EFFECT OF PP120 ON RECEPTOR-MEDIATED INSULIN ENDOCYTOSIS IS REGULATEDBY THE JUXTAMEMBRANE DOMAIN OF THE INSULIN-RECEPTOR

pp120, a substrate of the insulin receptor tyrosine kinase, does not u ndergo ligand-stimulated phosphorylation by the insulin-like growth fa ctor-1 (IGF-1) receptor. However, replacement of the C-terminal domain of the IGF-1 receptor beta-subunit with the corresponding segment of the insulin receptor restored pp120 phosphorylation by the chimeric re ceptor. Since pp120 stimulates receptor-mediated insulin endocytosis w hen it is phosphorylated, we examined whether pp120 regulates IGF-1 re ceptor endocytosis in transfected NIH 3T3 cells. pp120 failed to alter IGF-1 receptor endocytosis via either wild-type or chimeric IGF-1 rec eptors. Thus, the effect of pp120 on hormone endocytosis is specific t o insulin, and the C-terminal domain of the beta-subunit of the insuli n receptor does not regulate the effect of pp120 on insulin endocytosi s. Mutation of Tyr(960) in the juxtamembrane domain of the insulin rec eptor abolished the effect of pp120 to stimulate receptor endocytosis, without affecting pp120 phosphorylation by the insulin receptor. Thes e findings suggest that pp120 interacts with two separate domains of t he insulin receptor as follows: a C-terminal domain required for pp120 phosphorylation and a juxtamembrane domain required for internalizati on. We propose that the interaction of pp120 with the juxtamembrane do main is indirect and requires one or more substrates that bind to Tyr( 960) in the insulin receptor.