Effects of α-tubulin acetylation on microtubule structure and stability

Significance

Microtubules are polymers of αβ-tubulin that play important roles in the cell. Regulation of their dynamics is critical for function and includes the posttranslational modification of tubulin. While most of tubulin modifications reside in the flexible C-terminal tail of tubulin, acetylation of α-tubulin on K40 is localized to the inside of the microtubule, within the so-called αK40 loop. Using high-resolution cryo-EM maps of acetylated and deacetylated microtubules, in conjunction with molecular-dynamics methods, we found that acetylation restricts the range of motion of the αK40 loop. In the deacetylated state, the loop extends deeper into the microtubule lumen and samples a greater number of conformations that we propose increase its accessibility to the acetylase and likely influence lateral contacts.

Abstract

Acetylation of K40 in α-tubulin is the sole posttranslational modification to mark the luminal surface of microtubules. It is still controversial whether its relationship with microtubule stabilization is correlative or causative. We have obtained high-resolution cryo-electron microscopy (cryo-EM) reconstructions of pure samples of αTAT1-acetylated and SIRT2-deacetylated microtubules to visualize the structural consequences of this modification and reveal its potential for influencing the larger assembly properties of microtubules. We modeled the conformational ensembles of the unmodified and acetylated states by using the experimental cryo-EM density as a structural restraint in molecular dynamics simulations. We found that acetylation alters the conformational landscape of the flexible loop that contains αK40. Modification of αK40 reduces the disorder of the loop and restricts the states that it samples. We propose that the change in conformational sampling that we describe, at a location very close to the lateral contacts site, is likely to affect microtubule stability and function.