EFFECTS OF TRUNCATED ACTIVIN AND FGF RECEPTORS AND OF FOLLISTATIN ON THE INDUCING ACTIVITIES OF BVG1 AND ACTIVIN - DOES ACTIVIN PLAY A ROLEIN MESODERM INDUCTION

Activin and Vg1, two members of the TGF-beta family, are believed to p lay roles in mesoderm induction and axis formation in the amphibian em bryo. Both molecules are provided maternally, either as protein (activ in) or as RNA and protein (Vg1), and experiments with a truncated form of a type IIB activin receptor have led to the conclusion that activi n is required for induction of mesoderm in vivo. In this paper we firs t show that truncated versions of two different Xenopus activin recept ors also have severe effects on the activity of the mature region of V g1, suggesting that such receptors may block the function of several m embers of the TGF-beta family. We go on to demonstrate that follistati n, a secreted protein which binds activin and blocks its activity, doe s not interfere with Vg1 signalling. Furthermore, overexpression of fo llistatin mRNA in Xenopus embryos does not perturb mesoderm formation. Taken together, our data show that the effects of truncated activin r eceptors on Xenopus development can be explained by the inhibition of Vg1 activity, while the lack of effect of follistatin argues against a function for activin in mesoderm induction.