The types II and III transforming growth factor-β receptors form homo-oligomers

Affinity-labeling experiments have detected hetero-oligomers of the types I, II, and III transforming growth factor β (TGF-β) receptors which mediate intracellular signaling by TGF-β, but the oligomeric state of the individual receptor types remains unknown. Here we use two types of experiments to show that a major portion of the receptor types II and III forms homo-oligomers both in the absence and presence of TGF-β. Both experiments used COS-7 cells co-transfected with combinations of these receptors carrying different epitope tags at their extracellular termini. In immunoprecipitation experiments, radiolabeled TGF-β was bound and cross-linked to cells coexpressing two differently tagged type II receptors. Sequential immunoprecipitations using anti-epitope monoclonal antibodies showed that type II TGF-β receptors form homo-oligomers. In cells co-expressing epitope-tagged types II and III receptors, a low level of co-precipitation of the ligand-labeled receptors was observed, indicating that some hetero-oligomers of the types II and III receptors exist in the presence of ligand. Antibody-mediated cross-linking studies based on double-labeling immunofluorescence explored copatching of the receptors at the cell surface on live cells. In cells co-expressing two differently tagged type II receptors or two differently tagged type III receptors, forcing one receptor into micropatches by IgG induced co-patching of the receptor carrying the other tag, labeled by noncross-linking monovalent Fab′. These studies showed that homo-oligomers of the types II and III receptors exist on the cell surface in the absence or presence of TGF-β1 or -β2. In cells co-expressing types II and III receptors, the amount of heterocomplexes at the cell surface was too low to be detected in the immunofluorescence co-patching experiments, confirming that hetero-oligomers of the types II and III receptors are minor and probably transient species.