Regulation of Lethal giant larvae by Dishevelled

The establishment of polarity in many cell types depends on Lgl, the tumour suppressor product of lethal giant larvae, which is involved in basolateral protein targeting. The conserved complex of Par3, Par6 and atypical protein kinase C phosphorylates and inactivates Lgl at the apical surface; however, the signalling mechanisms that coordinate cell polarization in development are not well defined. Here we show that a vertebrate homologue of Lgl associates with Dishevelled, an essential mediator of Wnt signalling, and that Dishevelled regulates the localization of Lgl in Xenopus ectoderm and Drosophila follicular epithelium. We show that both Lgl and Dsh are required for normal apical-basal polarity of Xenopus ectodermal cells. In addition, we show that the Wnt receptor Frizzled 8, but not Frizzled 7, causes Lgl to dissociate from the cortex with the concomitant loss of its activity in vivo. These findings suggest a molecular basis for the regulation of cell polarity by Frizzled and Dishevelled.

The positional, structural, and sequence requirements of the Drosophila TLS RNA localization element

The subcellular localization of mRNAs is a key step in the polarization of cells in organisms from yeast to man. Here, we use a transgenic fly/in situ hybridization assay system to define the positional, structural, and sequence requirements of the TLS, a stem loop RNA sequence element that mediates the subcellular localization of K10 and Orb transcripts in Drosophila oocytes. We find that the TLS is a highly robust and modular element. It mediates efficient RNA localization regardless of sequence context or position within the transcript. Site-specific mutagenesis experiments indicate that the size and shape of the stem and loop regions are critical determinants of TLS activity. Such experiments also identify specific base residues that are important for TLS activity. All such residues map to the stem portion of the structure. Significantly, mutations at these residues interfere with TLS activity only when they alter the stereochemistry of the stem's minor groove. For example, mutation of the A:U base pair at position 3 of the TLS stem to G:C severely reduces TLS activity, while mutation of the same base pair to U:A has no effect. Extensive searches for TLS-like elements in other Drosophila mRNAs using sequence and structural parameters defined by our experiments indicate that the TLS is unique to K10 and Orb mRNAs. This unexpected finding raises important questions as to how the many hundreds of other mRNAs that are known or thought to exhibit K10 and Orb-like localization are localized.