F-actin, a model polymer for semiflexible chains in dilute, semidilute, and liquid crystalline solutions

J. Käs, H. Strey, J. X. Tang, D. Finger, R. Ezzell, E. Sackmann, P. A. Janmey

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Abstract

Single actin filaments were analyzed in solutions ranging from dilute (0.2 μg/ml), where filaments interact only with solvent, to concentrations (4.0 mg/ml) at which F-actin forms a nematic phase. A persistence length of ~1.8 μm and an average length of ~22 μm (Kaufmann et al., 1992) identify actin as a model for studying the dynamics of semiflexible polymers. In dilute solutions the filaments exhibit thermal bending undulations in addition to diffusive motion. At higher semidilute concentrations (1.4 mg/ml) three- dimensional reconstructions of confocal images of fluorescently labeled filaments in a matrix of unlabeled F-actin reveal steric interactions between filaments, 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 tube diameter (a) scales with monomer concentration c as (a) ∞ c((-0.5 ± 0.15)). The diffusion of filaments in semidilute solutions (c = (0.1-2.0) mg/ml) is dominated by diffusion along the filament contour (reptation), and constraint release by remodeling of the surrounding filaments is rare. The self- diffusion coefficient D(is parallel with) along the tube decreases linearly with the chain length for semidilute solutions. For concentrations >2.5 mg/ml a transition occurs from an isotropic entangled phase to a coexistence between isotropic and nematic domains. Analysis of the molecular motions of filaments suggests that the filaments in the aligned domains are in thermal equilibrium and that the diffusion coefficient parallel to the director D(is parallel with) is nearly independent of filament length. We also report the novel direct observation of u-shaped defects, called hairpins, in the nematic domains.

Original languageEnglish
Pages (from-to)609-625
Number of pages17
JournalBiophysical Journal
Volume70
Issue number2 I
DOIs
StatePublished - Feb 1996

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