Activin-A binds to a heterotrimeric receptor complex on the vascular endothelial cell surface. Evidence for a type 3 activin receptor.

The effect of transfection of the type 2 activin receptor, ACTR2, on binding of 125I-activin-A to the surface of bovine aortic endothelial cells (BAEC) was investigated. BAEC transfected either with full-length ACTR2 or with a truncated form of ACTR2 lacking the intracellular kinase domain (ACTR2T) displayed two classes of 125I-activin-A binding sites, one of high affinity (Kd = 250-254 pM) and one of low affinity (Kd = 6.5-16 nM). Affinity labeling of ACTR2-transfected BAEC with 125I-activin-A revealed labeled species of 55, 95, 100, and 160 kDa, all four of which were immunoprecipitated by an anti-ACTR2 monoclonal antibody. Only the 95- and 100-kDa species, however, were immunoprecipitated following denaturation of the affinity-labeled cell lysate with SDS. BAEC transfected with an epitope-tagged form of ACTR2T (ACTR2TMyc) displayed intense 55- and 70-kDa affinity-labeled forms of the truncated receptor, together with a 160-kDa species. As with the full-length receptor, the 160-kDa species associated non-covalently with ACTR2TMyc. These data indicate that, in vascular endothelial cells, ACTR2 forms a high affinity heterotrimeric receptor complex with activin-binding proteins characteristic of type 1 and type 3 activin receptors, and that formation of the complex does not require the kinase domain of ACTR2.