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Why longer microtubules get disassembled faster than shorter ones

Kinesin-8 is a protein that controls the size of the mitotic spindle, the apparatus that separates the chromosomes to the daughter cells prior to cell division. The spindle is made up of an array of long filamentous proteins called microtubules and kinesin-8 controls spindle length by disassembling the microtubules that are too long.

Using single-molecule techniques in which the activity of individual molecules can be visualized under the microscope, we worked out how this little protein, of dimension only a few tens of nanometers, can distinguish between a microtubule that is 10 microns long faster from one that is  5 microns long, even though both are 100-times larger then a single kinesin-8 (Varga et al. Cell 2009). The longer the microtubule, the more kinesin-8 land on it; the kinesin-8s then walk to the end of the microtubule; and after reaching the end each motor removes just a single protein subunit from the microtubule.

Thus the depolymerization rate is proportional to the flux of motor to the end, which is proportional to the number of motors that land on the microtubule, which is proportional to the length of the microtubule. In this way longer microtubules get disassembled faster than shorter ones.

 

original publication:
Vladimir Varga, Cecile Leduc, Volker Bormuth, Stefan Diez, Jonathon Howard
Kinesin-8 Motors Act Cooperatively to Mediate Length-Dependent Microtubule Depolymerization
Cell, Vol. 138, no. 6, pp. 1174-1183, 2009

Image Caption: Single kinesin-8 molecules (green) moving to the end of a microtubule (red) and shortening it. The length of the microtubule is 4 microns long. Each horizontal line corresponds to the track 10 s later (total time 2 1/2 minutes)