Chiral structure of F-actin bundle formed by multivalent counterions

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 Factin 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.

Semiflexible Biopolymers in Bundled Arrangements

“…Even though some of the aforementioned studies do not only involve analytical calculations but also computer simulations capturing the effects of counterion condensation [ 44 , 45 , 48 ], we would like to point to a number of publications approaching this topic via simulations [ 54 , 55 , 56 , 57 ].…”

Aggregation of flexible polyelectrolytes: Phase diagram and dynamics

“…[25][26][27][28][29][30][31][32][33][34][35][36][37][38][39][40][41] a) Electronic mail: anvym@imsc.res.in b) Electronic mail: rrajesh@imsc.res.in c) Electronic mail: vani@imsc.res.in At finite densities of PE chains, the effective attractive interactions among the PE chains can lead to aggregation, in addition to individual collapsed phases. Understanding counterion mediated aggregation of charged polymers is very relevant as the aggregation of biopolymers such as DNA and actin has been implicated to play an important role in biological functions such as cell scaffolding, DNA packaging, cytoskeletal organization [42][43][44][45][46][47][48][49] . In addition to biological polymers, recent studies have show that aggregation of synthetic polymers is crucial in their ability to function as biomimetic and functional materials [50][51][52][53] .…”

Aggregation dynamics of rigid polyelectrolytes

“…Many biologically relevant polymers such as DNA, actin and microtubules, have charged, rigid or semiflexible backbone structures, and may aggregate into bundles in the presence of counterions [1][2][3][4][5][6] . The aggregates of such biological polymers play an important role in cell scaffolding and possess superior mechanical properties compared to well known synthetic flexible polymers 7,8 .…”