MICROTUBULES AND VESICLES UNDER CONTROLLED TENSION

Microtubules trapped inside cell-sized vesicles (similar to 10-mu m di ameter) define shapes reminiscent of living cells. In this work, three configurations are studied: an individual microtubule, a linear bundl e, and a circular band. Mechanical properties are probed by varying th e vesicle membrane tension using micropipet aspiration. Quantitative r esults include measurements of the persistence length of a single micr otubule, 6.3+/-2.4 mm, and the stretching modulus of the vesicle membr ane, k(s) greater than or equal to 140 erg/cm(2). The number of microt ubules in a typical bundle is similar to 100, and the total length in a typical band is similar to 0.5 cm, A tension-induced first-order tra nsition from linear bundle to circular band is observed and modeled. T he loss of stability of the bent bundle is associated with a saddle-no de bifurcation.