Packing defects and the width of biopolymer bundles

TL;DR

This paper proposes that packing defects in biopolymer bundles create an effective repulsion that explains the observed well-defined width distribution of actin filament bundles, combining concepts from elasticity and instability theory.

Contribution

It introduces a theoretical model linking packing defects to bundle width regulation, providing a novel explanation for experimental observations.

Findings

Packing defects induce an effective repulsion in filament bundles.

The model explains the stable bundle width distribution.

Analogy to Rayleigh instability in charged droplets.

Abstract

The formation of bundles composed of actin filaments and cross-linking proteins is an essential process in the maintenance of the cells' cytoskeleton. It has also been recreated by in-vitro experiments, where actin networks are routinely produced to mimic and study the cellular structures. It has long been observed that these bundles seem to have a well defined width distribution, which has not been adequately described theoretically. We propose here that packing defects of the filaments, quenched and random, contribute an effective repulsion that counters the cross-linking adhesion energy and leads to a well defined bundle width. This is a two-dimensional strain-field version of the classic Rayleigh instability of charged droplets.