Structural basis of interprotofilament interaction and lateral deformation of microtubules.

The diverse functions of microtubules require stiff structures possessing sufficient lateral flexibility to enable bending with high curvature. We used cryo-electron microscopy to investigate the molecular basis for these critical mechanical properties. High-quality structural maps were used to build pseudoatomic models of microtubules containing 11-16 protofilaments, representing a wide range of lateral curvature. Protofilaments in all these microtubules were connected primarily via interprotofilament interactions between the M loops, and the H1'-S2 and H2-S3 loops. We postulate that the tolerance of the loop-loop interactions to lateral deformation provides the capacity for high-curvature bending without breaking. On the other hand, the local molecular architecture that surrounds these connecting loops contributes to the overall rigidity. Interprotofilament interactions in the seam region are similar to those in the normal helical regions, suggesting that the existence of the seam does not significantly affect the mechanical properties of microtubules.