New & Noteworthy
July 19, 2012
Traffic jams are a way of life in Lagos, Kuala Lampur, Berlin, Los Angeles, or pretty much anywhere with too few roads and too many cars. If only people would learn a thing or two from cells, then traffic jams might be a thing of the past. Which is surprising, considering how much traffic there is inside of a cell.
The inside of a cell is way more crowded than any human city. Proteins called kinesins are delivering cargo to where it needs to go by hurtling down microtubule highways through a crowded mass of macromolecules, membranes, and organelles. This all happens in a frenzy of activity at breakneck speed.
And yet there are not a lot of cellular traffic jams. We surmise this because we know that when there are lots of cellular traffic problems, diseases like ALS can result. So cells must have some way to prevent traffic problems.
In an attempt to figure out how cells prevent traffic problems, Leduc and coworkers first set out to find out how they can happen in the first place. They did this by setting up an in vitro system of microtubule highways and the purified yeast kinesin 8 protein, Kip3p. Using this system they figured out that traffic jams can happen when too many kinesins are on the microtubule at once (density-induced jams) or when they don’t get off the end of the microtubule fast enough (bottleneck-induced jams). These are equivalent to too many cars at rush hour, or to the obstacles of accidents or highway construction.
From these data they hypothesize that kinesins have evolved in ways that keep their density down and prevent bottlenecks. They suggest that bottlenecks are prevented by rapid dissociation from the ends of the microtubules and that density is kept down by having the kinesins be not too processive (i.e., not keep going and going and going…). So kinesins avoid traffic congestion by quickly getting on and off the highway both along its length and at the end.
They concluded all of this from their elegant “highway in a tube” assay. This system is ideally suited for studying how traffic jams might happen because it is relatively simple to change parameters like end dissociation rate and processivity by tweaking salt and/or protein concentrations. And it is very cool because traffic jams can be watched in real time. A cellular traffic helicopter report!
The basic idea was to generate the microtubule pathway in the presence of a slow hydrolyzing GTP analog and taxol such that the microtubules were not easily depolymerized by Kip3p. They then added various amounts of mCherry labeled Kip3p to a small amount of EGFP-labeled Kip3p and watched to see when the EGFP-labeled Kip3p slowed down or got stuck.
They saw that high concentrations of Kip3p led to pileups at the end of the microtubule. These pileups disappeared when the dissociation rate of Kip3p was increased by using higher salt concentrations. They also saw that at high concentrations, the Kip3p molecules slowed down as they got in the way of each other and that decreasing processivity eliminated this problem.
So the traffic situation in a cell and a city are remarkably similar. In both, keeping the numbers of cars or kinesins down and making sure they can quickly get around obstacles prevents traffic problems. Maybe civil engineers need to start looking at the cell for ideas about ways to deal with the daily grind of our commutes.
Ron Vale (UCSF) Part 1: Introduction to Motor Proteins