ALK1 regulates the internalization of endoglin and the type III TGF-β receptor

Complex formation and endocytosis of transforming growth factor-beta (TGF-beta) receptors play important roles in signaling. However, their interdependence remained unexplored. Here, we demonstrate that ALK1, a TGF-beta type I receptor prevalent in endothelial cells, forms stable complexes at the cell surface with endoglin and with type III TGF-beta receptors (T beta RIII). We show that ALK1 undergoes clathrin-mediated endocytosis (CME) faster than ALK5, type II TGF-beta receptor (T beta RII), endoglin, or T beta RIII. These complexes regulate the endocytosis of the TGF-beta receptors, with a major effect mediated by ALK1. Thus, ALK1 enhances the endocytosis of T beta RIII and endoglin, while ALK5 and T beta RII mildly enhance endoglin, but not T beta RIII, internalization. Conversely, the slowly endocytosed endoglin has no effect on the endocytosis of either ALK1, ALK5, or T beta RII, while T beta RIII has a differential effect, slowing the internalization of ALK5 and T beta RII, but not ALK1. Such effects may be relevant to signaling, as BMP9-mediated Smad1/5/8 phosphorylation is inhibited by CME blockade in endothelial cells. We propose a model that links TGF-beta receptor oligomerization and endocytosis, based on which endocytosis signals are exposed/functional in specific receptor complexes. This has broad implications for signaling, implying that complex formation among various receptors regulates their surface levels and signaling intensities.