XTRR-I IS A TGF-BETA RECEPTOR AND OVEREXPRESSION OF A TRUNCATED FORM OF THE RECEPTOR INHIBITS AXIS FORMATION AND DORSALISING ACTIVITY

We have previously cloned a type I serine/threonine kinase receptor fr om Xenopus, namely XTrR-I. We show here that XTrR-I is able to bind an d mediate the activity of TGF beta I, but is unable to mediate respons e to activin or BMP-4. We have made a truncated receptor construct tha t can act as a dominant negative mutant receptor, and this can block t he activity of TGF beta 2 but not that of activin. Overexpression of e ither the full-length or truncated receptor has a drastic effect on me soderm differentiation. The truncated receptor inhibits expression of notochord and muscle in mesodermalised animal caps, while the full-len gth receptor greatly increases the amount of notochord. In addition, t he truncated receptor blocks the axis duplicating activity of both sia mois and Xwnt8. We conclude that XTrR-I is involved in mediating a dor salising activity important for mesoderm differentiation. (C) 1998 Els evier Science Ireland Ltd. All rights reserved.