Structure of F–actin solutions

Abstract

For the purpose of investigating the structure of solutions of muscle protein F–actin and the effect of external force on this structure, simultaneous measurements of rigidity, viscosity, and birefringence were made under rotation and oscillation at very low rates of shear, and as a result a consistent picture of F–actin solutions has been obtained. The F–actin solutions of various concentrations are made by adding a small amount of magnesium ions to a salt‐free G–actin solution. Rigidity, viscosity, and degree of flow birefringence all increase in proportion to the F–actin concentration. Even at very low concentrations, F–actin solutions have a large rigidity. This rigidity is found to be due to a network structure of semiflexible F–actin filaments composed of very many G–actin molecules. The linkage‐to‐linkage length and the flexibility of filaments are estimated from experimental data. Rotation at very low rate of shear causes rigidity and viscosity to decrease rapidly. All results are reasonably explained by assuming that the filaments are elongated and then linkages are detached by shear of rotation. With a decrease in the number of linkages, F–actin filaments are completely oriented. Once attained as a result of shear, the orientation decreases very slowly on standing and gives an anomalous decay curve, probably as a result of setting of the network structure of oriented filaments. At high concentrations of F–actin, a spontaneous orientation occurs. Hysteresis phenomena of F–actin solutions observed under various mechanical treatments are all attributable to formation and detachment of linkages between semiflexible F–actin filaments.

Crowding‐induced organization in cells: spontaneous alignment and sorting of filaments with physiological control points

“…The spontaneous alignment of protein filaments in cellfree solution is well known: optical birefringence has been observed in deoxygenated solutions of sickle cell hemoglobin (Harris, 1950;Allison, 1957), in solutions of actin filaments (Kasai et al, 1960;Oosawa and Asakura, 1975;Coppin and Leavis, 1992;Newman et al, 1989;Suzuki et al, 1991) and in solutions of microtubules (Hitt et al, 1990). However, Onsager's theory cannot be applied directly to cellular systems for several reasons:…”

Nonmuscle Actin-Binding Proteins

“…Measurements of linear viscoelasticity have shown that actin filaments are relatively resistent to breakage under small shear stresses (Zaner & Stossel 1982), but a number of experiments have shown that breakage of actin filaments does occur at a low rate. In addition, it has long been known that actin solutions are thixotropic; that is, their viscosity is drasticaily decreased following shear (Kasai et al 1960), and this viscosity slowly increases with time (Jen et al 1982). Such calculations suggest that actin filaments should be over 1 mm long, but such lengths have Bever been observed (Wegner & Savko 1982;Frieden & Goddette 1983).…”

“…The relatively rigid, rod-like nature of actin filaments dictates that for actin binding proteins to create isotropic actin gels with a low volume fraction of polymer mass they must, at a minimum, stabilize the isotropy inherent in solutions of overlapping, semiflexible actin filaments (Kasai et al 1960 ;Zaner & Stossel 1983 ;Niederman et al 1983). Another way to produce isotropic gels is for the cross-linking species to cause filaments to branch at angles approaching 90°, thereby counteracting the tendency of long rod like fibers to align in parallel (Flory 1956).…”

Biomimetic F‐Actin Cortex Models

“…The physical chemical properties of actin have been the subject of many in vitro experiments since its discovery 35–49. Though in the early experiments the F‐actin was visualized using transmission electron microscopy (TEM) only, the mechanical properties of the gel they formed were already extensively studied using macroscopic rheology methods 37…”

“…These microdroplets can be considered as a most simplified mechanical model for cells, where the mesh size and the low‐scale morphology of the filaments resemble cellular actin, although the different volume and 3D structure may result in deviations in the mechanical behaviour of a cellular F‐actin cortex which is a rather 2D F‐actin network. Several rheological studies were published investigating the rheology of F‐actin31, 35, 37–44, 61, 69–72 without and during crosslinking using crosslinker molecules, such as myosin, α‐actinin, or fascin, and also divalent ions, such as Ca 2+ and Mg 2+ , etc 45–47. 49, 52, 54, 56, 59, 60, 62, 63, 65, 67, 68, 73–80 The general findings cover a very broad range of information, from which we would like to highlight two aspects.…”