Collective cargo hauling by a bundle of parallel microtubules: bi-directional motion caused by load-dependent polymerization and depolymerization

TL;DR

This paper presents a simplified theoretical model demonstrating how a bundle of parallel microtubules can produce bi-directional motion through load-dependent polymerization and depolymerization, without motor proteins.

Contribution

The study introduces a novel model showing load-dependent dynamics in microtubule bundles can cause bi-directional motion, challenging the need for motor proteins in chromosome segregation.

Findings

Microtubule bundles can generate bi-directional motion via load-dependent polymerization.

Indirect interactions among microtubules lead to diverse collective dynamic states.

Bi-directional motion can occur without external forces or motor proteins.

Abstract

A microtubule (MT) is a hollow tube of approximately 25 nm diameter. The two ends of the tube are dissimilar and are designated as `plus' and `minus' ends. Motivated by the collective push and pull exerted by a bundle of MTs during chromosome segregation in a living cell, we have developed here a much simplified theoretical model of a bundle of parallel dynamic MTs. The plus-end of all the MTs in the bundle are permanently attached to a movable `wall' by a device whose detailed structure is not treated explicitly in our model. The only requirement is that the device allows polymerization and depolymerization of each MT at the plus-end. In spite of the absence of external force and direct lateral interactions between the MTs, the group of polymerizing MTs attached to the wall create a load force against the group of depolymerizing MTs and vice-versa; the load against a group is shared equally by the members of that group. Such indirect interactions among the MTs gives rise to the rich variety of possible states of collective dynamics that we have identified by computer simulations of the model in different parameter regimes. The bi-directional motion of the cargo, caused by the load-dependence of the polymerization kinetics, is a "proof-of-principle" that the bi-directional motion of chromosomes before cell division does not necessarily need active participation of motor proteins.