Chiral Structure of F-actin Bundle Formed by Multivalent Counterions?

TL;DR

This study uses molecular dynamics simulations to investigate how multivalent counterions induce the formation of F-actin bundles, revealing detailed electrostatic interactions, structural arrangements, and the influence of twist rigidity on actin symmetry.

Contribution

It introduces a coarse-grained model that incorporates realistic F-actin features and analyzes the electrostatic mechanisms behind bundle formation and symmetry preferences.

Findings

Counterions concentrate in narrow gaps between F-actins.

Long-lived defects increase with bundle size.

The global energy minimum corresponds to 24/11 symmetry.

Abstract

The mechanism of multivalent counterion-induced bundle formation by filamentous actin (F-actin) is studied using a coarse-grained model and molecular dynamics simulation. Real diameter size, helically ordered charge distribution and twist rigidity of F-actin are taken into account in our model. The attraction between parallel F-actins induced by multivalent counterions is studied in detail and it is found that the maximum attraction occurs between their closest charged domains. The model F-actins aggregate due to the like-charge attraction and form closely packed bundles. Counterions are mostly distributed in the narrowest gaps between neighboring F-actins inside the bundles and the channels between three adjacent F-actins correspond to low density of the counterions. Density of the counterions varies periodically with a wave length comparable to the separation between consecutive G-actin monomers along the actin polymers. Long-lived defects in the hexagonal order of F-actins in the bundles are observed that their number increases with increasing the bundles size. Combination of electrostatic interactions and twist rigidity has been found not to change the symmetry of F-actin helical conformation from the native 13/6 symmetry. Calculation of zero-temperature energy of hexagonally ordered model F-actins with the charge of the counterions distributed as columns of charge domains representing counterion charge density waves has shown that helical symmetries commensurate with the hexagonal lattice correspond to local minima of the energy of the system. The global minimum of energy corresponds to 24/11 symmetry with the columns of charge domains arranged in the narrowest gaps between the neighboring F-actins.