AN INSULIN-RECEPTOR MUTANT (ASP(707)-]ALA), INVOLVED IN LEPRECHAUNISM, IS PROCESSED AND TRANSPORTED TO THE CELL-SURFACE BUT UNABLE TO BIND INSULIN

We have identified a homozygous mutation near the carboxyl terminus of the insulin receptor (IR) alpha subunit from a leprechaun patient, ch anging Asp(707) into Ala, Fibroblasts from this patient had no high af finity insulin binding sites, To examine the effect of the mutation on IR properties, the mutant IR was stably expressed in Chinese hamster ovary cells, Western blot analysis and metabolic labeling showed a nor mal processing of the mutant receptor to alpha and beta subunits, No i ncrease in high affinity insulin binding sites was observed on Chinese hamster ovary cells expressing the mutant receptor, and also, affinit y cross-linking of I-125-labeled insulin by di-succinimidyl suberate t o these cells failed to label the mutant alpha subunit, Biotinylation of cell surface proteins by biotin succinimidyl ester resulted in effi cient biotinylation of the mutant IR alpha and beta subunits, showing its presence on the cell surface, On solubilization of the mutant insu lin receptor in Triton X-100-containing buffers, I-125-insulin was eff iciently cross-linked to the receptor alpha subunit by disuccinimidyl suberate, These studies demonstrate that Ala(707) IR is normally proce ssed and transported to the cell surface and that the mutation distort s the insulin binding site, Detergent restores this site, This is an e xample of a naturally occurring mutation in the insulin receptor that affects insulin binding without affecting receptor transport and proce ssing, This mutation points to a major contribution of the alpha subun it carboxyl terminus to insulin binding.