Measuring cohesion between macromolecular filaments, one pair at a time: Depletion-induced microtubule bundling

TL;DR

This study measures the depletion-induced cohesion between microtubule filaments using optical trapping and imaging, revealing limitations of classical models and confirming pairwise additivity of interactions.

Contribution

The paper introduces an experimental method combining imaging and optical trapping to quantify filament interactions, highlighting deviations from classical depletion theory.

Findings

Depletion forces depend on polymer concentration and ionic strength.

Classical Asakura-Oosawa theory does not fully describe the data.

Depletion interactions are pairwise additive under tested conditions.

Abstract

In presence of non-adsorbing polymers, colloidal particles experience ubiquitous attractive interactions induced by depletion forces. Here, we measure the depletion interaction between a pair of microtubule filaments using a method that combines single filament imaging with optical trapping. By quantifying the dependence of filament cohesion on both polymer concentration and solution ionic strength, we demonstrate that the minimal model of depletion, based on the Asakura-Oosawa theory, fails to quantitatively describe the experimental data. By measuring the cohesion strength in two- and three- filament bundles we verify pairwise additivity of depletion interactions for the specific experimental conditions. The described experimental technique can be used to measure pairwise interactions between various biological or synthetic filaments and complements information extracted from bulk osmotic stress experiments.