Molecular and functional characterization of activin receptors.

Activins are multifunctional proteins with effects on a broad spectrum of cells and tissues. They are structurally related to a large family of growth and differentiation factors that includes the inhibins, the transforming growth factors b (TGFb), the bone morphogenetic proteins (BMP), Mullerian inhibitory substance, and a number of gene products that control the development of Drosophila and Xenopus. Although the cellular signaling mechanisms of these factors remain unclear, cDNAs encoding cell surface receptors for activin have been cloned. Those receptors are transmembrane serine kinases, suggesting a novel form of signaling. Overexpression of activin receptors in Xenopus embryos indicates that these molecules are functionally involved in the transmission of the activin signal.