Post-translational changes in tertiary and quaternary structure of the insulin proreceptor. Correlation with acquisition of function.

Tertiary and quaternary structural changes that occur during post-translational processing of the insulin proreceptor were examined in 3T3-L1 adipocytes. In pulse-chase experiments with [35S]methionine, labeled insulin receptor species, isolated by immuno- and insulin-affinity adsorption, were analyzed by sodium dodecyl sulfate (SDS)-polyacrylamide gel electrophoresis under conditions where intra- and intermolecular disulfide bonds remained intact or were cleaved by reduction. Reducing SDS-polyacrylamide gel electrophoresis confirmed that the insulin receptor is synthesized as a long-lived (t1/2 = 3 h) proreceptor precursor of 210 kDa which undergoes proteolytic cleavage and carbohydrate maturation to form the alpha- and beta-subunits of the mature receptor. The proreceptor acquires insulin binding activity through a subtle structural change (t1/2 = 45 min) detected only by an autoimmune antibody specific for an epitope of the active insulin binding site. Analysis of insulin receptor species by nonreducing SDS-polyacrylamide gel electrophoresis revealed that the proreceptor undergoes two additional structural changes not detected by reducing SDS-polyacrylamide gel electrophoresis. The proreceptor is synthesized as a monomer (M1) with an apparent molecular mass of 170 kDa that is converted by disulfide rearrangement to another monomeric form of 190-kDa apparent molecular mass (M2). N-Linked glycosylation is required for this transition, since aglycoproreceptor, synthesized in the presence of tunicamycin, does not undergo any detectable tertiary or quaternary structural changes. M2 self-associates to form a disulfide-linked proreceptor dimer (D) which is subsequently proteolytically processed, forming the mature, disulfide-linked alpha 2 beta 2 receptor tetramer. The mature receptor was distinguished from the three proreceptor species (M1, M2, and D) by its cell surface location and its ability to bind tightly to wheat germ agglutinin-agarose, indicating the presence of complex oligosaccharide chains. Subcellular fractionation indicated that both the M1 to M2 and M2 to D conversions occur in the endoplasmic reticulum. Separation of the nonreduced proreceptor species into "active" and "inactive" forms by affinity chromatography on insulin-agarose revealed that neither the transition of M1 to M2, nor of M2 to D, is correlated with the acquisition of insulin binding function. Rather, during its life-time, the M2 species acquires insulin binding activity and an epitope recognized by a binding site specific autoimmune antibody through a subtle structural change not detected by reducing or nonreducing SDS-polyacrylamide gel electrophoresis.