Effect of alternative glycosylation on insulin receptor processing

Abstract

The mature insulin receptor is a cell surface heterotetrameric glycoprotein composed of two α- and two β-subunits. In 3T3-L1 adipocytes as in other cell types, the receptor is synthesized as a single polypeptide consisting of uncleaved α- and β-subunits, migrating as a 190-kDa glycoprotein. To examine the importance of N-linked glycosylation on insulin receptor processing, we have used glucose deprivation as a tool to alter protein glycosylation. Western blot analysis shows that glucose deprivation led to a time-dependent accumulation of an alternative proreceptor of 170 kDa in a subcellular fraction consistent with endoplasmic reticulum localization. Co-precipitation assays provide evidence that the alternative proreceptor bound GRP78, an endoplasmic reticulum molecular chaperone. N-Glycosidase F treatment shows that the alternative proreceptor contained N-linked oligosaccharides. Yet, endoglycosidase H insensitivity indicates an aberrant oligosaccharide structure. Using pulse-chase methodology, we show that the synthetic rate was similar between the normal and alternative proreceptor. However, the normal proreceptor was processed into α- and β-subunits (t 1/2 = 1.3 ± 0.6 h), while the alternative proreceptor was degraded (t 1/2 = 5.1 ± 0.6 h). Upon refeeding cells that were initially deprived of glucose, the alternative proreceptor was processed to a higher molecular weight form and gained sensitivity to endoglycosidase H. This 'intermediate' form of the proreceptor was also degraded, although a small fraction escaped degradation, resulting in cleavage to the α- and β-subunits. These data provide evidence for the first time that glucose deprivation leads to the accumulation of an alternative proreceptor, which can be post-translationally glycosylated with the readdition of glucose inducing both accelerated degradation and maturation.