Microtubules switch occasionally into unfavorable configurations during elongation

Tubulin assembles to form a range of structures that differ by their protofilament and monomer helix-start numbers. The microtubule lattice is believed to accommodate these different configurations by skewing the protofilaments so that the lateral interactions between tubulin subunits are maintained. Here, we present the characterization of 14 types of microtubules, including six novel ones, through an extensive analysis of microtubules assembled in vitro from pure tubulin. Although the six new types represented only 1 % of the total length of the population examined ( approximately 17 mm), they define the limits of microtubule structure and assembly. Protofilament skewing is restricted to within +/-2 degrees. Outside this range, the restoring force induced by the skewed protofilaments is compensated by a longitudinal shift (less than +/-0.2 nm) between adjacent protofilaments. Configurations with theoretical protofilament skew angles larger than +/-4 degrees or that necessitate larger modifications of the microtubule surface lattice were not observed. Analysis of the microtubule types distribution reveals that it is sharply peaked around the less skewed conformations. These results indicate that both the flexibility of the protofilaments and the strength of their lateral interactions restrict the range of structures assembled. They also demonstrate that growing microtubules can occasionally switch into energetically unfavorable configurations, a behavior that may account for the stochastic nature of catastrophes.