Identification of a Drosophila activin receptor

Activins are cytokines of the transforming growth factor β superfamily that control various events during vertebrate embryo development and cell differentiation in the adult, and act through transmembrane receptors that contain a cytoplasmic protein-serine/threonine kinase domain. We describe the identification, deduced primary structure, and expression pattern of Atr-II, a receptor serine/threonine kinase found in Drosophila. With the exception of the spacing of 10 cysteine residues, the extracellular domain of Atr-II is very dissimilar from those of vertebrate activin receptors, yet it binds activin with high affinity and specificity. The kinase domain sequence of Atr-II is 60% identical to those of activin receptors from vertebrates, suggesting similarities in their signaling mechanisms. Maternal Atr-II transcript and its product are abundant in the oocyte. During development, the highest levels of Atr-II transcript and protein are observed in the mesoderm and gut. The possible role of an activin signaling system in Drosophila development is discussed.