Structural studies of cyanobacterial carboxysomes

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Cyanobacteria are a type of bacteria that, like plants, can use energy derived from sunlight to turn carbon dioxide and water into sugar. They are especially good at doing this under circumstances where the trace elements needed for life are very scarce, and for this reason are the most important "plants" in the open ocean. This makes cyanobacteria important players in global warming, as the open ocean is the main environment in which there is a significant net removal of carbon dioxide from the atmosphere. One of the specialisations that cyanobacteria have adopted for their lifestyle is to collect the enzyme, called RuBisCO, that is responsible for incorporating carbon dioxide into sugars, into unusually large structures called carboxysomes. Intriguing, carboxysomes look superficially very much like viruses, which also are comprised of a protein shell surrounding a functional core. This shell appears to work as a barrier that impedes the escape of carbon dioxide thus allowing RuBisCO, one of the slowest and least efficient enzymes known, to have a chance to capture it permanently with a chemical reaction. My lab plans to investigate how this shell functions by dissecting it biochemically and examining what each piece looks like and does. We will make each shell protein in bacteria, and then, after purifying the protein and growing it in crystalline form, use a technique called X-ray crystallography to build an atom-by-atom model. The shell proteins will also be investigated as complexes that will allow us to model how larger portions of the carboxysome are put together. Close examination of the derived models should give numerous clues as to how each protein contributes, generating ideas which we can then follow up by further experimental work.