Diabetes-associated mutations in insulin: consecutive residues in the B chain contact distinct domains of the insulin receptor.

How insulin binds to and activates the insulin receptor has long been the subject of speculation. Of particular interest are invariant phenylalanine residues at consecutive positions in the B chain (residues B24 and B25). Sites of mutation causing diabetes mellitus, these residues occupy opposite structural environments:  PheB25 projects from the surface of insulin, whereas PheB24 packs against the core. Despite these differences, site-specific cross-linking suggests that each contacts the insulin receptor. Photoactivatable derivatives of insulin containing respective p-azidophenylalanine substitutions at positions B24 and B25 were synthesized in an engineered monomer (DKP-insulin). On ultraviolet irradiation each derivative cross-links efficiently to the receptor. Packing of PheB24 at the receptor interface (rather than against the core of the hormone) may require a conformational change in the B chain. Sites of cross-linking in the receptor were mapped to domains by Western blot. Remarkably, whereas B25...