A NOVEL MESODERM INDUCER, MADR2 FUNCTIONS IN THE ACTIVIN SIGNAL-TRANSDUCTION PATHWAY

A functional assay to clone mouse mesoderm inducers has identified the mouse gene Mad related 2 (Madr2). Madr2 induces dorsal mesoderm from Xenopus ectoderm and can mimic the organizer in recruiting neighboring cells into a second axis. By analyzing the expression of a lacZ/Madr2 fusion protein, we find Madr2 confined to the nucleus in the deep, an terior cells of the second axis, whereas in epidermal and more posteri or cells the protein is cytoplasmically localized. This context-depend ent nuclear localization suggests that in certain regions of the embry o, Madr2 responds to a localized signal and amplifies this signal to f arm the second axis. furthermore, although Madr2 remains unlocalized i n ectodermal explants, addition of activin enhances the concentration of Madr2 in the nucleus. Significantly, a functional lacZ fusion to a carboxy-terminal portion of Madr2 is nuclear localized even in the abs ence of activin. This indicates that Madr2 contains a domain that fan activate downstream components and a repressive domain that anchors th e protein in the cytoplasm. Nuclear localization of Madr2 in response to activin, and the activin-like phenotypes induced by overexpression of Madr2 indicate that Madr2 is a signal transduction component that m ediates the activity of activin.