CYTOPLASMIC POLYADENYLATION OF ACTIVIN RECEPTOR MESSENGER-RNA AND THECONTROL OF PATTERN-FORMATION IN XENOPUS DEVELOPMENT

The activin receptor, a transmembrane serine-threonine kinase, is a ke y component necessary for pattern formation in early Xenopus developme nt. This protein interacts with members of the transforming growth fac tor beta family and stimulates cells of the marginal zone to different iate along the mesodermal pathway. In large part, this function of the activin receptor has been inferred from observations of phenotypes in duced by injected mRNA encoding wild-type or mutant forms of the prote in. Naturally occurring activin receptor mRNA is maternally inherited and contains within its 3' untranslated region an embryonic-type cytop lasmic polyadenylation element (CPE), an oligouridylic acid sequence t hat promotes cytoplasmic polyadenylation and resultant translational a ctivation. Based on the presence of this element, we predicted in a pr evious report that activin receptor mRNA expression in embryos might b e regulated by cytoplasmic polyadenylation (Simon and Richter, Mel. Ce ll. Biol. 14, 7867-7875, 1994). In this study, we have tested this hyp othesis and show that not only do endogenous and injected activin rece ptor mRNAs undergo cytoplasmic polyadenylation during embryogenesis, b ut also that this process is necessary for stimulating translation and inducing the morphological defects observed by mRNA overexpression. T he activin receptor CPE is bound by a M(r) 36 x 10(3) protein in vitro , and competition for this factor between mRNAs in vivo inhibits activ in receptor mRNA polyadenylation. This competition may be responsible for the lack of mesoderm formation observed in such injected embryos. These data suggest that cytoplasmic polyadenylation controls different iation and pattern formation in early Xenopus development. (C) 1996 Ac ademic Press, Inc.