Spatially Restricted Translation of the xCR1 mRNA in Xenopus Embryos

Abstract

The xCR1 protein is a maternal determinant and cofactor for nodal signaling in vertebrate embryos. The xCR1 protein accumulates specifically in the animal cells of Xenopus embryos, but maternal xCR1 mRNA is distributed equally throughout all embryonic cells. Here, we show that vegetal cell-specific translational repression of xCR1 mRNA contributes to this spatially restricted accumulation of the xCR1 protein in Xenopus embryos. xCR1 mRNA was associated with polyribosomes in animal cells but not vegetal cells. A 351-nucleotide region of xCR1 mRNA's 3′ untranslated region was sufficient to confer a spatially restricted pattern of translation to a luciferase reporter mRNA by repressing translation in vegetal cells. Repression depended upon the mRNA's 5′ cap but not its 3′ poly(A) tail. Furthermore, the region of xCR1 mRNA sufficient to confer vegetal cell-specific repression contained both Pumilio binding elements (PBEs) and binding sites for the CUG-BP1 protein. The PBEs and the CUG-BP1 sites were necessary but not sufficient for translation repression. Our studies of xCR1 mRNA document the first example of spatially regulated translation in controlling the asymmetric distribution of a maternal determinant in vertebrates.

ACKNOWLEDGMENTS

We thank Marv Wickens and Labib Rouhana for the GST-Xpum1 plasmid and advice on RNA binding experiments and Catherine Fox, Tricia Kiley, and Marv Wickens for comments on the manuscript. We thank Justin Spanier for assistance with the RNA binding experiments. We thank Caroline Hill for reagents and Betty Craig for use of the luminometer.

This work was supported by grants from the NIH (no. HD43996 to M.D.S.) and the James and Dorothy Shaw Fund of the Greater Milwaukee Foundation.