Polymorphism of F-actin assembly. 2. Effects of barbed end capping on F-actin assembly

Dynamic changes in the length distribution of actin filaments during polymerization can be modulated by barbed end capping proteins

During actin polymerization, it has been theorized that the actin filament length distribution initially grows in the form of a Gaussian before converting to produce that of an exponential. However, it has been difficult to demonstrate this experimentally. In this study, we use modern fluorescence microscopy techniques to observe the changing actin filament length distribution during and subsequent to the polymerization process. Nucleated actin filament growth using barbed end capping proteins (gelsolin and erythrocyte capping protein) leads to Gaussian length distributions that are relatively stable. As predicted, nucleated actin filament growth using actin/spectrin complexes follows a similar process until polymerization reaches equilibrium whereafter the Gaussian length distribution rapidly converts to that of an exponential. This study provides direct confirmation of the original theories for the mode of actin polymerization but raises doubts regarding the mechanism of the length distribution conversion from Gaussian to exponential.

Structure of autocatalytically branched actin solutions

The average branching number and cluster size in branched actin solutions with filament capping are evaluated using analytic theory and simulation methods. The average number of daughter branches per filament in steady state is much less than unity, regardless of the concentration of branching stimulant. Much more highly branched structures are obtained in the initial stages of polymerization.

Binding of dystrophin's tandem calponin homology domain to F-actin is modulated by actin's structure

Dystrophin has been shown to be associated in cells with actin bundles. Dys-246, an N-terminal recombinant protein encoding the first 246 residues of dystrophin, includes two calponin-homology (CH) domains, and is similar to a large class of F-actin cross-linking proteins including alpha-actinin, fimbrin, and spectrin. It has been shown that expression or microinjection of amino-terminal fragments of dystrophin or the closely related utrophin resulted in the localization of these protein domains to actin bundles. However, in vitro studies have failed to detect any bundling of actin by either intact dystrophin or Dys-246. We show here that the structure of F-actin can be modulated so that there are two modes of Dys-246 binding, from bundling actin filaments to only binding to single filaments. The changes in F-actin structure that allow Dys-246 to bundle filaments are induced by covalent modification of Cys-374, proteolytic cleavage of F-actin's C-terminus, mutation of yeast actin's N-terminus, and different buffers. The present results suggest that F-actin's structural state can have a large influence on the nature of actin's interaction with other proteins, and these different states need to be considered when conducting in vitro assays.

Flexible polymer-induced condensation and bundle formation of DNA and F-actin filaments

A simple semi-empirical theory is developed for the ionic strength dependence of the flexible polymer-induced condensation of semiflexible polyelectrolytes such as DNA and F-actin filaments. Critical concentrations of flexible polymer needed for condensation are calculated by comparing the free energies of inserting the semiflexible polyelectrolytes in a solution of flexible polymers, respectively, in their free state, and in their condensed state. Predictions of the theory are compared to experimental data on the condensation of DNA and F-actin filaments induced by the flexible polymer poly(ethylene oxide). The theory also predicts that reentrant decollapse is possible at low ionic strength and high concentrations of flexible polymer, as observed for DNA.

Distinct structural changes detected by X-ray fiber diffraction in stabilization of F-actin by lowering pH and increasing ionic strength

Lowering pH or raising salt concentration stabilizes the F-actin structure by increasing the free energy change associated with its polymerization. To understand the F-actin stabilization mechanism, we studied the effect of pH, salt concentration, and cation species on the F-actin structure. X-ray fiber diffraction patterns recorded from highly ordered F-actin sols at high density enabled us to detect minute changes of diffraction intensities and to precisely determine the helical parameters. F-actin in a solution containing 30 mM NaCl at pH 8 was taken as the control. F-actin at pH 8, 30 to 90 mM NaCl or 30 mM KCl showed a helical symmetry of 2.161 subunits per turn of the 1-start helix (12.968 subunits/6 turns). Lowering pH from 8 to 6 or replacing NaCl by LiCl altered the helical symmetry to 2.159 subunits per turn (12.952/6). The diffraction intensity associated with the 27-A meridional layer-line increased as the pH decreased but decreased as the NaCl concentration increased. None of the solvent conditions tested gave rise to significant changes in the pitch of the left-handed 1-start helix (approximately 59.8 A). The present results indicate that the two factors that stabilize F-actin, relatively low pH and high salt concentration, have distinct effects on the F-actin structure. Possible mechanisms will be discussed to understand how F-actin is stabilized under these conditions.