A mammalian homologue of the PDZ domain containing Caenorhabditis elegans protein PAR-6 was found in a yeast two-hybrid system screen as binding to the Rho family member Cdc42. PAR-6 contains a PDZ domain and in C. elegans it has been shown to be crucial for the asymmetric cleavage and establishment of cell polarity during the first cell divisions in the growing embryo. Mammalian PAR-6 interacted with Cdc42 and Rac1 both in the yeast two-hybrid system and in in vitro binding assays. Co-immunoprecipitation experiments, employing transiently transfected Cos-1 cells, further confirmed that Cdc42 and Rac1 are physiological binding partners for PAR-6. We found that, in epithelial Madin-Darby canine kidney cells (MDCK), endogenous PAR-6 was present in the tight junctions, as judged from its co-localisation with the tight junction protein ZO-1, however, PAR-6 was also detected in the cell nucleus. Stimulation of MDCK cells with scatter factor/hepatocyte growth factor induced a loss of PAR-6 from the areas of cell-cell contacts in conformity with their progressive breakdown. In C. elegans PAR-6 co-localises with PAR-3 and has been suggested to form a direct complex. In agreement with earlier studies, mammalian PAR-3 was found to be present in tight junctions of MDCK cells but, in contrast to PAR-6, the protein could not be detected in the nucleus. Furthermore, co-immunoprecipitation experiments, employing Cos-1 cells, demonstrated that mammalian PAR-6 and PAR-3 formed a direct complex. These findings, together with the reported roles of PAR-6 and PAR-3 in C. elegans, suggest that Cdc42 and Rac1 and PAR-6/PAR-3 are involved in the establishment of cell polarity in epithelial cells.
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