Posted on 01/14/2002 3:02:24 PM PST by Karl_Lembke
By Barry A. Palevitz
One of the enduring questions in biology is how eukaryotic cells arose from prokaryotic ancestors at least 2 billion years ago. Besides differences in genome organization, eukaryotic animals, plants, and fungi possess a much higher degree of cellular compartmentation in the form of membrane bound organelles than their distant bacterial and Archaean cousins. But how did such a plethora of cellular domains, each with a discrete role in metabolism, evolve?
To the extent that science proves anything, it answered the question for two eukaryotic organelles a long time ago. Mitochondria and chloroplasts evolved from endosymbiotic associations between an ancestral host cell and smaller prokaryotic partners. In the case of chloroplasts, the symbiont was a photosynthetic cyanobacterium; for mitochondria, most likely it was ana-proteobacterium.
The cytoplasm of eukaryotic cells is like chicken soup-it's chock full of organelles suspended like chunks of assorted vegetables and noodles in cytosolic broth. The broth also contains filaments of various dimensions that collectively comprise the cell's cytoskeleton. Like the bones of a large animal, the cytoskeleton provides a structural framework lending shape to cells and against which enzymatic 'muscles' work to elicit movement. That's how amoebae migrate, algae swim, stem cells divide, and cytoplasm streams relentlessly up, down, and across plant cells.
While the cytoskeleton is as much a hallmark of eukaryoticity as any mitochondrion or chloroplast, the origin of its filaments in deep time is more mysterious. Biologists assumed that genes for cytoskeletal proteins arose from prokaryotic precursors, but evidence in favor of the hypothesis was scarce, until recently.
Tubulin First on Stage
Microtubules comprise one component of the cytoskeleton responsible for a variety of movements including mitosis and meiosis. The 25 nm tubes consist of dimerica- and b-tubulin subunits that share about 40 percent sequence homology. Another form,y-tubulin, functions in microtubule formation.
But where did microtubules come from? It now appears that tubulins share a common ancestor with a protein called FtsZ, a key player in bacterial cell division.1 FtsZ is also present in plants, where it functions in chloroplast division,2 and a similar protein associates with mitochondria, at least in one alga.3 FtsZ polymerizes into filaments in the test tube in a process dependent on GTP. The same nucleotide is required for tubulin assembly into microtubules.1
Tubulins and FtsZ are clearly related, judging from similarities in three-dimensional structure. And although the proteins share only about 15 percent amino acid sequence identity overall, they're much more similar at the local level, particularly at the domain responsible for binding and cleaving GTP.4,5
Actin Into the Fold
Like the tubulins, actin-another essential component of the eukaryotic cytoskeleton-is a globular protein that binds nucleotide, in this case ATP. As actin monomers polymerize into 6-nm-wide microfilaments consisting of two helically wound protofilaments, the ATP, situated in a deep enzymatic cleft between two halves of the protein, hydrolyzes to ADP and inorganic phosphate.
It turns out that actin shares its ATPase domain with a family of proteins including hexokinase, the enzymatic kick starter of glycolysis, and several bacterial proteins. One of them is called MreB, a protein essential for generating or maintaining the rod shape of many bacteria. By examining structural similarities between eukaryotic actin and MreB from Thermotoga maritima, a research team at the Medical Research Council in Cambridge, England recently concluded that the two proteins are more closely related to each other than to other members of the family and undoubtedly share a common ancestor.6
The group showed that the three-dimensional shapes of actin and MreB are so similar they can be superimposed. The analogy with tubulin/FtsZ goes even further. Both proteins share considerable amino acid homology at several key sequences surrounding the ATP binding site, again situated deep in a cleft between two halves of the folded polypeptide chain.
Under the right conditions, MreB polymerizes into protofilaments that pair up lengthwise. The protein subunits are spaced about the same distance apart along the filaments as in polymeric actin, but MreB double filaments aren't nearly as helical.
The similarity between MreB and actin doesn't stop at structure and sequence. In a paper published earlier in 2001, a research group led by Jeffrey Errington at the University of Oxford, U.K. visualized MreB in the rod shaped cells of Bacillus subtilis using fluorescence and electron microscopy.7 MreB forms filamentous bands that encircle the cell in low helices, like reinforcing hoops. In an essay accompanying the Cambridge group's article, Duke University cell biologist Harold Erickson calculated that each band contains 10 protofilaments.8
When Errington's team genetically deprived cells of functional MreB, they became spherical. A search of genome databases showed that MreB is present in bacteria with nonspherical shapes, including rods. It's absent in spherical cocci. In other words, MreB has a cytoskeletal function. "I think it is quite convincing that MreB is the actin progenitor," says Erickson. "A key step, still unknown, going from bacteria to vertebrates is to develop a mechanism to make the double-helical actin filament from the single MreB protofilament structure."
