Serine carboxypeptidase-like (SCPL) proteins have been identified in organisms ranging from yeast to humans. Although the Arabidopsis genome alone is estimated to encode approximately 50 SCPL proteins, the functions of these enzymes, and similar proteins throughout the plant kingdom, are essentially unknown. Sinapoylmalate and sinapoylcholine are two of the major phenylpropanoid metabolites in Arabidopsis thaliana and are found in leaves and seeds, respectively. A TLC-based screen identified several mutant lines that accumulate the biochemical precursor of these compounds, sinapoylglucose, in their leaves and seeds in place of sinapoylmalate and sinapoylcholine, respectively. These mutations have been named SNG1 and SNG2 for sinapoylglucose accumulator. The SNG1 gene has been cloned and shown to encode sinapoylglucose:malate sinapoyltransferase (SMT), an SCPL protein. In order to begin to understand how proteins of this type can catalyze reactions of this type, an in-depth analysis of SMT and the reaction that it catalyzes will be conducted. Additional mutants have also been isolated in which SCPL genes flanking SMT are deleted. These mutants have novel biochemical phenotypes associated with the absence of these genes. In this project, the compounds that are affected by these mutations will be identified, the pathways by which they are synthesized will be elucidated, and the enzymes required for their accumulation will be characterized. Finally, the map-based cloning of the SNG2 gene is nearing completion and provides strong evidence that it encodes sinapoylglucose:choline sinapoyltransferase, yet another SCPL protein. The characterization of the reactions catalyzed by these enzymes will provide new insights into how these enzymes function. Ultimately, these experiments may lead to the ability to alter plant metabolism, and potentially enable the design of enzymes with novel catalytic abilities.
