Post-translational regulation of starch synthesis in higher plants

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The primary objective of this proposal is to enhance our understanding of the regulation of starch synthesis in plants and identify the biochemical mechanisms which contribute to the organisation of starch structure. Starch is a naturally occurring glucose polymer and is used widely in food and non-food industries. Variation in functionality is derived from traits such as the ratio of amylose to amylopectin, glucan chain length, branching and granule size. The two PIs were the first to demonstrate that enzymes of starch synthesis aggregate to form heteromeric protein complexes, catalysed by reversible protein-phosphorylation. Recently we have made use of genetic approaches to demonstrate that mutations in starch synthase IIa and starch branching enzyme IIb have an impact beyond the loss of enzyme activity, modifying the nature of the heteromeric protein-complexes with consequences for the size, structure and properties of the starch granule. We propose that protein phosphorylation is critical in determining physical association among enzymes of starch synthesis, affecting the structure of the glucan polymer produced. To test this hypothesis we will: a) determine the interaction domains between associated proteins; b) identify phosphorylation sites and conditions in vivo under which the phosphorylation state of each enzyme is modified; c) characterise the protein kinase(s) and phosphatase(s) responsible for post-translational modification of enzymes of starch synthesis; d) mutate the phosphorylation sites and protein-interaction domains of starch synthesising enzymes in vivo and characterise the structure of the resulting starch phenotypes. These studies will allow us to develop a model for the role of post-translational modification in the regulation of starch synthesis and structure. The programme represents a natural synergy of the two PIs, bringing together plant physiology, cell biology, biochemistry, genetics, and starch and protein chemistry to the study of one of the most significant pathways of plant metabolism. The knowledge generated has potential application to the tailoring of starches for specific applications in the food and industrial sectors.