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Cargo transport by several motors

In cells, organelles and vesicles are usually transported by cooperation of several motor proteins, including plus-end directed motor kinesin and minus-end directed motor dynein. Many biophysical models have been constructed to understand the mechanism of this motion. However, so far, the basic principle about it remains unclosed. In this paper, based on the recent experimental results and existing theoretical models, a spider-like model is provided. In this model, each motor is regarded as a bead-spring system. The bead can bind to or unbind from the track stochastically, and step forward or backward with fixed step size L and force dependent transition rates. The spring connects the bead to cargo tightly. At any time, the position of cargo is determined by force balance condition. The obvious characteristics of our model are that, the motors interact with each other and they do not share the external load equally. Our results indicate, the stall force of cargo, under which the mean velocity of cargo vanishes, usually decreases with the interactions between motors. If the cargo is pulled by several motors from same motor species, the stall force of cargo is bigger than that of the single motor, but usually smaller than their sum. However, if the cargo is pulled by two motors with different directionality, the stall force of cargo might bigger or smaller than the difference of the two single motor stall forces. The results imply, cooperation of several motors is helpful to pull big cargoes, though it is not so good as might be expected and no obvious help to improve the velocity.