ALK5

Most cell types express three sizes of receptors for TGFβ. These are designated Type I (53 kDa), Type II (7085 kDa), and Type III (250-350 kDa) . The Type III receptor, a proteoglycan that exists in membranebound and soluble forms, binds TGFβ1, TGFβ2, and TGFβ3 but does not appear to be involved in signal transduction. The Type II receptor is a membrane bound serine/threonine kinase that binds TGFβ1 and TGFβ3 with high affinity and TGFβ2 with a much lower affinity. The Type I receptor, originally known as ALK5 (Activin receptor-like kinase) is also a membranebound serine/threonine kinase that apparently requires the presence of the Type II receptor to bind TGFβ. Current evidence suggests that signal transduction requires the cytoplasmic domains of both the Type I and Type II receptors.