Published 25 September 2006. doi:10.1083/jcb.1747iti5
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 174, Number 7, 906-906
Cooperative motors
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Mixing OSM-3 and kinesin-II produces motoring speeds between the speeds of either motor alone.
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Two kinesin-2 motors, OSM-3 and kinesin-II, drive the intraflagellar transport (IFT) of cargo toward the tips of cilia. Although OSM-3 is quicker than kinesin-II, Pan et al. (page 1035) show that these motors cooperate to move things together. In a related paper, Imanishi et al. (page 931) find that OSM-3 folds up to slow down.
OSM-3 moves at approximately twice the speed of kinesin-II. Pan et al. found that, when mixed together in an in vitro microtubule gliding assay, both motors moved at an intermediate speed, suggesting that they work together rather than independently to pull cargo along microtubules.
Mathematical modeling suggested that, to produce an overall intermediate speed, the motors either take turns pulling a cargo or work in a concerted but competitive fashion, with OSM-3 hurrying kinesin-II along and kinesin-II holding OSM-3 back.
To distinguish between these possibilities, Pan et al. separated the two motors by fragmenting their shared IFT cargo particles. They then saw that one portion move at the rate of OSM-3 and the other at the rate of kinesin-II. Thus, the motors work together, tempering each others' speed along the cilium. At the very distal tip of the cilium, where IFT is faster, OSM-3 is known to work alone.
Meanwhile, Imanishi et al. found that OSM-3, like kinesin-1, folds over on itself such that the head and tail of the protein are in contact. Disruption of this interaction relieved autoinhibition of the motor domain and allowed for processive movement, as also occurs when cargo is bound.
Outside of their motor domains, kinesin-1 and OSM-3 are not homologous. The researchers thus hypothesize that this type of nonsequence-specific intramolecular fold might be a common mechanism by which motors remain still until they find a reason to move—the appearance of cargo. 
Rabiya S. Tuma
rabiya{at}nasw.org
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