Characterization of type II activin receptors. Binding, processing, and phosphorylation.

Activins bind to two classes of cell surface proteins, type I receptors of approximately 50-55 kDa and type II receptors of approximately 70-75 kDa. The two cloned type II activin receptors belong to a new subfamily of transmembrane protein serine kinases. Antibodies directed against each of these cloned receptors were generated and used in immunoprecipitation experiments to study the properties of the type II receptors in vivo. Precipitation of affinity-labeled receptors, formed by chemical cross-linking of 125I-activin A, resulted in coprecipitation of type I receptor complexes; denaturation of the lysates prior to interaction with the antibodies resulted in precipitation of only type II receptors. Treatment of both affinity-labeled and metabolically labeled receptors with peptide N-glycosidase F revealed the presence of N-linked carbohydrate chains. Metabolic labeling of cells with [32P]orthophosphate indicated that both type II receptors were phosphoproteins containing predominantly phosphoserine, with small amounts of phosphothreonine, but no detectable phosphotyrosine. Analysis of tryptic phosphopeptide maps of wild-type and kinase-defective mutants suggested that at least some of the phosphorylated sites arose from autophosphorylation.