Alternative glycosylation of the insulin receptor prevents oligomerizationand acquisition of insulin-dependent tyrosine kinase activity

Glucose deprivation leads to the synthesis of an aberrantly glycosylated (' alternative') and inefficiently processed form of the insulin proreceptor i n 3T3-L1 adipocytes. To further explore the effect of aberrant (rather than absent) N-linked glycosylation of the insulin receptor, we examined the re lationship of processing to function. Our studies show that the alternative form of the proreceptor does not oligomerize nor does it acquire the abili ty to undergo insulin-sensitive autophosphorylation. This along with an int eraction with the glucose-regulated stress protein GRP78/BiP implies inappr opriate folding/dimerization and retention in the ER. Glucose refeeding cau ses the post-translational modification of the alternative form of the pror eceptor to a novel 'intermediate' form which is independent of new protein synthesis. As little as 100 muM glucose (or mannose) can induce this modifi cation. In vitro digestion of the alternative and intermediate proreceptors with SPC1/furin shows that both the alpha- and beta -subunit domains are g lycosylated, albeit aberrantly. This implies that the aberrantly glycosylat ed proreceptor could serve as a substrate for SPC1 in a physiological setti ng if the receptor was able to interact with the enzyme in the appropriate compartment (i.e., the trans-Golgi network). Based on inhibitor studies, ho wever, both the alternative and intermediate forms of the proreceptor appea r to be primarily targeted to the proteasome for degradation. (C) 2000 Else vier Science B.V. All rights reserved.