Structure-function studies of the cyanobacterial carboxysome and its role in the carbon concentration mechanism

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Cyanobacteria are photosynthetic bacteria, and function ecologically as microscopic plants that remove carbon dioxide from the atmosphere and incorporate it into biological molecules. Despite their "primitive" nature and tiny size, they occur in almost all conceivable habitats, especially in the open ocean where their astronomical numbers form the base of the food chain. Cyanobacteria are responsible for a large fraction of the "credit" side of the planet's overall carbon budget, making them key players in phenomena such as global warming. One of the specializations that make cyanobacteria especially efficient at carbon fixation, is their ability to organize the enzyme RuBisCO (responsible for the critical step where carbon dioxide is captured) into structures called carboxysomes. Carboxysomes are, at 100 to 200 nm in diameter, enormous by cellular standards, and are comprised mostly of RuBisCO, surrounded by a thin protein shell with virus-like, icosahedral geometry. This shell appears to work, at least in part, as a barrier to carbon dioxide escape allowing RuBisCO (one of the slowest and least efficient enzymes known) to have a chance to capture it as a sugar. The shell is built from a handful of small proteins that behave as regular hexagonal tiles, capable of assembling into sheets of arbitrary size. It is of fundamental importance to understand the detailed principles that make this aspect of photosynthesis in cyanobacteria so efficient. My lab will investigate the architectural principles governing carboxysomal structure by making all of the components separately, and then investigating how they assemble. We will also be using X-ray crystallography to build atomic-scale models of the individual components, and studying how they interact to build larger sub-assemblies.