F-ACTIN, A MODEL POLYMER FOR SEMIFLEXIBLE CHAINS IN DILUTE, SEMIDILUTE, AND LIQUID-CRYSTALLINE SOLUTIONS

Single actin filaments were analyzed in solutions ranging from dilute (0.2 mu g/ml), where filaments interact only with solvent, to concentr ations (4.0 mg/ml) at which F-actin forms a nematic phase. A persisten ce length of similar to 1.8 mu m and an average length of similar to 2 2 mu m (Kaufmann et al., 1992) identify actin as a model for studying the dynamics of semiflexible polymers. In dilute solutions the filamen ts exhibit thermal bending undulations in addition to diffusive motion . At higher semidilute concentrations (1.4 mg/ml) three-dimensional re constructions of confocal images of fluorescently labeled filaments in a matrix of unlabeled F-actin reveal steric interactions between fila ments, which account for the viscoelastic behavior of these solutions. The restricted undulations of these labeled chains reveal the virtual tube formed around a filament by the surrounding actin. The average t ube diameter [a] scales with monomer concentration c as [a] proportion al to c-((0.5 +/- 0.15)). The diffusion of filaments in semidilute sol utions (c = (0.1-2.0) mg/ml) is dominated by diffusion along the filam ent contour (reptation), and constraint release by remodeling of the s urrounding filaments is rare. The self-diffusion coefficient D-paralle l to along the tube decreases linearly with the chain length for semid ilute solutions. For concentrations >2.5 mg/ml a transition occurs fro m an isotropic entangled phase to a coexistence between isotropic and nematic domains. Analysis of the molecular motions of filaments sugges ts that the filaments in the aligned domains are in thermal equilibriu m and that the diffusion coefficient parallel to the director D-parall el to is nearly independent of filament length. We also report the nov el direct observation of u-shaped defects, called hairpins, in the nem atic domains.