Glycosylation of human truncated FcεRI α chain is necessary for efficient folding in the endoplasmic reticulum

Abstract

The high affinity immunoglobulin E (IgE) receptor is an αβγ2 tetrameric complex. The truncated extracellular segment (αt) of the heavily glycosylated α chain is sufficient for high affinity binding of IgE. Here we have expressed various αt mutants in eukaryotic and prokaryotic cells to analyze the role of glycosylation in the folding, stability, and secretion of αt. All seven N-linked glycosylation sites in αt are glycosylated and their mutations have an additive effect on the folding and secretion of αt. Mutation of the seven N-glycosylation sites (Δ1-7 αt) induces misfolding and retention of αt in the endoplasmic reticulum. Similarly, tunicamycin treatment reduces substantially the folding efficiency of wild-type αt. In contrast, no difference in folding efficiency is detected between wild-type αt and Δ1-7 αt expressed in Escherichia coli. In addition, maturation of N-linked oligosaccharides and addition of O-linked carbohydrates are not required for either the transport or the IgE-binding function of αt. Furthermore, complete enzymatic deglycosylation does not affect the stability and the IgE-binding capacity of αt. Therefore, glycosylation is not intrinsically necessary for proper folding of αt but is required for folding in the endoplasmic reticulum. Our data are compatible with the concept that specific interactions between N-linked oligosaccharides and the folding machinery of the endoplasmic reticulum are necessary for efficient folding of αt in eukaryotic cells.