  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.
