promotes export of GluA2 protein with an immature glycosylation pattern.
