Posted on 01/14/2002 3:01:36 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.
References
So, why is it that an infinitely more complicated pattern and structure in biology must have happened by chance?
They're basically looking for a pattern that's not already seen in nature - isn't that right, RA?
(And there's the rub re: proteins. Protein structures & sequences are seen in nature!)
So, why is it that an infinitely more complicated pattern and structure in biology must have happened by chance?
Actually, what they're looking for is not so much complexity as regularity and patterns. The most complex signal there is is random noise. The heavens are filled with random noise, but no one attributes its complexity to intelligence or design.
Evolution, while claiming to be intellectual and logical, seems very illogical to me.
OK, let's see your logical analysis.
That might be an interesting application. Maybe some researchers will try it in the near future.
It would be interesting to see this software applied to this sort of research. A decade ago, protein folding was a hot research topic, and they were trying to work out approaches to it. I haven't followed the research at all.
For SETI research, we are looking for an extremely narrowband CW signal. This alone signifies a non-natural origin.
Not so much a pattern, but a type of signal not found in nature; narrowband CW. :)
IIRC, you recently explained the reasoning behind this. Isn't it both because narrowband CW is not found from known natural sources, and because it is found from known intelligent sources (human-made radio stations)?
You are most correct! :)
Based on what we actually know at this point, it appears we are alone. It's all ours.
Sagan has passed on and taken his purple-hazy visions with him. The peace and quiet we now experience allows us to look around with fresh eyes. We see no evidence whatsoever of extraterrestrial civilizations. None. Zero. Nada.
Works the other way as well. To say there must be ET is just as baseless.
Not talking odds, not talking feelings. There is no evidence of ET. Not the same as saying there is no ET, just saying there is no evidence of ET. Not like saying there must be ET when there is no evidence of ET.
No, to say right NOW that there are no other intelligent civilizations out there besides ourselves, is just arrogance talking, not scientific proof. ...... I am certain that there are others out there, it's just a matter of time.
Beautiful! I don't even have to compose my own answer! Let me give it a shot: Soon, we shall know for sure if there is a God out there, but to say now, that He does not exist, is speaking without any facts.
No, to say right NOW that there is no God out there is just arrogance talking...I am certain that there is a God out there, and it's just a matter of time. Many of us know now that He lives, but it is just a matter of time (death) before everyone knows.
God is within and without. This is not a belief, I know this. So do you.
But . . .
We're talking ET here.
So we can say now that two proteins must be related because they share a similar function in one domain and they have a whopping 15% homology!
I've done a little study of my own. I found that a yugo shares many of the same functions as a Honda Civic. They both have four wheels, a motor, a steering wheel, a gas pedal, a brake, headlights, etc. In fact, I'm sure that they share more than a 15% homology.
I have concluded that long ago, a steel mill blew up and over billions of years, yugos evolved. After another billion years, Honda Civics have evolved from yugos. If we carry this evolution even farther, we get a Hummer!
All organisms share some sort of homology. If one function is to be performed (like the cleaving of GTP), it would make sense that God used a very similar-looking molecule to accomplish the task.
The development of cellular machinery is not so much an account of complicated machinery appearing out of nowhere, but more of existing bits and pieces being fitted together to solve different problems.
While this article discusses protein similarities, the question still remains: what is the MECHANISM of the change? How long did it take? Is anyone in control of it? I believe that God has taken these bits and pieces and put them together to create distinct organisms. These organisms all share the same building blocks and need to perform some of the same functions, so of course some of the molecules will look similar. This is analagous to the car example. A Honda Civic shares a lot of the same parts with a Honda Accord. It does not mean one "evolved" from the other, it means they had the same DESIGNER!
Think about it. If God didn't design this world and its inhabitants so that they would be somewhat similar, it wouldn't function the way it does. Just because things have similarities does not mean that they were not designed that way on purpose. If God didn't use the same 20 amino acids to make up all the proteins in this world, we wouldn't be able to eat anything.
The design of this world is so intricate and ingenious that I am in awe. I can't reconcile the word "accident" with the things I see happening in the human body, let alone the interworkings of the world with its inhabitants and the inhabitants with each other.
Actually, both would work almost equally well as a paradigm to discuss and categorize biological data.
Your earlier comments are quite astute ("The point is, real scientists, doing real research, don't care what you believe.").
Nor do they care what a bunch of religious zealots who take biology and evolution as a substitue religion believe.
Yes and no. When it comes to proteins that are as separated as those from bacteria and mammalian it can be difficult.
How do you quantify similarity of shape and structure?
I am not sure if this is a rhetorical question or not. If so what is your point? If not, I will address it.
No, he's right - you sequence the amino acids and count the matches. I've done runs on proteins from organisms as different as E. coli and rhesus monkeys. This does not, however, measure secondary and tertiary structure, which is what you really need if you're to determing biological activities, but the amino count won't get you there by itself.
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