Strength and stability of active ligand-receptor bonds: A microtubule attached to a wall by molecular motor tethers

Abstract

We develop a stochastic kinetic model of a preformed attachment of a microtubule (MT) with a cell cortex, in which the MT is tethered to the cell by a group of active motor proteins. Such an attachment is a particularly unique case of ligand-receptor bonds: The MT ligand changes its length (and thus binding sites) with time by polymerization-depolymerization kinetics, while multiple motor receptors tend to walk actively along the MT length. These processes, combined with force-mediated unbinding of the motors, result in an elaborate behavior of the MT connection to the cell cortex. A fundamental challenge in this context is to understand how such a preformed attachment maintains its integrity long enough in spite of the ongoing turnover of the MT subunits from its depolymerizing plus end and withstands potentially disruptive effects arising from enhanced rates of detachment of the tethering motors because of external tensions. We present results for the strength and lifetime of the system through the well-established force-clamp and force-ramp protocols when external tension is applied to the MT. The simulation results reveal that the MT-cell attachment behaves as a catch bond or slip bond depending on system parameters. We provide analytical approximations of the lifetime and discuss implications of our results on in vitro experiments.