Question

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Answer 1

Answer:

During cell division, mitotic spindles are assembled by

microtubule-based motor proteins1, 2

. The bipolar organization

of spindles is essential for proper segregation of chromosomes,

and requires plus-end-directed homotetrameric motor proteins

of the widely conserved kinesin-5 (BimC) family3

. Hypotheses

for bipolar spindle formation include the 'push−pull mitotic

muscle' model, in which kinesin-5 and opposing motor proteins

act between overlapping microtubules2, 4, 5

. However, the

precise roles of kinesin-5 during this process are unknown.

Here we show that the vertebrate kinesin-5 Eg5 drives the

sliding of microtubules depending on their relative orientation.

We found in controlled in vitro assays that Eg5 has the

remarkable capability of simultaneously moving at 20 nm s

-1

towards the plus-ends of each of the two microtubules it

crosslinks. For anti-parallel microtubules, this results in

relative sliding at 40 nm s

-1

, comparable to spindle pole

separation rates in vivo6

. Furthermore, we found that Eg5 can

tether microtubule plus-ends, suggesting an additional

microtubule-binding mode for Eg5. Our results demonstrate

how members of the kinesin-5 family are likely to function in

mitosis, pushing apart interpolar microtubules as well as

recruiting microtubules into bundles that are subsequently

polarized by relative sliding. We anticipate our assay to be a

starting point for more sophisticated in vitro models of mitotic

spindles. For example, the individual and combined action of

multiple mitotic motors could be tested, including minus-enddirected motors opposing Eg5 motility. Furthermore, Eg5

inhibition is a major target of anti-cancer drug development,

and a well-defined and quantitative assay for motor function

will be relevant for such developments

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