...Dogterom and colleagues now describe a new optical-tweezers-based method that can be used to observe the assembly dynamics of individual microtubules at molecular resolution.

In Nature, Dogterom and colleagues now describe a new optical-tweezers- based method that can be used to observe the assembly dynamics of individual microtubules at molecular resolution. Microtubules — a crucial part of the cytoskeleton in all eukaryotic cells — are dynamic protein polymers that are known to self-assemble from tubulin dimers. However, there has been a lack of information about the molecular events that underlie the growth and shrinkage of microtubules, because studies in vitro and in vivo have been limited to measurements of average growth and shrinkage rates.

To obtain information on microtubule dynamics at the resolution of single tubulin dimers (8 nm), Dogterom and co-workers developed a technique based on optical tweezers that allows dynamic microtubule plus ends to grow and shrink against a microfabricated barrier. A bead was attached to an axoneme — a rigid bundle of multiple stabilized microtubules — and the bead–axoneme construct was suspended in an optical trap near a barrier. Microtubule growth was initiated by the addition of tubulin and GTP, and microtubule growth was measured by monitoring bead displacement.

In the presence of tubulin and GTP, the authors observed 20–30-nm stepwise increases in microtubule length. This is significantly larger than the 8-nm size of a tubulin dimer, which indicates that microtubule assembly might not always occur through the addition of individual dimers. When the microtubule-associated protein XMAP215 was included in the assay, the growth was markedly enhanced such that rapid length increases of 40–60 nm were observed. These observations indicate that “small tubulin oligomers are able to add directly to growing microtubules and that XMAP215 speeds up microtubule growth by facilitating the addition of long oligomers.” In the future, it will be interesting to use this technique to investigate how other types of microtubule-associated protein regulate microtubule dynamics.