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2. Ral and sec5 interaction is novel and involved in asymmetric cell polarity

We have shown that Ral is involved in the asymmetric localisation of neosynthesized proteins in polarised epithelial cells. This function is mediated by a direct link of Ral to Sec5, a protein that participates to the dynamic formation of the exocyst - or more accurately of variable exocyst complexes. Secondly, we have generated genetic data that place Ral in a circuit downstream of the Rap GTPase - a scheme that is conflicting with observations in ex vivo grown immortalised cell lines.

A protein, sec5 that is a part of the exocyst was identified. This partnership was validated in vitro and in vivo (by co-immunoprecipitation): sec5 is a bona fide and most probably direct partner of activated Ral. What is the possible function of this interaction? We have explored two aspects of exocyst function. We have shown that Ral via interaction with the exocyst triggers secretion in chromaffin cells. In polarised cells (differentiated MDCK cells) where the exocyst is nearby tight junctions, we are finding that the interaction between Ral and the exocyst involves Ral in the establishment and/or maintenance of the apical /baso-lateral polarity barrier.

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