DIFFERENTIAL REQUIREMENT FOR TYPE-I AND TYPE-II TRANSFORMING-GROWTH-FACTOR-BETA RECEPTOR KINASE-ACTIVITY IN LIGAND-MEDIATED RECEPTOR ENDOCYTOSIS

Transforming growth factor beta (TGF beta) superfamily polypeptides re gulate cell growth and differentiation by binding to single pass serin e/threonine kinases referred to as TGF beta type I and type II recepto rs, Signal propagation is dependent upon heteromeric (type I-type II) complex formation and transphosphorylation of the type I receptor by t he type II receptor. While many of the phosphorylation events necessar y for receptor signaling have recently been characterized, the role of TGF beta receptor kinase activity in modulating receptor endocytosis has not been addressed. To that end, we have used chimeric receptors c onsisting of the extracellular domain of the granulocyte/macrophage co lony-stimulating factor alpha and beta receptors spliced to the TGF be ta type I and type II transmembrane and cytoplasmic domains to address the specific role of type I and/or type II receptor kinase activity i n TGF beta receptor internalization, downregulation, and signaling. To inactivate chimeric receptor kinase activity, point mutations in the ATP binding site were made at amino acids 232 and 277 in the type I an d type II receptor, respectively. Either of these mutations abolished plasminogen activator inhibitor 1 protein expression stimulated by gra nulocyte/macrophage colony-stimulating factor activation of chimeric h eteromeric type I-type II TGF beta receptors, They did not, how ever, modulate TGF beta signaling stimulated through the endogenous TGF beta receptor. Although TGF beta receptor signaling was dependent upon the kinase activity of both chimeric receptors, the initial endocytic res ponse was distinctly regulated by type I and/or type II receptor kinas e activity. For instance, while heteromeric receptor complexes contain ing a kinase-inactive type I receptor were endocytosed similarly to wi ld type complexes, the kinase activity of the type II TGF beta recepto r was necessary for optimal internalization and receptor down-regulati on. Furthermore, these responses were shown to occur independently of type II receptor autophosphorylation but require a type II receptor ca pable of transphosphorylation.