Even though I’m the CEO of a tiny department of a very large organization, it’s time to thank those unsung divisions that make it all possible. It’s been a very good year. Thanks in part to our work, the boss is a lot more adept at using the pedal when he plays the piano.
First: thanks to the guys in shipping and receiving. Kinesin moves the stuff out and Dynein brings it back home. Think of how far they have to go. The head office sits in area 4 of the cerebral cortex and K & D have to travel about 3 feet down to the motorneurons in the first sacral segment of the spinal cord controlling the gastrocnemius and soleus, so the boss can press the pedal on his piano when he wants. Like all good truckers, they travel on the highway. But instead of rolling they jump. The highway is pretty lumpy being made of 13 rows of tubulin dimers.
Now chemists are very detail oriented and think in terms of Angstroms (10^-10 meters) about the size of a hydrogen atom. As CEO and typical of cell biologists, I have to think in terms of the big picture, so I think in terms of nanoMeters (10^-9 meters). Each tubulin dimer is 80 nanoMeters long, and K & D essentially jump from one to the other in 80 nanoMeter steps. Now the boss is shrinking as he gets older, but my brothers working for players in the NBA have to go more than a meter to contract the gastrocnemius and soleus (among other muscles) to help their bosses jump. So split the distance and call the distance they have to go one Meter. How many jumps do Kinesin and Dynein have to make to get there? Just 10^9/80 — call it 10,000,000. The boys also have to jump from one microtubule to another, as the longest microtubule in our division is at most 100 microns (.1 milliMeter). So even in the best of cases they have to make at least 10,000 transfers between microtubules. It’s a miracle they get the job done at all.
To put this in perspective, consider a tractor trailer (not a truck — the part with the motor is the tractor, and the part pulled is the trailer — the distinction can be important, just like the difference between rifle and gun as anyone who’s been through basic training knows quite well). Say the trailer is 48 feet long, and let that be comparable to the 80 nanoMeters K and D have to jump. That’s 10,000,000 jumps of 48 feet or 90,909 miles. It’s amazing they get the job done.
Second: Thanks to probably the smallest member of the team. The electron. Its brain has to be tiny, yet it has mastered quantum mechanics because it knows how to tunnel through a potential barrier. In order to produce the fuel for K and D it has to tunnel some 20 Angstroms from the di-copper center (CuA) to heme a in cytochrome C oxidase (COX). Is the electron conscious? Who knows? I don’t tell it what to do. Now COX is just a part of one of our larger divisions, the power plant (the mitochondrion).
Third: The power plant. Amazing to think that it was once (a billion years or more ago) a free living bacterium. Somehow back in the mists of time one of our predecessors captured it. The power plant produces gas (ATP) for the motors to work. It’s really rather remarkable when you think of it. Instead of carrying a tank of ATP, kinesin and dynein literally swim in the stuff, picking it up from the surroundings as they move down the microtubule. Amazingly the entire division doesn’t burn up, but just uses the ATP when and where needed. No spontaneous combustion.
There are some other unsung divisions to talk about (I haven’t forgotten you ladies in the steno pool, and your incredible accuracy — 1 mistake per 100,000,000 letters [ Science vol. 328 pp. 636 – 639 ’10 ]). But that’s for next time.
To think that our organization arose by chance, working by finding a slightly better solution to problems it face boggles this CEO’s mind (but that’s the current faith — so good to see such faith in an increasingly secular world).