Forschungs- & Entwicklungsinformationsdienst der Gemeinschaft - CORDIS

Final Activity Report Summary - EPIADHESION (Regulation of cell adhesion, polarity and proliferation in mammalian epithelial celles by proteins of the discs large and crumbs group)

Genetic studies in Drosophila and C. elegans have identified three groups of evolutionarily conserved proteins required for the establishment of cell polarity in a variety of biological contexts:
(i) Par3, Par6, aPKC (in some cases the activity of aPKC is regulated by small GTPases, such as Cdc42);
(ii) Crb3 (Crumbs), PALS1 (MPP5), PATJ, which are both located predominantly apically; and
(iii) Dlg1, Scribble, Lgl located mostly on the basolateral surfaces.
During epithelial morphogenesis, these proteins participate in a complex network of inter-dependent interactions that define the position and functional organization of adherens junctions and tight junctions. The loss of epithelial integrity is a key feature of many human cancers and several of these proteins exhibit tumour suppressor activity; however, the biochemical pathways through which they control polarity are poorly understood.

During the course of our studies, we identified a biochemical and functional link between human Dlg1 and Crb3 polarity complexes, which is mediated via the previously uncharacterised MAGUK-p55 subfamily member MPP7. MPP7 was identified by mass spectrometry as a major protein co-immunoprecipitating specifically with endogenous hDlg1 from polarised MCF7 mammary breast epithelial cell extracts and this interaction requires the L27N domain of MPP7 and the L27 domain of hDlg1. Using confocal microscopy, we showed that MPP7 localises at the lateral site in polarised epithelial cells and partially colocalises with markers for adherens and tight junctions. Importantly, MPP7 targets via its interaction with hDlg1 to the lateral surface in epithelial cells, as in hDlg1-depleted cells, MPP7 fails to localize to the plasma membrane.

In addition, the transmembrane protein Crb3 can recruit wild-type MPP7, but not MPP7 with a deleted SH3-HOOK domain, to the cell membrane and this relocalisation is mediated via a physical interaction between MPP7 and PALS1. Interference with MPP7 function using retroviral shRNA results in a delay in the formation of functional tight junctions as judged by transepithelial electrical resistance measurements. These data identify a new biochemical link between polarity protein complexes that plays an important role in epithelial polarity and tight junction integrity.

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