The MAGUK Protein MPP7 Binds to the Polarity Protein hDlg1 and Facilitates Epithelial Tight Junction Formation

Medical Research Council Laboratory for Molecular Cell Biology, Cancer Research UK Oncogene and Signal Transduction Group, University College London, London WC1E 6BT, England.
Molecular Biology of the Cell (Impact Factor: 4.47). 06/2007; 18(5):1744-55. DOI: 10.1091/mbc.E06-11-0980
Source: PubMed


Three groups of evolutionarily conserved proteins have been implicated in the establishment of epithelial cell polarity: the apically-localized proteins of the Par (Par3-Par6-aPKC-Cdc42) and Crumbs groups (Crb3-PALS1-PATJ) and the basolaterally localized proteins of the Dlg group (Dlg1-Scribble-Lgl). During epithelial morphogenesis, these proteins participate in a complex network of interdependent interactions that define the position and functional organization of adherens junctions and tight junctions. However, the biochemical pathways through which they control polarity are poorly understood. In this study, we identify an interaction between endogenous hDlg1 and MPP7, a previously uncharacterized MAGUK-p55 subfamily member. We find that MPP7 targets to the lateral surface of epithelial cells via its L27N domain, through an interaction with hDlg1. Loss of either hDlg1 or MPP7 from epithelial Caco-2 cells results in a significant defect in the assembly and maintenance of functional tight junctions. We conclude that the formation of a complex between hDlg1 and MPP7 promotes epithelial cell polarity and tight junction formation.

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    • "Second, Dlg1 associates through its L27N domain with a number of related MAGUK family proteins, including Lin2/ CASK, MPP2, MPP3, and MPP7 (Bohl et al., 2007). MPP7 colocalizes with Dlg1 to the lateral membrane, and gene silencing of either MPP7 or Dlg1 delays the formation of functional tight junctions, suggesting that they function together in this context (Stucke et al., 2007). By contrast, CASK colocalizes with Dlg1 along the basal membrane, indicating that the interactions between other MAGUK proteins and Dlg1 might be mutually exclusive. "
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    • "Protein spots of interest were excised from the gels and digested with trypsin as described previously (Vandahl et al., 2001). The digested protein spots were applied for automated nano-liquid chromatography tandem mass spectrometry (LC-MS/MS) analysis using an Ultimate (Dionex, Amsterdam, the Netherlands) in-line connected to an electrospray ionization (ESI) Esquire HCT ion trap (Bruker Daltonics, Bremen, Germany) according to Stucke et al. (2007). MS/MS fragmentation spectra were converted to Mascot generic files (mgf) using the automation engine software (version 3.2, Bruker Daltronics) and searched, using a lab-installed version of mascot (version 2.1.0) "
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    • "MAGUK proteins are found at areas of cell-cell contact, where they are essential for multi-protein complex assembly. MPP7 forms a tripartite complex with Discs Large 1(DLG1) and Lin7, and is necessary for maintenance of cell polarity [32,33]. Interestingly, HNF4A, the gene responsible for MODY1, has also been shown to be important in formation of tight junctions [34]. "
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