Organizing a functional junctional complex requires specific domains of the Drosophila MAGUK Discs large

Abstract

Discs large (Dlg) was the first identified member of an increasingly important class of proteins called membrane-associated guanylate kinase homologs (MAGUKs), which are often concentrated at cell junctions and contain distinct peptide domains named PDZ1-3, SH3, HOOK, and GUK. Dlg is localized at and required for the formation of both septate junctions in epithelial cells and synaptic junctions in neurons. In the absence of Dlg, epithelia lose their organization and overgrow. We tested the functions of each domain of Dlg in vivo by constructing transgenic flies expressing altered forms of the protein. In the first set of experiments each domain was examined for its ability to correctly target an epitope-tagged Dlg to pre-existing septate junctions. Based on these results the Hook domain is necessary for localization of the protein to the cell membrane and the PDZ2 is required for restricting the protein to the septate junction. In the second set of experiments each domain was tested for its role in growth regulation and organization of epithelial structure. These results show that PDZ1 and GUK are apparently dispensable for function, PDZ2 and PDZ3 are required for growth regulation but not for epithelial structure, and SH3 and HOOK are essential for both aspects of function. The results demonstrate the functional modularity of Dlg and clarify the functions of individual MAGUK domains in regulating the structure and growth of epithelial tissue.

The APC-hDLG complex negatively regulates cell cycle progression from the G0/G1 to S phase

“…This ®nding is consistent with studies on Drosophila dlg showing that the GUK domain and the amino-terminal region containing the PDZ domains are required for the tumor suppressor function of dlg (Woods and Bryant, 1991;Woods et al, 1996). Furthermore, in studies using transgenic¯ies, it was shown that the second and third PDZ domains, the SH3 domain, and a region between the SH3 and GUK domains called the HOOK domain are all required for rescue of the overgrowth phenotype of imaginal disc epithelium in dlg-null larvae (Hough et al, 1997). Since the PDZ, SH3 and GUK domains are believed to constitute protein-interaction interfaces, structural alterations in these domains may abrogate the ability of hDLG to interact with key regulatory or e ector molecules.…”

“…Since the PDZ, SH3 and GUK domains are believed to constitute protein-interaction interfaces, structural alterations in these domains may abrogate the ability of hDLG to interact with key regulatory or e ector molecules. In addition, the HOOK domain and the second PDZ domain have been shown to be important for the correct localization of some proteins at the membrane and at the septate junction, respectively (Hough et al, 1997). It is also well known that MAGUKs are involved in the clustering of membrane proteins (Kornau et al, 1995;Kim et al, 1995).…”

Epithelial polarity and proliferation control: links from the Drosophila neoplastic tumor suppressors

“…Interestingly, structure-function analyses of nTSG proteins suggest that polarity and proliferation control are to some extent separable, with distinct regions of the multidomain Dlg and Scrib proteins contributing to the two functions (Hough et al 1997;J. Zeitler, C. Hsu, and D. Bilder, unpubl.).…”

Scribble protein domain mapping reveals a multistep localization mechanism and domains necessary for establishing cortical polarity

“…Studies in Drosophila, C. elegans and mammalian epithelial cells have demonstrated that apical/basal cell polarity is created through membrane domains and junctional complexes with characteristic positions along the apical/basal axis (Bryant, 1997). In vertebrates, these are the tight junctions, adherens junctions and a basolateral domain, from apical to basal, respectively.…”