Novel Roles for Acyl-Synthetases and Acyl-Thioesterases in Plant Lipid Metabolism

Acyl lipids are a diverse and important class of biomolecules. In plants, acyl lipids include ubiquitous compounds such as phospholipids, galactolipids, and triacylglycerols, as well as more specialized compounds including cutin and suberin monomers, cuticular waxes, and oxylipins. All acyl lipids are derived from fatty acids, which generally require activation for metabolism to either a CoenzymeA (CoA) or an acyl carrier protein (ACP) thioester. Acyl-synthetases and acyl-thioesterases, enzymes that activate and deactivate fatty acids, respectively, provide important checkpoints in lipid metabolism. Historically, the activation/deactivation paradigm of acyl-synthetases and acyl-thioesterases has been quite simple. However, various more sophisticated roles of these enzymes classes have been suggested, which is in line with the fact that a typical plant genome encodes many more of these enzymes than a simple activation/deactivation model would require. In this thesis I explore some of these more specialized roles of fatty acyl-synthetases and fatty acyl-thioesterases using the model plant Arabidopsis thaliana. First, I present evidence that Arabidopsis long-chain acyl-CoA synthetases LACS1, LACS2, and LACS3 are involved in trans-membrane movement of fatty acids, possibly playing a role in directing flux of fatty acids into particular biosynthetic pathways. This finding has implications in our understanding of the trafficking of ubiquitous fatty acyl-CoA substrates in plant cells, the conclusions of which can be extrapolated to other LACS proteins. Second, I characterize a novel family of Arabidopsis acyl-ACP thioesterases called ALT1, ALT2, ALT3, and ALT4. These genes were found to be expressed in various tissues throughout the plant, each with a unique expression pattern. When expressed heterologously in E. coli, each ALT protein generated a unique set of medium-chain fatty acids and/or medium-chain β-ketofatty acids. The occurrence of medium-chain fatty acids and β-ketofatty acids in various tissues of Arabidopsis plants provides clues regarding the biological roles of ALT1-4. Potential ALT homologues are found in nearly all plant genomes, and the encoded proteins are likely involved in specialized metabolism. This research opens many doors for future research, both in terms of basic research of plant lipid metabolism as well as applied uses of specialized high-value acyl lipids.