Posted on 02/14/2003 2:57:23 PM PST by CalConservative
Protein Machine Does Gymnastics 02/13/2003
Scientists are bringing into sharper focus an amazing molecular motor named dynein. Dynein is responsible for much of the movement in the cell: the whiplike action of sperm tails, the sweeping action of cilia, and the ferrying of cargo down the microtubule intracellular railroad. The UK research team of Stan Burgess et al in the Feb. 13 issue of Nature imaged thousands of the little molecules (large by protein standards, with a molecular mass of over 500,000) that work something like railroad handcars. They have a ring-shaped hexagonal head of six AAA proteins to which is added a C-terminal domain. Emerging out of one side and in the same plane as the ring is a stalk, which has a structure on the end that attaches to the microtubule. Emerging out the other end is a stem that attaches to whatever cargo needs to be transported. The stem is fastened to the ring by a linker, that seems to act like a ratchet on a gear during the cycle.
How does it work? Though the details are still fuzzy, it appears that ATP hydrolysis occurs in the central ring, or head domain; i.e., energy is extracted from ATP, producing ADP and phosphate, putting the machine into a “cocked state”. This causes a conformational change (parts moving in relation to one another) resulting in a 34o rotation of the ring relative to the linker. The head domain rolls in relation to the stem, producing mechanical spring energy. Since the stalk and stem have some flexibility, they are “capable of storing elastic strain energy when the molecule develops force against a load.“ The movement pops out the ADP, and then the mechanism springs back to its cocked position; the so-called “power stroke.” Simultaneously, another ATP energy pellet enters the engine for the next cycle.
The angle between the stalk and stem thus changes back and forth in a rocking fashion, producing mechanical leverage, as the linker continually engages and disengages in the central ring, like a hook catch on a gear. As a result, the dynein motor slides down the microtubule monorail in 15-nanometer jumps. But that’s not all; there is two-way communication between the tip of the stalk and the engine in the head, and even more amazing regulatory mechanisms that tell the motor where and how fast to go.
In their News and Views write-up on the paper, entitled “Molecular motors: A magnficent machine,” Richard B. Vallee and Peter Höök consider this a remarkable gymnastic ability that is rarely seen in motor proteins. The dynein machines actually use the chemical energy stored in ATP to produce force and carry out work. They point out that this action occurs many times per second in the molecular motor.
It is truly exciting to see the inner workings of cellular processes, long hidden from view, coming to light. They are more wonderful than we could have imagined. Burgess et al do not speculate about the evolution of the dynein machines other than to note that parts of the ring are “conserved” (unevolved). Vallee and Höök, however, speculate that this highly-efficient system evolved from the family of AAA proteins which also have similar ring structures. But this is pure guesswork. The argument runs out of steam if you follow the logic. In the film Unlocking the Mystery of Life, Scott Minnich rebutted this so-called “co-option” hypothesis, the idea that molecular machines evolved by borrowing parts from other machines. First he pointed out that while some of the parts are similar to others, there are many that are unique, so where are you going to borrow them from? Paraphrasing, he said: “They have to explain each part as coming from some other part, until you are eventually borrowing parts from nothing. So you can only carry that argument so far.”
Minnich hastened to add something even more amazing, the assembly instructions for these machines. This is even more of a problem for evolution, because a host of genes and regulatory enzymes control the assembly of all the protein parts, and they of necessity are even more complex than the final product – just as the architect of a house is more complex than the house. Genes and other protein machines carry out the assembly in a precise sequence analogous to home construction. There is even remote signalling involving significant action at a distance. So not only is the machine itself irreducibly complex: the genetic instructions that assemble the machine are even more complex. Jonathan Wells adds, “What you have, then, is irreducible complexity all the way down.”
Your ability to read these words right now depends on dynein motors at work in every cell of your body, zipping down monorail tracks at high speed, carrying zip-coded cargo to precise docking points throughout the cellular factory. Do we really need an evolutionary story about how these systems came from nothing? Can’t we just study them with sheer wonder and fascination at their design?
Psalm 139:14 I praise you because I am fearfully and wonderfully made;
your works are wonderful,
I know that full well.
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