At present, several proteins are known to affect epithelial cell polarity during development. However, their sig-nalling pathways and mechanisms are still elusive. In C. elegans the gut forms a simple tube made up of 20 cells arranged in a series of nine rings (1). To perform its function, the cells have to develop an apico-basal axis that is manifested in their internal architecture, surface polarity and the formation of specialised junctions. In C. elegans the zonula adherens (ZA) forms a continuous belt around the apex of epithelial cells, defining the border between apical and baso-lateral domains. The ZA is the only type of junction which has been detected in the em-bryo at the ultrastructural level so far (2,3). Our primary interests are to determine which components of this junction are required for the integrity of the gut epithelium in C. elegans and which adaptor molecules mediate the interaction of polarity cues with the executive cytoskeleton.
dlg-1 (discs large) is restricted to the ZA of all embryonic epithelia (4), which contrasts with the localisation of the Drosophila and vertebrate homologues in septate and tight junctions, respectively. The molecular nature of DLG-1 makes it a likely candidate to participate in the organization of protein scaffolds that control the assembly of junction components into the ZA. We have started to screen for direct binding partners of DLG-1, using two different domains as baits in a yeast two-hybrid screen. Proper localization of DLG-1 requires the baso-lateral LET-413 protein (3), the Drosophila scrib orthologue (5), but is independent of the catenin-cadherin system (6). High quality immunofluorescence analysis of
dlg-1 and catenin/cadherin knockout embryos revealed severe defects in epithelial integrity, demonstrating the necessity of both sytems for correct development. Analysis of
band-4.1/ERM (ezrin-radixin-moesin) proteins, a family that is defined as membrane-cytoskeleton linkers, has revealed an essential function of
erm-1 for correct localization of the junctional complex in the gut epithelium of C.elegans. (1) Sulston et al., 1983, Dev Biol 100,
p64; (2) Leung et al., 1999, Dev Biol 216,
p114; (3) Legouis et al., 2000, Nat Cell Biol 7,
p415; (4) Bossinger et al., 2001, Dev Biol 230,
p29; (5) Bilder and Perrimon, 2000, Science 289,
p113; (6) Costa et al., 1998, JCB 141,
p297.