In its ATP-driven movement along a microtubule, Kinesin-1 (formerly conventional kinesin or KHC) can take over a hundred steps without dissociating. This behavior is termed processive motility, analogous to the processive polymerase activity of RNA and DNA polymerases, and it is thought to be important for long-distance transport of vesicles and organelles. Processivity of Kinesin-1 was demonstrated by Howard and coworkers (1), who showed that microtubules can move on single Kinesin-1 motors in the conventional gliding assay, and by Block and coworkers (2), who showed that glass beads with only one motor attached moved approximately 1 micron before detaching from a microtubule. In nice agreement with the motility experiments, Hackney (3) has demonstrated that Kinesin-1 hydrolyzes on the order of 100 ATP molecules per encounter with a microtuble.
The key feature of Kinesin-1 that enables processive motility is its two heads. Single-headed Kinesin-1 mutants do not show processive motility (4,5), and they hydrolyze few ATPs per diffusional encounter (6). The model used to account for the processivity of Kinesin-1 is the hand-over-hand model, in which the two heads bind alternately while walking towards the plus end of the microtubule. To prevent the motor from diffusing away from the microtubule, at least one head must be bound at all times, hence one head must not release until the second head binds to the next binding site. One prediction from the hand-over-hand model is that if one of the two heads is cut off, the remaining head will bind to microtubules but will release only very slowly.
Using a heterodimer in which one of the head domains was cut off, we showed that single-headed Kinesin-1 can still move micotubules as long as many motors are working together (4). However, when only one single-headed motor interacts with a microtubule, the motor binds and releases very slowly. This slow release is just what the hand-over-hand model predicts if the release of one head from the microtubule is contingent on the binding of the second head. We interpret the ability of multiple single-headed motors to move microtubules to the ability of the other motors to pull bound heads off of the microtubule, in effect acting as the second head from a distance. Others have also recently obtained evidence that one-headed Kinesin-1 derivatives move by a nonprocessive mechanism along microtubules (5). Understanding the structural and biochemical interactions between the two heads that leads to the processivity of Kinesin-1 is one of the active areas of investigation in the kinesin field.
Contributed by Will Hancock
|1.||Howard, J., A.J. Hudspeth and R.D. Vale. 1989. Nature 342, 154-158.|
|2.||Block, S.M., L.S.B. Goldstein and B.J. Schnapp. 1990. Nature 348, 348-352.|
|3.||Hackney, D.D. 1995. Nature 377, 448-450.|
|4.||Hancock, W.O. and J. Howard. 1998. J. Cell Biol. 140, 1395-1405.|
|5.||Young, E.C., H.K. Mahtani and J. Gelles. 1998. Biochemistry 37, 3467-3479.|
|6.||Jiang, W. and D.D. Hackney. 1997. J. Biol. Chem. 272, 5616-5621.|
View links to recent papers on processivity of the kinesin motor proteins from the PUBMED database.
Learn how to build a moveable beam optical trap by Steve Block.
Learn more about optical tweezers by Alex Knight and Justin Molloy.
Learn about Total Internal Reflection Fluorescence Microscopy (TIRFM) by Alex Knight and Justin Molloy.
Go to the Motility Page.