Polymer Induced Bundling of F-actin and the Depletion Force

TL;DR

This paper investigates how polyethylene glycol induces reversible bundling of F-actin through depletion forces, influenced by molecular weight and ionic strength, with implications for understanding cytoskeletal organization.

Contribution

It provides a quantitative analysis of PEG-induced F-actin bundling and explores the roles of depletion forces and ionic conditions in this process.

Findings

Bundling occurs above a critical PEG concentration C_o.

Hysteresis observed in bundle dissolution influenced by ionic strength.

Little polymer associates with the actin bundles.

Abstract

The inert polymer polyethylene glycol (PEG) induces a "bundling" phenomenon in F-actin solutions when its concentration exceeds a critical onset value C_o. Over a limited range of PEG molecular weight and ionic strength, C_o can be expressed as a function of these two variables. The process is reversible, but hysteresis is also observed in the dissolution of the bundles, with ionic strength having a large influence. Additional actin filaments are able to join previously formed bundles. Little, if any, polymer is associated with the bundle structure. Continuum estimates of the Asakura-Oosawa depletion force, Coulomb repulsion, and van der Waals potential are combined for a partial explanation of the bundling effect and hysteresis. Conjectures are presented concerning the apparent limit in bundle size.