Quantitation of liquid-crystalline ordering in F-actin solutions

Abstract

Actin filaments (F-actin) are important determinants of cellular shape and motility. These functions depend on the collective organization of numerous filaments with respect to both position and orientation in the cytoplasm. Much of the orientational organization arises spontaneously through liquid crystal formation in concentrated F-actin solutions. In studying this phenomenon, we found that solutions of purified F-actin undergo a continuous phase transition, from the isotropic state to a liquid crystalline state, when either the mean filament length or the actin concentration is increased above its respective threshold value. The phase diagram representing the threshold filament lengths and concentrations at which the phase transition occurs is consistent with that predicted by Flory's theory on solutions of noninteracting, rigid cylinders (Flory, 1956b). However, in contrast to other predictions based on this model, we found no evidence for the coexistence of isotropic and anisotropic phases. Furthermore, the phase transition proved to be temperature dependent, which suggests the existence of orientation-dependent interfilament interactions or of a temperature-dependent filament flexibility. We developed a simple method for growing undistorted fluorescent acrylodan-labeled F-actin liquid crystals; and we derived a simple theoretical treatment by which polarization-of-fluorescence measurements could be used to quantitate, for the first time, the degree of spontaneous filament ordering (nematic order parameter) in these F-actin liquid crystals. This order parameter was found to increase monotonically with both filament length and concentration. Actin liquid crystals can readily become distorted by a process known as "texturing." Zigzaging and helicoidal liquid crystalline textures which persisted in the absence of ATP were observed through the polarizing microscope. Possible texturing mechanisms are discussed.

Active polar fluid flow in finite droplets

“…This means that the magnitude of the polarisation can be defined as |p| = 1 without loss of generality. This assumption is valid for networks of high concentration, since the nematic order of actin networks increases with filament concentration [24]. Variations in the order parameter (|p|) can be included as in [22] where numerical methods are used to simulate a crawling keratocyte fragment.…”

Multi-functions of hydrogel with bilayer-based lamellar structure

“…However, most conventional soft and wet hydrogels usually show extremely poor functions due to their amorphous structure, i.e., random cross-linking polymer chain at molecular level, in contrast with the natural bio-tissue that possesses well-defined hierarchy structure from molecular level to macroscopic scale. The well ordered anisotropic structures of the biological tissues enable highly elaborate functions of living organisms [11][12][13][14][15]. For example, actins and myosin show a liquid crystalline anisotropic structure in a muscle sarcomere, which contributes the smooth motion of muscle fibers and muscle contraction in a specific direction [16][17][18].…”

Models for the Length Distributions of Actin Filaments: II. Polymerization and Fragmentation by Gelsolin Acting Together

“…5. Actin filaments interact with other filaments in vivo through crosslinking and bundling; these interactions are mediated by a variety of actinassociated proteins, and depend on the lengths of the filaments relative to intermolecular distances (Suzuki et al, 1991;Coppin and Leavis, 1992;Furukawa et al, 1993;Wachsstock et al, 1993Wachsstock et al, , 1994.…”