The (a) probability distribution and (b) free energy profile for the colchicine-bound tubulin as a function of intradimer rotation.

<p>Intradimer rotation of the endpoint structures, the “straight” taxol-liganded, zinc-stabilized protofilament tubulin and “bent” α₁β₁ heterodimer from the T2R-colchicine complex, are denoted at θ∼0° <i>(blue dash)</i> and θ∼12° <i>(red dash)</i>, respectively. Intradimer rotations observed in crystal structures of colchicine-bound tubulins are denoted as dotted lines at θ∼7.8° (both α₁β₁ and α₂β₂ heterodimers of T2R-colchicine-ustiloxin complex). Individual probability histograms corresponding to each umbrella window is shown in Figure S2b in <a href="http://www.ploscompbiol.org/article/info:doi/10.1371/journal.pcbi.1003464#pcbi.1003464.s001" target="_blank">Text S1</a>.