Kinesin: Fast, Efficient, Essential, and Mysterious
Perhaps you've seen the video from Discovery Institute of the miniature walking machine known as kinesin. This microscopic marvel gets the cell's protein products distributed to their final destinations, among other things.
This movement is extremely efficient, and by itself is a wonder.
But there's another wonder. A single kinesin can pull its cargo at up to 800 nanometers per second along its microtubule highway, depending on its load and the amount of ATP available. That's almost a micron per second. To give you a sense of scale, one bacterium is about a micron in length, and a typical animal cell is roughly 10 microns. That means that, under optimal conditions, and if kinesin is unobstructed, it could travel the length of an average cell in about 12.5 seconds. If it partners with other kinesin molecules, it can move even faster.
This is a good thing for our neurons. A neuron has its nucleus and its biosynthetic apparatus in the cell body. This is where proteins and organelles that the neuron needs are made. But these proteins and organelles are used out at the very tip of a long thin process called an axon, which extends from the body of the neuron all the way to the place it sends its signals.
In the brain there are billions of neurons with axons connecting one region to another (the different kinds are shown above). In the peripheral nervous system, axonal processes can reach up to several feet in length. In the case of nerves running to your toes, axons must reach all the way from your lumbar spine. So any motor delivering essential items needs to be fast.
How do the proteins and organelles get from where they are made to where they are used? And how do things that need to be recycled return to the cell body? It's by means of kinesin (for outward bound travel) and another motor protein called dynein (for inward bound travel). Remarkably, dynein and kinesin cooperate, rather than compete -- otherwise there would be a constant tug of war in the cell. They "know" where packages are meant to go, and which motor protein should do the job. How that happens is a whole other story.
This isn't just abstract stuff. Many neurodegenerative diseases have been linked to mutations of kinesin, including Alzheimer's disease. So this little workhorse of the cell is not only a marvel, but essential.
If you'd like to see a short time-lapse video of axonal transport in hippocampal neurons, go here.
Finally, in order for kinesin to work, we need the motor protein itself, the microtubule highway it walks along, and some mysterious means of connecting and directing where its cargos get carried. This system appears to be essential for a variety of eukaryotic cell functions. At least 11 kinesin families appear to have existed in the putative last common ancestor for all eukaryotes. Yet evidence for kinesin motor proteins in bacteria is sketchy at best. So we have yet another complex system necessary for eukaryotic cellular function that seems to have appeared out of nowhere.