TGF-beta receptors.

TGF-beta binds specifically and with high affinity to all cell types with few exceptions. The binding parameters vary because the dissociation constants range from 1 pM to 60 pM. There may be as few as 200 or as many as 100,000 receptors per cell, depending on the cell type. Several cell surface proteins that specifically bind TGF-beta 1 and TGF-beta 2 have been characterized as putative receptors and have been classified on the basis of their size. Type I receptors have an Mr of approximately 50,000, type II receptors an Mr of approximately 80,000. Both type I and II receptors are glycoproteins and TGF-beta-induced responses appear to be mediated by one or both types. The most abundant and largest of the cell surface TGF-beta binding proteins is a membrane-bound proteoglycan that is a dimer of subunits each with Mr of approximately 250,000 (beta-glycan). This protein is plentiful on primary cells of mesenchymal origin but absent on primary epithelial and endothelial cells. It does not appear to be associated with TGF-beta-mediated cell responses and its function remains unknown. There are multiple cellular responses to TGF-beta and accordingly there appear to be multiple pathways for signal transduction. A guanine nucleotide binding protein-dependent pathway is involved in transmission of the signal for at least one TGF-beta-induced response while there is evidence that other responses are mediated through an independent pathway. TGF-beta receptor purification and cloning efforts will be rewarded with valuable information on the mechanisms of signal transmission.