We observe quasistatic deformation of lipid vesicles from within, due to the polymerization of confined microtubules. A pair of long, narrow membrane sleeves appears, sheathing the microtubule ends as they grow. Spontaneous buckling reveals that the force generated can be greater than 2 pN. The evolution of shape and magnitude of force are consistent with a simple theory for the membrane free energy. We consider a model of the force generating mechanism in which thermal fluctuations of the membrane are “rectified” by the binding of tubulin dimers to the microtubule end.

• Received 30 September 1996

DOI:https://doi.org/10.1103/PhysRevLett.79.4497