Cooperative regulation of AJM-1 controls junctional integrity in Caenorhabditis elegans epithelia.

The function of epithelial cell sheets depends on the integrity of specialized cell-cell junctions that connect neighbouring cells. We have characterized the novel coiled-coil protein AJM-1, which localizes to an apical junctional domain of Caenorhabditis elegans epithelia basal to the HMR-HMP (cadherin-catenin) complex. In the absence of AJM-1, the integrity of this domain is compromised. Proper AJM-1 localization requires LET-413 and DLG-1, homologues of the Drosophila tumour suppressors Scribble and Discs large, respectively. DLG-1 physically interacts with AJM-1 and is required for its normal apical distribution, and LET-413 mediates the rapid accumulation of both DLG-1 and AJM-1 in the apical domain. In the absence of both dlg-1 and let-413 function AJM-1 is almost completely lost from apical junctions in embryos, whereas HMP-1 (α α-catenin) localization is only mildly affected. We conclude that LET-413 and DLG-1 cooperatively control AJM-1 localization and that AJM-1 controls the integrity of a distinct apical junctional domain in C. elegans. D uring animal development, specialized junctional domains are crucial for the function of epithelial cell sheets. In both vertebrates and invertebrates, adherens junctions are thought to regulate cell-cell adhesion and dynamic changes in cell morphology Here we show that the novel coiled-coil protein AJM-1 (for 'apical junction molecule') is required for the integrity of epithelial junctions of C. elegans and that it localizes to an apical junctional domain. (AJM-1 was originally called JAM-1 (refs 13, 14) but has been renamed to avoid confusion with the vertebrate transmembrane tight junction protein, JAM-1.) This domain is basal to the HMR-HMP(cadherin-catenin) complex; on the basis of the localization of the Discs large homologue DLG-1 to the same domain, it might be required for maintaining a tight apical seal between epithelial cells at apical junctions. Furthermore, we show that AJM-1 directly binds DLG-1, which is required for the proper distribution of AJM-1 around the junctional belt but not for general cell polarity. In addition, we show that in embryos lacking LET-413 the patterns of both DLG-1 and AJM-1 are equally disrupted, including a delay in concentration of these proteins at a narrow apical domain. Almost complete loss of junctional AJM-1 is observed in the absence of both LET-413 and DLG-1, whereas HMP-1 (α-catenin) localization is reduced but junctional. We propose a model in which LET-413 and DLG-1 control the integrity of a distinct apical subdomain by cooperatively regulating the localization of AJM-1. Results AJM-1 encodes a novel coiled-coil protein localizing to C. elegans apical junctions. As an initial step in understanding the molecular composition of apical junctions in C. elegans, we characterized the antigen recognized by the MH27 antibody. The antibody had been previously shown to stain apical borders of C. elegans epitheli