Role of External Forcing in Directional Instability of Microtubule Transport

Abstract

The many cellular processes like cell division and axonal, vesicle and organelle transport are largely driven by cytoskeletal elements like Microtubule (MT) and molecular motors. There are two group of motors - Kinesin and Dynein, which walks and transport towards plus and minus end of MT respectively. However, in presence of both motors on single cargo or MT can lead to Tug-of-war scenarios and shows stochastic switching in transport direction. These scenarios can have following type of geometries, a single cargo or MT transported by two antagonistic motors and other an aster being transported by a single type of motor. The motor-MT system can be compared to Magneto-elastic system (an example of Duffing oscillator) where motor and MT corresponds to magnet and metallic strip respectively. On external forcing, Magneto-elastic system shows Force Induced Stability (FIS). It would be interesting to know whether in vivo motor-MT system uses Force Induced Directionality (FID). We perform in silico gliding assay using Cytosim, a Langevin-Brownian particle simulator. Gliding assay setup geometry consists of MT gliding on immobilized mix motors. Control simulations were performed in absence of external forcing. Cytosim is modified for implementing the forcing using fluid flows to study the effect of external forcing. In order to test the predictions from simulations, we attempt to purify kinesin and perform mix multi motor gliding assay and study effect of forcing using optical trap. Tug-of-war and stochastic switching were observed in control simulations. The developed parameters, Directionality and bias are able to quantify directional instability in MT transport and can distinguish between forced and non-forced scenarios. The effect of external forcing is studied and results shows signs of FID for forcing (frequency - 10Hz and amplitude - 1 um) value. Further analysis is required to find optimal forcing parameters which can led to FID in motor-MT system and to understand the underlying phenomenon and relate to in vivo scenarios like cytoplasmic flows, actin polymerization.