REGULATION OF ACTIVIN TYPE-I RECEPTOR FUNCTION BY PHOSPHORYLATION OF RESIDUES OUTSIDE THE GS DOMAIN

Activin signals through a heteromeric complex of receptor serine kinas es by inducing type II receptor-mediated phosphorylation, and conseque nt activation, of the type I receptor, Type I receptor phosphorylation occurs at a glycine- and serine-rich site in the juxtamembrane domain ; phosphorylation at that site correlates with signaling, Investigatio n of type I activin receptor mutants impaired for GS domain phosphoryl ation revealed that, in the presence of elevated amounts of type II ac tivin receptor, GS domain phosphorylation is not required for signalin g. The type I receptor showed activin-dependent phosphorylation of sev eral tryptic phosphopeptides, suggesting that phosphorylation of recep tor I at sites both within and outside the GS domain is required for f ull signaling. (C) 1997 Federation of European Biochemical Societies.