Flux occurs even in monopolar spindles, which lack sliding forces.

On page 173, Cameron et al. find that kidney cells do not depend on sliding forces to generate flux in their mitotic spindles.

Flux is the poleward translocation of spindle microtubules relative to the poles that often helps chromosomes reach separate poles. Flux can be achieved by pushing microtubules outward from the center, by pulling them in from their ends, or by a combination of the two.

In fly S2 cells and in frog extracts, Eg5 is found at the central spindle, where antiparallel microtubules overlap. By cross-linking these microtubules and sliding them toward the poles, Eg5 has been proposed to create flux.

As Eg5 pushes from the middle, kinesin 13 lops off tubulin subunits at the poles. Kinesin 13 inhibition lengthens spindles but does not prevent the sliding component of flux, suggesting that central pushing forces are the major components of flux. But Cameron and colleagues find that, in marsupial kidney cells, flux continues without pushing forces.

The authors made careful measurements of flux rates in PtK1 cells and found only a 25% dampening in the absence of Eg5 activity. Even monopolar spindles, which lack overlapping antiparallel microtubules, maintained almost normal flux rates.

Since pushing forces were not needed, the group tried to knock out pulling forces as well. Kinesin 13, however, was not easily removed from the equation. RNAi is not yet possible in PtK1 cells, and antibody-mediated inhibition was unsuccessful. The group even tried knocking the motor off poles, but it still clung to microtubule minus ends. Thus, although pulling forces seem to be dominant, in this cell type at least, it is unclear whether kinesin 13 is the motor responsible.

Future studies should also address what factors determine the flux method in different cell types. Perhaps spindle–cortex interactions, which are lacking in frog egg extracts, are necessary for pulling forces. Or maybe the use of centrosome- rather than chromatin-driven spindle formation mechanisms has different effects on the motors that propel flux.