Description
Most cell types express three sizes of receptors for TGF-beta. These are designated Type I (53 kDa), Type II (70-85 kDa), and Type III (250-350 kDa). The Type III receptor, a proteoglycan that exists in membrane-bound and soluble forms, binds TGF-beta 1, TGF-beta 2, and TGF-beta 3 but does not appear to be involved in signal transduction. The Type II receptor is a membrane-bound serine/threonine kinase that binds TGF-beta 1 and TGF-beta 3 with high affinity and TGF-beta 2 with a much lower affinity. The Type I receptor is also a membrane-bound serine/threonine kinase that apparently requires the presence of the Type II receptor to bind TGF-beta. Current evidence suggests that signal transduction requires the cytoplasmic domains of both the Type I and Type II receptors.