ANALYSIS OF MICROTUBULE RIGIDITY USING HYDRODYNAMIC FLOW AND THERMAL FLUCTUATIONS

We report the use of two independent new methods to measure the flexur al rigidity of microtubules. Microtubules were grown off axonemal piec es adhering to a glass coverslip. In the first method, a hydrodynamic flow was applied to microtubules and the flexural rigidity was derived from the analysis of the bending shape of the microtubules at equilib rium in the flow. In the second method, the flexural rigidity was deri ved from the thermal fluctuations of the free end of axoneme-bound mic rotubules. With both methods, the flexural rigidity of standard GDP mi crotubules was estimated to be 0.85 +/- 0.2 x 10(-23) newtons x m(2) w hich corresponded to a persistence length of 2 +/- 0.2 mm. Binding of ligands known to affect the biochemical properties of microtubules aff ected their rigidity. The structural analogs of inorganic phosphate A1 F(4)(-) and [BeF3-, H2O], which bind to the site of the gamma-phosphat e of GTP on GDP microtubule and reconstitute the GDP-P-i microtubule i ntermediate state of GTP hydrolysis, cause an similar to 3-fold increa se in microtubule flexural rigidity and persistence length. Taxol and taxotere, antitumoral microtubule stabilizing drugs, in contrast cause a decrease in flexural rigidity and appear to affect the three-dimens ional superstructure of microtubules, which can no longer be considere d as semi-flexible rods. The relationship between the mechanical prope rties of microtubules and their biological function is discussed.