Tyrosine 1062 of Ret-MEN2A is a docking site for Rai protein. (A) Lysates from serum-starved Cos cells transiently cotransfected with cDNAs encoding p52Rai and different Ret mutants were immunoprecipitated (IP) with anti-Ret antibodies and immunoblotted with either anti-Rai (α-Rai) (upper panel), anti-pTyr (α-pTyr) (middle panel), or anti-Ret (α-Ret) (lower panel) antibodies. WB, Western blot. (B) Serum-starved Cos cells transiently transfected with p52Rai were lysed and incubated with immobilized GST or the GST fusion proteins representative of unphosphorylated (Ret-COOH) or tyrosine-phosphorylated (pYRet-COOH) Ret. GST-bound proteins were revealed by using anti-Rai antibodies. (C) Cos cells were transfected with Ret-MEN2A or Ret-MEN2A Y1062F cDNAs and serum starved. Lysates were incubated with immobilized GST or the indicated Rai GST fusion proteins. Immunoblotting with anti-Ret antibodies revealed GST-bound proteins. +, present; −, absent. (D) Lysates from Cos cells transiently transfected with Ret-MEN2A and serum starved were incubated with Rai GST-PTB in the presence of increasing amounts of an unphosphorylated (p1062) or a tyrosine-phosphorylated (pp1062) peptide spanning Ret tyrosine 1062. GST-bound proteins were visualized with anti-Ret antibodies.