TGF‐β receptors

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Abstract
The nature and role of cell surface proteins that bind members of the TGF‐β family has been investigated. TGF‐β, activins, and BMPs each bind to receptors of 55 kDa (type I) and 70 kDa (type II). In the TGF‐β 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/threonine 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‐β. In addition to the signaling receptors, many cell types express the TGF‐β binding proteoglycan betaglycan. Betaglycan has been purified, molecularly cloned, and shown to bind TGF‐β 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‐β1 or TGF‐β2. 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‐β and their target cells.