THE TYPE-II AND TYPE-III TRANSFORMING GROWTH-FACTOR-BETA RECEPTORS FORM HOMO-OLIGOMERS

Affinity-labeling experiments have detected hetero-oligomers of the ty pes I, II, and m transforming growth factor beta (TGF-beta) receptors which mediate intracellular signaling by TGF-beta, but the oligomeric state of the individual receptor types remains unknown. Here we use tw o types of experiments to show that a major portion of the receptor ty pes II and III forms homo-oligomers both in the absence and presence o f TGF-beta. Both experiments used COS-7 cells co-transfected with comb inations of these receptors carrying different epitope tags at their e xtracellular termini. In immunoprecipitation experiments, radiolabeled TGF-beta was bound and cross-linked to cells co-expressing two differ ently tagged type II receptors. Sequential immunoprecipitations using anti-epitope monoclonal antibodies showed that type II TGF-beta recept ors form. homo-oligomers. In cells co-expressing epitope-tagged types II and III receptors, a low level of co-precipitation of the ligand-la beled receptors was observed, indicating that some hetero-oligomers of the types II and III receptors exist in the presence of ligand. Antib ody-mediated cross-linking studies based on double-labeling immunofluo rescence explored co-patching of the receptors at the cell surface on live cells. In cells cc-expressing two differently tagged type II rece ptors or two differently tagged type III receptors, forcing one recept or into micropatches by IgG induced co-patching of the receptor carryi ng the other tag, labeled by noncross-linking monovalent Fab'. These s tudies showed that homo-oligomers of the types II and III receptors ex ist on the cell surface in the absence or presence of TGF-beta 1 or -b eta 2. In cells co-expressing types II and III receptors, the amount o f heterocomplexes at the cell surface was too low to be detected in th e immunofluorescence co-patching experiments, confirming that hetero-o ligomers of the types II and III receptors are minor and probably tran sient species.