Assembly of C-elegans apical junctions involves positioning and compactionby LET-413 and protein aggregation by the MAGUK protein DLG-1

Specialised subapical junctions play a critical role in maintaining epithel ial cell polarity and tissue integrity, and provide a platform for intracel lular signalling. Here we analyse the roles of C. elegans genes let-413 and dlg-1, a homologue of Drosophila lethal discs large, in the assembly of th e C. elegans apical junction (CeAJ), and provide the first characterisation of this structure, We have identified dlg-1 as an essential gene in an RNA interference screen against C. elegans homologues of genes encoding protei ns involved in tight or septate junction formation. We show that DLG-1 colo calises with the junctional protein JAM-1 at CeAJs in a unit distinct from HMP-1/alpha -catenin, and apical to the laterally localised LET-413, Loss o f dlg-1 activity leads to JAM-1 mislocalisation and the disappearance of th e electron-dense component of the CeAJs, but only mild adhesion and polarit y defects. In contrast, loss of let-413 activity leads to the formation of basally extended discontinuous CeAJs and strong adhesion and polarity defec ts. Interestingly, in LET-413-deficient embryos, CeAJ markers are localised along the lateral membrane in a manner resembling that observed in wildtyp e embryos at the onset of epithelial differentiation. We conclude that the primary function of LET-413 is to correctly position CeAJ components at a d iscrete subapical position. Furthermore, we propose that DLG-1 is required to aggregate JAM-1 and other proteins forming the electron-dense CeAJ struc ture. Our data suggest that epithelial adhesion is maintained by several re dundant systems in C. elegans.