pEg6, a spire family member, is a maternal gene encoding a vegetally localized mRNA in Xenopus embryos

In Xenopus, during oocyte maturation and the segmentation period, cell cycle progression is independent of new transcription, but requires de novo translation. This suggests that the completion of oocyte maturation and then the rapid cell division period is controlled exclusively at a post-transcriptional level by specific gene products. To isolate these maternal genes, a differential screening of a Xenopus egg cDNA library was performed. Several cDNAs were isolated which correspond to mRNA polyadenylated in eggs and deadenylated in embryos, and these constitute the founders members of the Eg family of mRNAs.

We report here the characterization of Eg6 mRNA as a novel maternal gene expressed in Xenopus egg until gastrula stage. The Eg6 transcript is initially concentrated in the vegetal cytoplasm of the egg, and later the distribution of the transcript marks the posterior vegetal end of developing embryos. pEg6 is a multidomain protein with a kinase non-catalytic C-lobe domain of unknown function, a cluster of four WH2 (Wiskott-Aldrich syndrome protein homology 2) domains and a modified FYVE zinc-finger motif. The amino acid sequence of pEg6 is related to PEM-5 (posterior end mark-5), from an ascidian maternal mRNA, and spire, a Drosophila protein required to establish dorsal-ventral and anterior-posterior axes of polarity and recently described as an actin nucleation factor. In Xenopus and Schizosaccharomyces pombe cells pEg6 expression induces filamentous actin clusters and is associated with vesicular structure.

These data suggest that pEg6 acts as a vegetally localized factor contributing to the actin nucleation process during Xenopus early development.