Regulation of epithelial junctions stability and turnover by the RhoG exchange factor SGEF

PROJECT SUMMARY Epithelial cells form tightly packed sheets of uniformly polarized cells, with an apical membrane contacting the environment, lateral membranes held together by specialized cell–cell junctions, and basal membranes anchored to other cells or the extracellular matrix. The establishment of apicobasal polarity in

epithelial cells is coordinated by three highly conserved protein complexes: PAR, Crumbs, and Scribble. These polarity complexes contain proteins that act as scaffolds to recruit other binding partners, including the Rho GTPases, to build spatially distinct signaling complexes. A major barrier to understand the formation of the

epithelium is our poor understanding of the mechanisms that control the function of the Scribble complex. For over two decades, the Scribble complex was known to function as a module from genetic studies, but it was unclear how the members of the complex interacted with each other, and their link to the regulators of adhesion

and polarity. To close this gap, my lab is focused in defining the precise composition, mechanism of formation and regulation of the Scribble complex. We have recently shown that SGEF, a RhoG-specific GEF, is a new component of the Scribble complex that acts as a scaffold to form a ternary complex by interacting directly with

Scribble and Dlg1, and plays a role during junction formation, and E-cadherin stability. Based on these findings, the objective of this proposal is to define the molecular mechanisms that regulate the function of the Scribble/SGEF/Dlg1 complex during cell-cell junctions’ assembly, maintenance, and turnover. We will test the

central hypothesis that the Scribble complex targets SGEF to the basolateral membrane where it regulates the E-cadherin stability and endocytosis in a catalytic dependent manner. We propose the following specific aims to test this hypothesis. Aim 1. Characterize the spatiotemporal regulation of SGEF-mediated RhoG activation by

the Scribble complex. Here we will test our working hypothesis that binding to Scribble and Dlg1 targets and/or activates SGEF to cell junctions, which promotes the localized activation of RhoG, which is essential for the regulation of E-cadherin expression levels; Aim 2. Define the role of the SGEF complex in the regulation of E-

cadherin stability and endocytosis. Our working hypothesis is that SGEF is recruited to the basolateral membrane through its interaction with Scribble and Dlg1, where it regulates the stability of E-cadherin at the plasma membrane. This approach is innovative, as it provides mechanistic insight on a new function for the Scribble complex.

This contribution will be significant because it will shed light on the fundamental mechanisms controlling cell-cell adhesion and its role in the establishment of polarity in epithelial cells and tissues. Understanding how cell adhesion and polarity are established and maintained in normal cells is important for situations in which loss of

adhesion and polarity represent a problem, such as epithelial cancers, which comprise 85% of all human cancers.