| Epithelial cells are integrated into tissues by the formation of intercellular contacts between adjacent cells. Great efforts have been expended in understanding the architectural and molecular structure of cell-cell contacts, but far less is known about their dynamics and assembly. Dramatic changes in actin dynamics and structure follow the initiation of cell contact assembly between two adjacent epithelial cells, suggesting a role for members of the Rho family of GTPases in this process. This work describes in detail the isolation and characterization of Rac1 complexes in epithelial cells and defines their role in the formation of cell-cell contacts between cells.; A Rac1-PAK complex is assembled in response to serum stimulation, and functions to drive formation of lamellae and membrane ruffles. Characterization of precursor complexes in the cytosol reveals the probable mechanism by which the Rac1-PAK complex is assembled and translocated to the membrane. The other membrane complexes, an 11S and a 16S complex, act in a common pathway to regulate actin dynamics. The 11S and 16S Rac1 complexes both drive formation of an Arp2/3-based actin meshwork. In response to cell-cell adhesion, 11S complexes become trapped at sites of cell-cell adhesion, forming 16S complexes through protein-protein interactions. Evidence indicates that it is a protein-protein interaction with annexin II, a membrane-associated protein that binds actin and anionic lipids, that serves to localize Rac1 to sites of nascent cell-cell contact. This pathway is important in the rapid expansion and strengthening of nascent cell-cell junctions. |
