  Membrane tubes are important elements for living cells to organize many
functions. Experiments have found that membrane tube can be extracted from
giant lipid vesicles by a group of kinesin. How these motors cooperate in
extracting the fluid-like membrane tube is still unclear. In this paper, we
propose a new cooperation mechanism called two-track-dumbbell model, in which
kinesin is regarded as a dumbbell with an end (tail domain) tightly bound onto
the fluid-like membrane and the other end (head domain) stepping on or
unbinding from the microtubule. Taking account of the elasticity of kinesin
molecule and the exclude volume effect of both the head domain and the tail
domain of kinesin, which are not considered in previous models, we simulate the
growth process of the membrane tube pulled by kinesin motors. Our results
indicate that motors along a single microtubule protofilament can generate
enough force to extract membrane tubes from vesicles, and the average number of
motors pulling the tube is about 8~9. These results are quite different from
previous studies (Ref. \cite{camp.08}), and further experimental tests are
necessary to elucidate the cooperation mechanism.
