Cargo transport by several motors

In a cell, organelles and vesicles are usually transported by cooperation of several motor proteins, including plus-end-directed motor kinesin and minus-end-directed motor dynein. In recent years, many biophysical models have been constructed to understand the mechanism of this transport; however, so far, its basic principle has not been completely understood. In this paper, we will present a model that is based on recent experimental results and existing theoretical models. In this model, each motor is regarded as a head-spring system. The head can bind to or detach from the track stochastically, and step forward or backward with a fixed step size L and force-dependent transition rates. The spring connects the head to the cargo tightly. The position of the cargo is determined by the force-balance condition. An obvious characteristic of our model is that the motors interact with each other and do not share the external load equally. Results indicate that the basic properties of the cargo, including its mean velocity and stall force (definitions are given in the text), are greatly affected by the intermotor interaction. Stated simplistically, the mean velocity and (average) stall force decrease with the intermotor interaction. Therefore, in a sense, the intermotor interaction is negative to the cooperate motion of motors. However, if the cargo is pulled by motors from the same species, its (average) stall force is usually larger than that of the single motor; this means that under external load, the velocity of cargo transported by several motors might be higher than the single-motor cases. We can infer from this that the cooperation of motors is beneficial to draw big cargoes, while it is not as beneficial as one might expect to improve motion velocity when the external load is low.