Animal 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!
Bacteria 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.
This 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!