In the lab we were discussing whether prokaryotes have true membrane bound organelles. By membrane bound organelles I mean that the membrane surrounding the organelle is not contiguous with the cell's delimiting membrane. Magnetosomes and other such organelles are though to be invaginations of the delimiting membrane, but this idea is controversial. Why should we care? Well we are not concerned about semantics (i.e. what is an organelle) but rather the implications of organellar biogenesis. You see once you have organelles composed of non-contiguous membranes you need to invoke a way to pinch and fuse membranes. As far as I know there hasn't been any description of membrane fusion or membrane fission in prokaryotes (except during cytokinesis and spore formation).
Then there's the case of cells that have membranes surrounding their nucleiods. Just today, a good friend sent me a paper from PNAS that describes such a critter. It's name is Gemmata obscuriglobus and it's a gram negative prokaryote. Like other gram negative proks, this organism is surrounded by inner and outer peripheral membranes. But the weird thing about this bug is that it has a double membrane around its DNA mass, just like the nuclear envelope present in eukaryotes.
Now from pictures in the paper (reproduced here) it appears that the outer "nucleiod" membrane is contiguous with one of the peripheral membranes.
A couple of questions.
1) Are the inner and outer membranes separate or are they contiguous. Another way of phrasing this question is whether the membrane COMPLETLY surrounds the nuclear mass or whether the nucleiod sheet incompletely surrounds the DNA. In this gap the inner and outer membranes would be connected. From the paper:
Although freeze-fracture micrographs suggest that the nuclear body is an entirely rather than partially bounded structure, definitive evidence of this will require extensive serial sectioning.
2) If the membrane does surround the DNA mass, how are particles (such as mRNA transcripts, ribosomes and newly synthesized proteins) transported across those membranes?
3) Is there a separation of functional domains? In eukaryotes, RNA synthesis and processing occurs within the nucleus, while mRNA translation happens in the cytoplasm. The nuclear envelope thus separates where you make and where you utilize mRNA. How about in Gemmata obscuriglobus? From the micrographs there are clearly ribosomes in the nucleiod and some of these are even bound to the membrane. So it would appear that there is translation in the nucleiod. Is anything else separated?
4) Any other functions for this extra memebrane? Perhaps this creature wants to increase the plasma membrane area. Or perhaps a fraction of the membrane (the nucleiod sheet) has to be in close proximity to the membrane while the rest of the membrane has to be physically separated from the DNA.
I'll have to read up on this (if I have the time). If anyone knows more about this creature leave a little note.
- Log in to post comments
This is neat. Not only do most prokaryotes have limited "organelle" formation, but they typically also have a very limited biogenesis pathway for lipids/phospholipids. It's likely that if G. obscuriglobus has these orgenelle like moieties (or at least, a nucleus like moiety), their lipid biogenesis pathways are probably going to be substantially different from say your friendly neighborhood E. coli. That might be something really, really interesting to study. Anyway, for starters, it should be possible to check the lipid composition of the outer membrane and the "nucleoid" membrane.....and that might help answer if the two are contiguous, or distinct.
The separation between nuclear membrane and other organelles isn't always that simple. Our group have been working on a novel ERAD E3 ligase for a while now and have noticed that a lot of endoplasmic reticulum related proteins are present on both the ER and the nuclear membrane. Perhaps something similar is the case with this organism, there maybe not be a simple nuclear compartment but an interconnected nuclear ER system.
MartinC,
Are you referring to Doa10?
http://scienceblogs.com/transcript/2006/11/doa10_a_paper_with_lots_of_g…
But don't forget that since the nuclear membrane is an extension of the ER lots of proteins will naturally diffuse from one compartment to the other.
In contrast the ER and nuclear envelope are distinct from Golgi, endosomes, plasma membrane and mitos. Because of this physical (or topological) separation, euks need machines to pinch off membranes from one compartment and fuse these membranes with the other compartments to move proteins and lipids around (although some ABC transporters are thought to directly pump lipids between two apposing membranes). In the case of this bacterium it looks like all the membranes are interconnected - that is different from euks where the plasma membrane is separate from the ER-nuclear envelope.
In a related note, someone in the lab pointed out that this critter looks almost like a sporulating prok where the outer mother cell hasn't burst. Still this would imply a physical separation between the nucleoid mass and the rest of the cytoplasm.
Alex, unfortunately we are not so lucky to be working with yeast, its a human E3 called RFP2 we are characterizing - its a RBCC protein with no counterparts in yeast so initially we didn't have a clue as to its function.
I heard a talk about this group of bacteria (they are called the plantomycetes and Gemmata is one of them) where they mentioned that transcription and translation are decoupled. I can't find a good reference but it seems that at least RNA is found throughout the cell whereas the DNA is only within the "nucleus."
See a nice review here.
What's interesting about the whole group is that many variations of intracellular membrane structures are produced. One novel Planctomycete group are the so called anammox bacteria which carryout the anaerobic oxidation of ammonium within specialized organelles called anammoxosomes composed of specialized ladderane lipids and almost.
All about anammox.
Just stumbled on this page and to answer the questions you posed... Question 1 is as far as I know correct, so far we see it as a totally sealed membrane.
I can personally answer your second question since it was the focus of my own studies. Gemmata has nuclear pore complex like structures similar to eukaryotes on this nuclear membrane and it seems to have a few different sizes as well. In my studies I've classed them into 4 sizes and I've had a look at Gemmata obscuriglobus (the one you have pictured) and also a related Gemmata sp. Wa1-1.
Your 3rd question, all we know is that the ribosomes are scattered throughout the whole cell in the riboplasm (all the dark little spots you see in the cell are those) and I do have pictures which you can see them line up along the nuclear membrane. As to function and such - has yet to be studied.
There's still much to be done on this organism and as far as I know there are only a few labs in the world taking a look at this organism. I did my study in the Fuerst lab.