Molecular characterisation of an ADP-dependent regulatory protein

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Conversion of carbon dioxide to sugars by plants is essential to life on earth. In the majority of plants this process competes with non-productive reaction with oxygen. One group of plants, the C4 plants, have developed a strategy to avoid this: this strategy allows the plants to survive in hot, dry and arid conditions which would not otherwise be viable for plant growth. These plants include the common crop plants, maize and sugarcane. Research is presently underway to determine if this pathway can be engineered into other plants to generate hardier crop plants and this research will feed into this objective. Our research addresses how one part of this alternative strategy of making sugars is controlled. In particular, we are interested in how an unusual protein acts to switch off the pathway in the dark. The protein, pyruvate,orthophosphate dikinase (PPDK) regulatory protein, or PDRP for short, inactivates another protein, PPDK, thereby preventing the plant from making the compound phosphoenolpyruvate from pyruvate. This prevents the plant from 'fixing' carbon dioxide into sugars in the dark (when there is no light to provide energy via photosynthesis). The regulatory protein, PDRP, can perform this vital function because it is dependent upon the presence of its substrate, ADP, for activity. This accumulates in the dark as a direct result of photosynthesis shutting down. We are interested in the details of the way in which the regulatory protein uses this molecule to inactivate PPDK. We will develop new ways to study this process and make mimics of the target which will help us further unravel the precise manner in which it occurs. The target protein is inactivated because a phosphate group is transferred from the substrate ADP to the protein, thereby blocking the part of PPDK required to perform its normal function. It is worth noting that using this particular molecule, adenine diphosphate (ADP), to perform this reaction is highly unusual, normally adenine triphosphate (ATP) is used instead.