Patterns of cellular activities based on protein sorting in cell motility, endocytosis and cytokinesis

In order to move persistently, a cell has to harmonize its protrusion and retraction with attachment and detachment from the substrate. Time-series analyses based on fluctuations in these activities are being used in combination with advanced imaging techniques to unravel the network of protein-protein interactions that tune the activities in a motile cell and co-ordinate them in space and time. Fusions with the green fluorescent protein have helped to visualize the recruitment of cytoskeletal proteins from a soluble pool and their transient assembly into supramolecular structures. Using a series of mutants deficient in specific cytoskeletal proteins has revealed common themes and interrelationships between cell motility, endocytosis and cytokinesis. For instance, a phagocytic cup competes with leading-edge formation, and recruits the same actin-associated proteins. Cytokinesis is based on the fine tuning of activities in the microtubule system and the actin network in the cell cortex. Cells dividing on a substrate apply tension to the surface on which they adhere, as determined by the silicone rubber technique. Actin-associated proteins are sorted during cytokinesis either to the extensions formed at the poles of a dividing cell or to the cleavage furrow. A major effort will be required to elucidate the mechanisms that dictate the pattern of local activities and drive the translocation of proteins in cell motility, endocytosis and cytokinesis.