The process of gelation of filamentous actin and the role of actin binding proteins will be investigated under a variety of conditions using non-perturbative dynamic light scattering and viscoelastic measurements. Recent measurements at low actin concentrations using microsphere diffusion probes will be extended to study the network pore sizes and interactions within actin gels under conditions designed to mimic the cellular cytoplasm. Additional experiments will be conducted in order to clarify the role of actin in maintaining and controlling the mechanical properties of the cytoplasm of non-muscle cells. It is generally recognized that the protein actin is one of the most common proteins in nature and is present in most, if not all, eukaryotic cells. Its form ranges from the thin filament in muscle cells to mixtures of the globular monomeric form (G-actin) and polymerized actin in non-muscle cells. Recent progress has begun to reveal the role of actin in determining the structure and mechanical properties of the cytoplasmic matrix of most non-muscle cells. Actin's function in cellular processes hinges on its ability to reversibly polymerize as well as to form a viscoelastic gel, often in conjunction with specific proteins present in the cytoplasm. The mechanisms by which these various actin- binding proteins control the polymerization-depolymerization and sol-gel-sol transformations of actin are not yet well understood.
