TGF-beta receptors.

The nature and role of cell surface proteins that bind members of the TGF-beta family has been investigated. TGF-beta, activins, and BMPs each bind to receptors of 55 kDa (type I) and 70 kDa (type II). In the TGF-beta system, these receptors are implicated in the mediation of multiple responses. A member of the type II receptor family has been cloned that encodes four alternatively spliced versions of a transmembrane serine/threonin kinase receptor related to the recently cloned mouse activin receptor and C-elegans daf-1 gene. Inhibitors of serine/threonine kinase activity block transcriptional and growth inhibitory responses to TGF-beta. In addition to the signaling receptors, many cell types express the TGF-beta binding proteoglycan betaglycan. Betaglycan has been purified, molecularly cloned, and shown to bind TGF-beta via its core protein and basic fibroblast growth factor via its heparan sulfate chains. In addition to receptors I and II and betaglycan, some cells express a newly identified set of membrane proteins that specifically bind either TGF-beta 1 or TGF-beta 2. Three of the four isoform-restricted binding proteins are bound to the membrane via phospholipid anchors. Like betaglycan, these proteins might function to regulate the interaction between TGF-beta and their target cells.