The size and position of mitotic spindles is determined by the lengths of their constituent microtubules. Regulation of microtubule length requires feedback to set the balance between growth and shrinkage. Whereas negative feedback mechanisms for microtubule length control, based on depolymerizing kinesins and severing proteins, have been studied extensively, positive feedback mechanisms are not known. Here, we report that the budding yeast kinesin Kip2 is a microtubule polymerase and catastrophe inhibitor in vitro that uses its processive motor activity as part of a feedback loop to further promote microtubule growth. Positive feedback arises because longer microtubules bind more motors, which walk to the ends where they reinforce growth and inhibit catastrophe. We propose that positive feedback, common in biochemical pathways to switch between signaling states, can also be used in a mechanical signaling pathway to switch between structural states, in this case between short and long polymers.
Cells contain an extensive network of long filaments called microtubules, which are made of a protein called tubulin and are essential for a wide variety of processes such as enabling cells to divide and move. Microtubules also serve as tracks along which motor proteins transport molecules from one part of the cell to another.
In yeast cells, a motor protein called Kip2 transports its cargo to one end (known as the “plus end”) of the microtubules. This is the fastest-growing end of the microtubule, although it frequently switches between phases of growth and shrinkage. Previous research has shown that cells containing reduced amounts of Kip2 have much shorter filaments than normal cells. One suggested explanation of these results is that Kip2 controls microtubule growth by transporting a protein that regulates filament length to the plus end of the microtubule.
However, by adding purified Kip2 to microtubules Hibbel et al. have now shown that Kip2 on its own can increase the length of filaments. The microtubules also switch much less frequently between growth and shrinkage in the presence of Kip2. This is because Kip2 moves along filaments at a speed that is greater than the rate at which microtubules grow. This sets up a positive feedback loop that causes microtubule growth to accelerate, as more copies of Kip2 can bind to longer microtubules. Each of these motor proteins can then move to the plus end of the microtubule and help the filament to grow even longer.
Several challenges remain. What is the molecular mechanism by which Kip2 increases the rate at which tubulin subunits are added to the microtubule end: does Kip2 carry tubulin dimers to the end in a shuttle-type mechanism? How does Kip2 prevent other proteins from promoting shrinkage? What stops microtubules from growing when they reach the end of the cell?