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Arabidopsis ECERIFERUM2-LIKE proteins are components of the fatty acid elongation machinery required for the production of cuticular waxes Haslam, Tegan M.

Abstract

Very-long-chain fatty acids (VLCFAs) are essential molecules produced by all plant cells, and are precursors of diverse primary and specialized metabolites. VLCFAs are elongated by a fatty acid elongation (FAE) complex of four core enzymes located on the endoplasmic reticulum, which sequentially adds two carbon units to a growing acyl-CoA chain. Identification and characterization of FAE enzymes in Arabidopsis thaliana has revealed that three of the four enzymes act as generalists, contributing to all metabolic pathways that require VLCFAs. A fourth component, the condensing enzyme, provides substrate specificity and determines the chain length of product synthesized by the entire complex. Chain length is important because it defines what downstream pathway a VLCFA can be used for. In Arabidopsis, characterized condensing enzymes can only elongate VLCFAs up to 28 carbons in length, despite the predominance of 29- and 31-carbon components in plant cuticular wax. This suggests that elongation beyond 28 carbons is unique and requires different protein components. The wax-deficient mutant eceriferum2 (cer2) of Arabidopsis lacks waxes longer than 28 carbons, suggesting that CER2 is specifically required for VLCFA extension past this threshold length. Molecular characterization of the CER2 gene, both in planta and by heterologous expression in Saccharomyces cerevisiae, demonstrates that CER2 is a component of the elongation machinery required to synthesize 30-carbon cuticular wax precursors (Chapter 3). Five homologous CER2-LIKE genes were identified in Arabidopsis; these CER2-LIKEs have similar metabolic functions to CER2, but different expression patterns and substrate specificities (Chapter 4). CER2-LIKEs form a distinct clade of the BAHD acyltransferase superfamily. However, structural predictions and site-directed mutagenesis reveal fundamental differences in the mechanism of activity of CER2-LIKEs relative to characterized BAHDs (Chapter 5). My results suggest that CER2-LIKEs enable condensing enzymes to accept longer VLCFA substrates. I suggest several mechanisms to explain the activity of CER2-LIKEs. This peculiarity of elongation is unique to land plants, and to the production of cuticular wax precursors. Because the acquisition of cuticles as barriers to transpirational water loss was key to plant colonization of land, CER2-LIKE activity is an important specialization of plant lipid metabolism.

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