Last week in Part 3, we discussed the cytoskeleton of cells and how it is involved in cell shape, support and the transportation of everything within the cell. Today we are going to look at a couple of groups of motor proteins that are involved in moving things along the cytoskeleton.
With the development of video-enhanced light microscopy, researchers were able to study microtubules while in a solution of cell extracts. They were studying the movements of organelles and other particles along the microtubules when they discovered two groups of microtubule-dependent motor proteins known as cytoplasmic dyneins and kinesins.
The cytoplasmic dyneins function in the processes of mitosis and organelle movement. The kinesins function in the processes mitosis, meiosis, organelle movement and in the transport of synaptic vesicles along nerve axons.
Both of these groups of motor proteins contain two heavy chains and several light chains. Each heavy chain is comprised of an ATP binding head. When the heavy chain ATP heads are combined, they form an ATPase motor that binds to the microtubule. The light chains appear like tails of rod-like structures that have a binding affinity to various cell parts. Each specific light chain only binds to a specific cell organelle or particle of which is then transported along the microtubule.
The researchers introduced microscopic polystyrene beads coated with the different motor proteins into the solutions containing microtubules to see how they moved along them. Those beads coated with plain cytoplasm extracts moved in both directions along the microtubules. Axon sourced kinesin coated beads all moved towards the positive end and the dynein coated beads all moved towards the negative end of the microtubules. Further studies showed that non-axon sourced kinesins can move in both directions.
If you recall in last weeks article, the positive and negative charges of the microtubule ends can reverse polarity. This reversal of polarity not only lengthens or shortens the microtubule itself, but it also changes the direction that the cytoplasmic dynein and kinesin motor proteins travel along it.
The two large heavy chains act somewhat like feet in that they move along the microtubule, carrying their specific load from one location to another within the cell. Imagine if you can a pathway with a number of people each carrying a specific load on their head, moving in both directions. One group of people only moves in one direction while the other group of people only move in the opposite direction. Each person will pick up their object and carry it so far down the path and then let hand it over to another person on another path to carry it elsewhere. Then consider the fact that the pathway is moving like a conveyor belt and depending upon the circumstances, can change the direction it is moving. Thus, when the pathway changes direction, the people also change the direction they move.
Scientists have yet to figure out how all of this movement and reversal of movement and transportation is coordinated to function so smoothly throughout the entire cell on a constant basis. In our world, it would require a super computer and a number of programmers to write the programs, a number of technicians to operate the super computer, a number of mechanical engineers to design the conveyor belt system, a number of construction people to not only build the initial system, but to constantly add on and remove parts as the pathway grows and shrinks, and then all of the labor force to actually carry the loads from one place to another.
Can you believe that this complex molecular transport system, which is far more efficient than any human factory ever created, just happened to have randomly appeared? How can evolution explain walking molecules that carry things from one place in a cell to another and then have the pathway change directions to bring them back? Or how is it call coordinated and controlled?
Only the infinite wisdom of our Creator God could have designed of all of this microscopic detail that is found within our so-called simple cells. Obviously, smaller is not simpler.