  The presence of membrane tubules in living cells is essential to many
biological processes. In cells, one mechanism to form nanosized lipid tubules
is via molecular motor induced bilayer extraction. In this paper, we describe a
simple experimental model to investigate the forces required for lipid tube
formation using kinesin motors anchored to DOPC vesicles. Previous related
studies have used molecular motors actively pulling on the membrane to extract
a nanotube. Here, we invert the system geometry; molecular motors are used as
static anchors linking DOPC vesicles to a two-dimensional microtubule network
and an external flow is introduced to generate nanotubes facilitated by the
drag force. We found that a drag force of ~7 pN was sufficient for tubule
extraction for vesicles ranging from 1 to 2 um in radius. By our method, we
found that the force generated by a single molecular motor was sufficient for
membrane tubule extraction from a spherical lipid vesicle.
