A Structural Bistability in the Microtubule Lattice Revealed by AFM Imaging of the Inside of Microtubules

Abstract

Microtubules (MTs) are an important part of the cytoskeleton. In the living cell, MTs are non-equilibrium polymers with complex chemical and mechanical properties. During polymerization, tubulin dimers are believed to form an intermediate open sheet structure, which eventually closes into a tube. This implies flexibility in the lateral bonds between protofilaments. Previous research from Wang and Nogales (2005, Nature, 435:911) has shown a bistability in these bonds under non-physiological conditions, leading to the pairing of protofilaments with alternating binding angles. We have here used atomic force microscopy (AFM) to image the fine structure of taxol-stabilized MTs that were flattened inside-out against the substrate surface. We were able to reach single-monomer resolution, and observed a highly reproducible lateral zigzag pattern of protofilaments, demonstrating that a structural bistability also exists in regular microtubules. To estimate the angular orientation of protofilaments in the flattened MTs, we used the tubulin atomic structure to reconstruct the AFM images.