Oligomers of tubulin can polymerize!
This is an important step for studying microtubule dynamics and is in my humble opinion among the most important papers on the microtubule front since Mitchison first described dynamic instability (Mitchison, 1984). As the title suggests this paper provides insight into microtubule polymerization at a molecular level. Prior to this work by the Dogterom laboratory we have only been unable to study how individual tubulin molecules polymerize within a microtubule. Many researchers (including myself) have speculated that tubulin under certain circumstances (i.e. in the presence of microtubule plus-end tracking proteins - +TIPs) might add as an oligomer that might then act as a stabilizing cap to facilitate further polymerization. This paper uses optical tweezer technology to show that MTs polymerized exclusively from tubulin grows in step sizes of up to 30 nm (a tubulin dimer is only 8 nm so this is clearly oligomers of some sort can polymerize). They go on to show that XMAP215 increases the step size to 60 nm thus supporting those hypotheses that +TIPs can add tubulin oligomers. Finally, they go the extra step using physics to demonstrate that the steps are not the result of open sheet closure but instead actual addition of tubulin oligomers. This work is clearly only a stepping stone leaving many unresolved questions. For example are the oligomers a single protofilament of tubulin or does a tubulin oligomer add within multiple protofilaments. Additionally this work provides a method to study the effects of +TIPs on MT assembly as opposed to the reckless hypothesizing with which we are familiar.