Vgl mRNA, a maternal message encding a member of the transforming growth factor (3 superfamily, undergoes localization to the vegetal cortex of Xenopus laevis oocytes during a narrow period of oogenesis. A 340-nudweotde sequence has been identified in Vgl RNA that directs its vegetal localization [Mowry, K. L. & Melton, D. A. (1992) Science 255, [991][992][993][994]. To understand how cis-and trans-acting factors are involved in Vgl mRNA localization, we have looked for specific interactions in vitro between oocyte proteins and Vgl mRNA. S100 extracts of late-stage oocytes contain a proteinbinding activity that protects specific regions of labeled Vgl mRNA from degradation by RNase T1. The use of different regions of Vgl RNA in competition reactions reveals two binding sites, both in the first half of the 3' untranslated region of Vgl message. UV crosslinking predominantly labels a 69-kDa protein; saturation analysis and competitor studies indicate that this protein binds with a high affinity to the downstream site, which corresponds to the 340-nucleotide vegetal localization sequence. Binding to this region is inhibited by another vegetally localized message, transforming growth factor (35 but is not inhibited by an animally localized RNA, An2. These data indicate that vegetally localized mRNAs share a binding motif that helps them achieve their intracellular distribution through specific RNA-protein interactions.The polarity of Xenopus oocytes is evident from very early stages and is perhaps presaged by an asymmetry detectable in primordial germ cells (1, 2). As oogenesis proceeds, this polarity is elaborated by a series of morphological events that occur along the animal-vegetal axis, such as asymmetric distribution of yolk platelets, migration of pigment granules to the animal hemisphere, accumulation of polar granules at the vegetal pole, and the displacement ofthe germinal vesicle to the animal hemisphere (3,4). On a molecular level, this organization is reflected in a small class of RNA molecules that are localized to particular regions of the oocyte cytoplasm (5, 6). The heterogeneity of the single-cell egg is then translated into cellular differences in the developing embryo by early cleavage planes, helping to create the cell-cell interactions that contribute to producing the body axis. Despite the importance of polarity in oocytes, little is understood about its generation, molecular nature, or interpretation.Vgl mRNA, a member of the transforming growth factor ,8 (TGFB) superfamily, is localized to the vegetal cortex of late-stage oocytes and remains in the vegetal hemisphere of early embryos until after gastrulation (5, 7). Synthesized early during oogenesis and distributed homogeneously throughout stage I and II oocytes, Vgl message accumulates along the vegetal cortex of oocytes during a short period in the middle of oogenesis, where it remains until maturation (8-10). Localization of Vgl mRNA appears to occur in at least two steps, the first being translocation ofthe message to the vegeta...
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