Localized maternal mRNA related to transforming growth factor β mRNA is concentrated in a cytokeratin-enriched fraction from Xenopus oocytes

The localized maternal RNA Vg1 resides in the cortical region of the vegetal pole of fully grown Xenopus oocytes and is inherited by only a subset of blastomeres in the early embryo [Weeks, D.L. & Melton, D.A. (1987) Cell 51, 861-867]. Because RNA-cyttoskeletal interactions may play a role in RNA localization, we have examined the association of Vg1 RNA with components of the oocyte's cytoskeleton. Gel and immunoblot analysis of a detergent-insoluble fraction revealed a greatly simplified protein pattern composed largely of cytokeratins and vimentin. In sharp contrast to the nonlocalized histone H3 mRNA, Vg1 RNA was concentrated some 35- to 50-fold in this insoluble fraction. Extractions at higher salt concentrations yielded preparations further enriched in cytokeratins and in the Vg1 RNA. Upon ovulation, Vg1 RNA is released into the soluble fraction. This change in Vg1 RNA distribution coincides with the observed breakdown of cortical cytokeratin filaments [Klymkowsky, M.W., Maynell, L.A. & Polson, A.G. (1987) Development 100, 543-557] and the loss of Vg1 RNA from the cortical region. Our findings are consistent with the hypothesis that RNA-cytoskeletal interactions are involved in the localization and segregation of information during development.