The soluble ectodomain of RetC634Y inhibits both the wild-type and the constitutively active Ret

Substitution of Cys-634 in the extracellular domain of the Ret tyrosine kinase receptor causes its dimerization and activation of its transforming potential. To gain further insight into the molecular basis leading to Ret activation we purified a mutant protein consisting of the entire ectodomain of the Ret carrying a Cys-634?¨Tyr substitution (EC-RetC634Y). The protein is glycosylated, like the native one, and is biologically active. By using an in vitro cell system we show that EC-RetC634Y inhibits the membrane-bound receptor RetC634Y, interfering with its dimerization. Furthermore, we demonstrate that EC-RetC634Y competes with the wild-type Ret receptor for ligand binding. The results presented support the notion of the possible involvment of glial cell line-derived neurotrophic factor (GDNF) with multiple endocrine neoplasia type 2A (MEN2A) tumours, and describe a useful tool for generating molecular mimetics directed towards specific mutations of the ret oncogene.