Fig. 4: Correlation between colchicine-site dynamics and global tubulin backbone rearrangements.

A Relaxation of azo-CA4: dark to 100 ns. B Movements of βT7 loop: 100 ns to 1 ms. C Collapse of the colchicine site: 1 ms to 100 ms. The three rows depict conformational changes at different spatial levels: Binding pocket in the upper row, tubulin backbone in the middle row, and longitudinally aligned tubulin dimers in the lower row. Starting structures are shown in gray. Backbone movements within tubulin (middle row) are represented by modevectors between consecutive time delays. Every second Cα-backbone atom in the main secondary structural elements with a displacement of at least 0.5 Å is shown. The N-terminal nucleotide-binding domains, the C-terminal domains, and the intermediate domains for both α and β tubulin are shown in blue, red, and green, respectively. The release pathway (red spheres) is indicated to illustrate the directional movements of the tubulin subunits. The global conformational changes (lower row) are visualized by three longitudinally aligned tubulin dimers (“Straight” tubulin dimers as found in the main shaft of a microtubule are shown in surface representation (dark gray, α-tubulin; light gray, β-tubulin; PDB ID 5SYE); “Curved” tubulin dimers (α-tubulin in blue and β-tubulin in cyan) at the indicated time delay and in relation to the previous state are shown in black). Deviations demonstrate the conformational plasticity of tubulin during azo-CA4 relaxation in its binding pocket and βT7 loop movements of β-tubulin, as well as a directed curvature adjustment after azo-CA4 release. Arrows indicate the main directions of movements in the indicated time delays.