Credit: Image courtesy of Paul Maddox, University of North Carolina, Chapel Hill, USA. Scale bar, 5 μm.

Behind the scenes, much of what appears static in the cell is actually the steady state of a dynamic system. The cytoskeleton is a key example: cytoskeletal polymers such as actin filaments and microtubules regularly undergo polymerization and depolymerization, which is often not visible if the overall length of the polymer remains unchanged. To really get a handle on these polymer 'fluxes', you need to ask what is going on with individual polymer subunits — a challenge that has been met by fluorescent-speckle microscopy. In the August issue of The Journal of Cell Biology, Maddox et al. apply this technology to the study of kinetochore microtubules, and from their observations they provide new insights into the microtubule dynamics that drive spindle mechanics.

During mitosis, kinetochore microtubules produce force that eventually pulls chromosomes towards the spindle poles. Numerous studies have looked at the contributions made during this process by kinetochore microtubule poleward flux versus the force generated at the kinetochore itself. However, the results have varied depending on the system examined and were sometimes limited by the inability to distinguish kinetochore microtubules from other spindle microtubules.

Maddox et al. set out to distinguish between these two models by using fluorescent-speckle microscopy on spindles in Xenopus egg extracts. The premise of this method is that low levels of fluorescent-tubulin subunits will co-polymerize with non-labelled subunits into a microtubule polymer and so provide reference marks that allow the movements of individual subunits within the polymer to be followed (shown in green in the figure; kinetochores are selectively labelled red.). Taking this approach, Maddox et al. asked whether kinetochore microtubules do indeed flux poleward.

They saw that during metaphase, kinetochore microtubules do flux, and thereby create tension at the kinetochore. After anaphase entry, these microtubules switch to depolymerization at the kinetochore. This, combined with the poleward flux, seems to drive chromosomes poleward. Variable switching between persistent polymerization and depolymerization was often observed in anaphase. The authors propose that switching to polymerization provides a 'clutch' mechanism that prevents microtubule detachment following strong force.

From these studies, new implications for spindle dynamics emerge. In particular, the authors propose that all kinetochores are fundamentally bistable, and that different behaviours such as chromosome oscillations depend on different flux rates within the microtubule polymer. This work therefore provides an illustrative example of the power of fluorescent-speckle microscopy and the key tool it has become for analysing cytoskeletal function in live cells.