  Starting from the hypothesis that the tubulin dimer is a conformationally
bistable molecule - fluctuating between a curved and a straight configuration
at room temperature - we develop a model for polymorphic dynamics of the
microtubule lattice. We show that tubulin bistability consistently explains
unusual dynamic fluctuations, the apparent length-stiffness relation of grafted
microtubules and the curved-helical appearance of microtubules in general.
Analyzing experimental data we conclude that taxol stabilized microtubules
exist in highly cooperative yet strongly fluctuating helical states. When
clamped by the end the microtubule undergoes an unusual zero energy motion - in
its effect reminiscent of a limited rotational hinge.