More Acts to Follow
The story doesn't end with MreB; there's more to find out. Scientists want to know if MreB is also present in eukaryotes-associated with mitochondria and chloroplasts-as is FtsZ. According to Katherine Osteryoung, a plant biologist at Michigan State University in East Lansing who identified two FtsZ genes in the mustard plant Arabidopsis,2 "there's no obvious indication of MreB in plants that I've found or am aware of."
Actin normally functions along with the motor enzyme myosin to produce cellular motion, while microtubules utilize two other motor families called dynein and kinesin related proteins. Researchers now wonder whether MreB and FtsZ work in conjunction with bacterial motors. According to Erickson, "none have been turned up in genetic screens for cell division (or other activities), and none have been identified by sequence gazing. My bet is that kinesin and myosin evolved in eukaryotes, after the evolution of microtubules and eukaryotic actin filaments."
Still, Osteryoung is pleased with the latest results: "To someone interested in these issues, establishment of the prokaryotic origins of two major eukaryotic cytoskeletal proteins is enormously satisfying. I look forward to the day when evolutionary intermediates... from MreB to actin and FtsZ to tubulin, perhaps awaiting discovery in some obscure and primitive eukaryote, will more fully reveal the evolutionary steps by which key components of the eukaryotic cytoskeleton acquired their present-day structures and functions."
Barry A. Palevitz (palevitz@dogwood.botany.uga.edu) is a contributing editor for The Scientist.
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Anotrher example of your inability to retain simple fact or adhere to reality.
Look at the record. You will see your comment in response to mine.
And again, you avoid the subject like the plague. The topic is evolution and biology.
When you know nothing about it it is of course easier for you to avoid the issue.
Which is certainly not my fault.
All these posts end up not being about biology because the people participating don't know anything about it and are arguing theology, not biology.
No. Discussions of science are about science.
However, we aren't all cutting edge researchers attempting to study protein evolution from a perspective of a billion years but that doesn't mean we can't understand the meaning of such research or discuss it intelligently.
One would think so. In theory what you say is true.
But there is no data to support it.
Hasn't happened here yet except for very rare occurances, most likely anamolous.
I often misspell words with an o sound -- like anomalous as anomolous or phosphatase as phosphotase.
I also misspell when writing fast or in cramped settings. To me that is of little consequence.
How come you didn't answer the question about divergent vs convergent evolution? Did you not know? Have you ever taken a biology class or class on evolution?
The question arises, what do people believe and why?
You are free to take it any way you want.
You are free to be clear.
You come on and make an overbearing arrogant comment. So I call you on it.
I never understand why people are so bombastic and self-righteous on the subject to, and above and beyond, the point of rude arrogance, yet then demonstrate they have no or little knowledge or understanding of biology or evolution itself.
It makes me wonder where the strong fundamentalist belief in evolution comes from.
It can't be based on scientific examination because the knowledge and understanding of the field is not there.
My hypothesis is that it comes more from a religious belief, or what can be seen as an anti-religious belief. It seems in a lot of cases the basis for an adherence (dogmatic) is more that the person simply doesn't believe in creation myths or the theology associated with it and uses what is actually a scientific discipline or paradigm as a substitute creation myth.
OK.
The overall topic (microfilament evolution) is divergent evolution, but they discuss some convergent evolution (endosymbiotic theory) as well.
See lexcorp's answer in 18 ( here) to see an example of a very bad answer.
Why is it the people who know the least are the most dogmatic and overbearing and rude on the subject?
I believe 100% that God exists and that He is my Savior and Creator, however, I'm not 100% sure HOW He did what He did. We will never know for sure, because if we did, faith could not exist.
I try to avoid supporting one theory over another simply because there is not enough concrete evidence to support one over the other. There is plenty of evidence to support micro-evolution, but there is no evidence for macro-evolution. I believe that evolution occurs on a small scale (birds evolved from other birds, etc.), but I can't believe that one type of animal could evolve into another.
I think I'll wait until someone can prove beyond all doubt EXACTLY HOW we got here. Until then, I'll be following the path God has set before me. I would rather be on the safe side.
If I'm wrong, and God doesn't exist, when I die I will simply cease to exist. I can't be disappointed because I am non-existent. However, if I'm right, and God DOES exist, where does that leave all those who decided to turn their backs on Him? I'll let you guys think about that one.
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