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Title: Related Arabidopsis serine carboxypeptidase-like sinapoylglucose acyltransferases display distinct but overlapping substrate specificities

Author
item FRASER, CHRISTOPHER - PURDUE UNIVERSITY
item THOMPSON, MICHAEL - PURDUE UNIVERSITY
item SHIRLEY, AMBER - BASF, NORTH CAROLINA
item Ralph, John
item SCHOENHERR, JESSICA - PURDUE UNIVERSITY
item SINLAPADECH, TAKSINA - PURDUE UNIVERSITY
item HALL, MARK - PURDUE UNIVERSITY
item CHAPPLE, CLINT - PURDUE UNIVERSITY

Submitted to: Plant Physiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/22/2007
Publication Date: 8/1/2007
Citation: Fraser, C.M., Thompson, M.G., Shirley, A.M., Ralph, J., Schoenherr, J.A., Sinlapadech, T., Hall, M.C., Chapple, C. 2007. Related Arabidopsis serine carboxypeptidase-like sinapoylglucose acyltransferases display distinct but overlapping substrate specificities. Plant Physiology. 144(4):1986-1999.

Interpretive Summary: Plant secondary metabolites account for a great amount of the biochemical diversity that exists in the plant kingdom. These compounds have been estimated to number up to 200,000, with the majority of them yet to be studied in detail. Plant secondary metabolites include compounds that possess many important properties and function in a host of physiologically important roles. They have also proven to be invaluable to humanity, playing long-standing roles in medicine and agriculture. A common metabolic step in the production of numerous secondary metabolites involves the attachment of an organic acid via an activated donor molecule. In this paper we report the characterization of three additional Arabidopsis proteins that are “sinapoylglucose-dependent sinapoyltransferases,” and the characterization (by nuclear magnetic resonance spectroscopy) of the crucial activated compounds involved. The functions of the proteins were identified through the characterization of mutant plants. One protein is involved in the synthesis of anthocyanins, often highly colored and UV-protectant compounds in plants. Overall, they represent additional members of an emerging class of enzymes that catalyze “acyltransferase” reactions in plant secondary metabolism, reaffirming that plants utilize glucose esters to activated acids in their metabolism for further biochemical reactions. Such research is ultimately aimed at improving the utilization of plant resources.

Technical Abstract: The Arabidopsis genome encodes fifty-one proteins annotated as serine carboxypeptidase-like (SCPL) enzymes. Nineteen of these SCPL proteins are highly similar to one another, and represent a clade that appears to be unique to plants. Two of these proteins have been characterized to date: sinapoylglucose: malate sinapoyltransferase (SMT) and sinapoylglucose:choline sinapoyltransferase (SCT). The observation that two of the least related proteins within this clade are acyltransferases rather than true serine carboxypeptidases suggests that some or all of the remaining members of this group may have similar activities. In fact, the gene that encodes SMT (SNG1: At2g22990) is one of five SCPL genes arranged in tandem to form a cluster on chromosome 2. In this study, an analysis of deletion mutant lines lacking one or more genes in this SCPL gene cluster reveals that three of these genes also encode sinapoylglucose-dependent acyltransferases. At2g23000 encodes sinapoylglucose: anthocyanin acyltransferase (SAT), an enzyme that is required for the synthesis of the sinapoylated anthocyanins in Arabidopsis. At2g23010 encodes an enzyme capable of synthesizing 1,2-disinapoylglucose from two molecules of sinapoylglucose, an activity shared by SNG1 and At2g22980. Sequence analysis of these SCPL proteins reveals pair-wise percent identities that range from 71% to 78%, suggesting that their differing specificities for acyl acceptor substrates are due to changes in a relatively small subset of amino acids. Thus, the study of these SCPL proteins provides an opportunity to examine structure-function relationships in enzyme evolution and may ultimately shed light on the role of evolution of hydroxycinnamate ester metabolism and the SCPL gene family in Arabidopsis and other flowering plants.