Progress 10/01/04 to 09/30/05
Outputs
We have completed the first crystal structures of the carboxysome shell subunits from the cyanobacterium Syn. 6803. This was the principal goal for the first year. The three dimensional atomic structures, published in Science in August 2005, provide the first detailed view of how the carboxysome is organized and how it might function to enhance CO2 fixation. The protein subunits are organized into hexagonal units. These hexagonal units are organized into molecular sheets. Narrow pores through the middle of the hexamers appear to allow transport of substrates and products into and out of the carboxysome, which is composed of some 10,000 protein subunits forming a closed shell. Preliminary progress has also been made towards understanding certain other key proteins, such as CcmL and RbcX, which also play important roles in carboxysome and RuBisCO function. Both of those proteins have been crystallized. The CcmL protein appears to form pentameric structures. If this holds
true, it would provide a possible explanation for how the carboxysome shell bends or closes up; insertion of pentagonal elements into a hexagonal sheet causes curvature. The RbcX protein acts as a chaperone to help the RuBisCO enzyme fold up properly after it is synthesized. The structure of RbcX, which will be finalized and published in the second year of the grant, will be the first look at this novel chaperone.
Impacts
Our ongoing studies are providing insight into how the process of CO2 fixation is greatly enhanced in efficiency in many microorganisms, by encapsulating the CO2-fixing enzymes inside the carboxysome shell. The structural studies are providing information about how the carboxysome functions in atomic level detail. This will ultimately open up possibilities for engineering novel structures and compartments with potential argricultural and environmental applications.
Publications
- Kerfeld, C.A., Sawaya, M.R., Tanaka, S., Nguyen, C.V., Phillips, M., Beeby, M., Yeates, T.O. 2005. Protein structures forming the shell of primitive bacterial organelles. Science 309, 936-8.
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