Oscillator

Bacterial “Organelles”

cell organelles.pngAnimal cells are made up of many smaller membrane-bound compartments called organelles that perform highly specialized functions necessary for life. Incredibly, several of these organelles have been shown to be evolutionarily related to free-living bacteria, captured and incorporated inside a larger cell billions of years ago in a complex mutually beneficial relationship, known as endosymbiosis (a partnership between two species where one of the species is inside the other). The mitochondria that power our cells, generating energy by breaking down sugars are in fact relatives of regular old bacteria, even having retained some bacterial genes that they replicate on their own. In some (very very rare) cases, the bacteria-ness of your own mitochondria can actually be bad for you, activating an aggressive immune response after a serious trauma releases the contents of lots of mitochondria into the bloodstream!

i-e514a6c2edc41d6acdc554fd5392dd83-494px-Average_prokaryote_cell-_en.svg-thumb-250x203-42353.pngBacteria themselves are much smaller and much simpler cells, performing many of the same cellular functions without the spatial organization of organelles, all the cell’s enzymes and genetic material are instead floating freely in the cell. Some types of bacteria, however, do have compartments that have specialized functions, separating certain enzymatic activities from the rest of the cytoplasm. These compartments are surrounded by a protein shell, not a membrane, so they aren’t technically organelles, but they’re still pretty amazing.

Many species of photosynthetic bacteria (the precursor to the chloroplast organelle that makes plants photosynthetic) have protein-bound compartments that separate the carbon dioxide capturing machinery from the rest of the cell, called carboxysomes. The enzyme that captures the carbon dioxide and turns its carbon atoms into chemical forms that the cell can use is called RuBisCO and it kind of sucks. Every carbon atom in a photosynthetic cell comes from this enzyme’s function, but the reaction happens much much slower than most enzymatic reactions and if there’s too much oxygen around it doesn’t happen at all. In the carboxysome, RuBisCO is so tightly packed that oxygen can barely fit through the cracks, and the high concentration of the enzyme can help to overcome some of the inefficiency of the reaction. The carboxysome protein shell is made up of interconnecting proteins shaped like hexagons and pentagons that link together to form a complex polygon, kind of like a soccer ball.

i-8a344bfbfd6fb2dc0fb0509df533fb4c-800px-Carboxysome-thumb-510x194-42357.pngi-03255abd191187ea01447beb19d1af8f-512px-Phage-thumb-200x234-42364.jpgThis soccer-ball protein geometry is also used by some species of viruses to form a protective shell. Some carboxysomes have a geometry that is more complex than the standard icosahedral viral capsid, indicating that carboxysomes may have not actually originated from the same common ancestor as the virus, but that their similarity is the result of convergent evolution. However, the question of whether carboxysomes are the result of endosymbiosis between bacteria and viruses that have evolved over billions of years is still open. This matryoshka doll concept of evolution and cellular substructure, with animal cells housing bacteria housing viruses is wonderful and fascinating, pointing to a rich diversity of interspecies cooperation in nature.

There’s still a lot that we don’t know about carboxysomes, and there’s a lot of active research going on in my lab about how carboxysomes are formed and controlled inside of photosynthetic cells, with the goal of being able to engineer special protein substructures inside any bacterial cell. Stay tuned for more on that shortly!

Comments

  1. #1 Joe
    March 10, 2010

    Also there are membrane-bound organelles in bacteria, e.g., magnetosomes.
    These protein-shell ones are also cool.

  2. #2 Christina
    March 10, 2010

    Oh yeah! There’s someone in my lab working on magnetosomes too (it’s a big lab)! Maybe something for another future blog post :)

  3. #3 frog
    March 10, 2010

    all the cell’s enzymes and genetic material are instead floating freely in the cell.

    They’re not separated into membrane-bound organelles (in general), but that doesn’t mean that they’re “floating freely”. You have invaginations with proteins trapped in those regions for starters; but even more generally, everything in cytoplasm is packed tightly — and trapped by its interactions with other proteins, and the dimensionality of its co-operative proteins (1, 2, or 3).

    It’s very, very organized — not “floating freely”, just not as clearly divided as in eukaryotes. There’s a big world between membrane-bound organelles and primitive bags of cytoplasm which supposedly existed 4 billion years ago.

  4. #4 Lucas
    March 10, 2010

    Great teaser at the end, leaving us to wonder what kind of awesome hacks are possible by employing these carboxysomes for our wishes ;).
    I’m still struggling with something though: if oxygen cannot fit through the cracks, how would the carbon dioxide (which is bigger) get in? The structure of the monomers seem to be too simple for elaborate mechanisms like the selectivity filters in ion channels.. Are funky nano-physics to blame?

  5. #5 frog
    March 10, 2010

    all the cell’s enzymes and genetic material are instead floating freely in the cell.

    They’re not separated into membrane-bound organelles (in general), but that doesn’t mean that they’re “floating freely”. You have invaginations with proteins trapped in those regions for starters; but even more generally, everything in cytoplasm is packed tightly — and trapped by its interactions with other proteins, and the dimensionality of its co-operative proteins (1, 2, or 3).

    It’s very, very organized — not “floating freely”, just not as clearly divided as in eukaryotes. There’s a big world between membrane-bound organelles and primitive bags of cytoplasm which supposedly existed 4 billion years ago.

  6. #6 Christina
    March 10, 2010

    @frog you’re right, it’s definitely an oversimplification to go from “no organelles” to “freely floating.” Not to give it away but some of the caeboxysome work in my lab is about how they are spatially regulated by the cytoskeleton, and this kind of control happens even without carboxysomes.

    @Lucas I’m afraid I oversplifies this part too, it’s less that oxygen can’t get in and more that you’re hugely concentrating carbon dioxide because the carboxysomes also associate with carbonic anhydrase.

    I was having a hard time figuring out how much detail I needed to include in this post, seems like I underestimated! Thanks for pointing out these important issues!

  7. #7 Lucas
    March 11, 2010

    Thanks for the explanation.
    The balance between simplicity and detail can be tricky, but I like that the post is so accessible. It gave me a chance to think for myself and ask questions!

The site is undergoing maintenance presently. Commenting has been disabled. Please check back later!