Structural intermediates in microtubule assembly and disassembly: how and why?

Microtubules are cytoskeletal polymers made of repeating alphabeta-tubulin heterodimers that play essential roles in all eukaryotic cells. The complex dynamic behavior of microtubules, which is ultimately due to the tubulin subunit structure and its intrinsic GTPase activity, is key to the functions of these ubiquitous polymers. Microtubule assembly and disassembly do not take place by simple helical growth and shrinkage via individual subunits, but rather involve transient polymer intermediates, distinct from the microtubule, without parallel in other biological self-assembly systems. The discovery of these intermediates a decade ago has been followed recently by quantitative descriptions of their structure and their relationship to nucleotide state.