Patterns of localization and cytoskeletal association of two vegetally localized RNAs, Vg1 and Xcat-2.

In Xenopus, localization of a rare class of mRNAs during oogenesis is believed to initiate pattern formation in the early embryo. We have determined the pattern of RNA localization for one of these RNAs, Xcat-2, which encodes a putative RNA-binding protein related to Drosophila nanos (Mosquera, L., Forristall, C., Zhou, Y. and King, M. L. (1993) Development 117, 377-386). Xcat-2 is exclusively localized to the mitochondrial cloud in stage I oocytes, moves with this body into the vegetal cortex during stage II and, later, partitions into islands consistent with it being a component of the germ plasm. As previously shown, Vg1 is not localized to the vegetal cortex until stage IV and distributes to all vegetal blastomeres during development. We found a direct correlation between the localized condition of these RNAs and their recovery in a detergent-insoluble fraction. We present evidence suggesting that differential RNA binding to a cytoskeletal component(s) in the vegetal cortex determines the pattern of inheritance for that RNA in the embryo.

Isolated vegetal cortex from Xenopus oocytes selectively retains localized mRNAs.

The process of converting an egg to a multicellular organism often begins with a polarization of information within the oocyte. The cytoskeleton plays an important role in maintaining such asymmetries. Here we describe the isolation and characterization of cortices, rich in cytoskeletal elements, from the animal and vegetal poles of stage VI Xenopus oocytes. Ultrastructural and immunocytochemical analyses revealed that the 10-microns-thick isolates included a cytokeratin network, mitochondria, and other elements, found in the oocyte cortex. Furthermore, three vegetally localized mRNAs, Vg1, Xcat-2, and Xcat-3, were all found to be specifically retained within the isolated vegetal cortices, in sharp contrast to histone RNA. Our findings support the view that the vegetal cortex represents a unique cytoskeletal domain in which a rare class of maternal mRNAs becomes localized.

A mRNA localized to the vegetal cortex of Xenopus oocytes encodes a protein with a nanos-like zinc finger domain.

mRNAs concentrated in specific regions of the oocyte have been found to encode determinants that specify cell fate. We show that an intermediate filament fraction isolated from Xenopus stage VI oocytes specifically contains, in addition to Vg1 RNA, a new localized mRNA, Xcat-2. Like Vg1, Xcat-2 is found in the vegetal cortical region, is inherited by the vegetal blasomeres during development, and is degraded very early in development. Sequence analysis suggests that Xcat-2 encodes a protein that belongs to the CCHC RNA-binding family of zinc finger proteins. Interestingly, the closest known relative to Xcat-2 in this family is nanos, an RNA localized to the posterior pole of the Drosophila oocyte whose protein product suppresses the translation of the transcription factor hunchback. The localized and maternally restricted expression of Xcat-2 RNA suggests a role for its protein in setting up regional differences in gene expression that occur early in development.