Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Acyl lipids in Arabidopsis and all other plants have a myriad of diverse functions. These include providing the core diffusion barrier of the membranes that separates cells and subcellular organelles. This function alone involves more than 10 membrane lipid classes, including the phospholipids, galactolipids, and sphingolipids, and within each class the variations in acyl chain composition expand the number of structures to several hundred possible molecular species. Acyl lipids in the form of triacylglycerol account for 35% of the weight of Arabidopsis seeds and represent their major form of carbon and energy storage. A layer of cutin and cuticular waxes that restricts the loss of water and provides protection from invasions by pathogens and other stresses covers the entire aerial surface of Arabidopsis. Similar functions are provided by suberin and its associated waxes that are localized in roots, seed coats, and abscission zones and are produced in response to wounding. This chapter focuses on the metabolic pathways that are associated with the biosynthesis and degradation of the acyl lipids mentioned above. These pathways, enzymes, and genes are also presented in detail in an associated website (ARALIP: http://aralip.plantbiology.msu.edu/). Protocols and methods used for analysis of Arabidopsis lipids are provided. Finally, a detailed summary of the composition of Arabidopsis lipids is provided in three figures and 15 tables.

Acetyl-coenzyme A carboxylase (ACCase, EC 6.4.1.2) catalyzes the synthesis of malonyl-coenzyme A, which is utilized in the plastid for de novo fatty acid synthesis and outside the plastid for a variety of reactions, including the synthesis of very long chain fatty acids and flavonoids. Recent evidence for both multifunctional and multisubunit ACCase isozymes in dicot plants has been obtained. We describe here the isolation of a tobacco (Nicofiana fabacum L. cv bright yellow 2 [NTI]) cDNA clone (E3) that encodes a 58.4-kD protein that shares 80% sequence similarity and 65% identity with the Anabaena biotin carboxylase subunit of ACCase. Similar t o other biotin carboxylase subunits of acetyl-COA carboxylase, the E3-encoded protein contains a putative ATP-binding motif but lacks a biotin-binding site (methionine-lysine-methionine or methioninelysine-leucine). The deduced protein sequence contains a putative transit peptide whose function was confirmed by its ability to direct in vitro chloroplast uptake. The subcellular localization of this biotin carboxylase has also been confirmed to be plastidial by western blot analysis of pea (Pisum sativum), alfalfa (Medicago safiva L.), and castor (Ricinus communis L.) plastid preparations. Northern blot analysis indicates that the plastid biotin carboxylase transcripts are expressed at severalfold higher levels in castor seeds than in leaves.

Acetyl-coenzyme A carboxylase (ACCase) catalyzes the formation of malonyl-coenzyme A, which is used in the plastid for fatty acid synthesis and in the cytosol for severa1 pathways including fatty acid elongation and flavonoid synthesis. Two overlapping Arabidopsis genomic clones were isolated and sequenced to determine the entire ACCase-coding region. Thirty introns with an average size of 94 bp were identified by comparison with an alfalfa ACCase cDNA sequence. l h e 10-kb Arabidopsis ACCase gene encodes a 251-kD polypeptide, which has 80% amino acid sequence identity with alfalfa ACCase and about 40% identity with ACCase of rat, chicken, yeast, and the diatom Cyclotella. No chloroplast transit peptide sequence was observed, suggesting that this Arabidopsis gene encodes a cytosolic ACCase isozyme. ACCase gene transcripts were detected by RNase protection assays in Arabidopsis root, leaf, silique, and seed. Cenomic DNA blot analysis revealed the presence of a second related Arabidopsis ACCase gene.

Two forms of acetyl-CoA carboxylase (ACCase) have been characterized in pea (Pisum sativum L.) leaves; a heteromeric chloroplast enzyme and a homomeric, presumably cytosolic enzyme. The biotin carboxylase (BC), biotin carboxyl carrier protein (BCCP), and beta-carboxyltransferase (CT) subunits of the plastidial-ACCase have recently been characterized and cloned. To further characterize the carboxyltransferase, an improved assay for CT was developed and used to follow its partial purification. CT activity co-purifies with ACCase activity during gel permeation chromatography. However, upon anion-exchange chromatography or native PAGE, CT separates from the BC and BCCP subunits of plastidial-ACCase and ACCase activity is lost. In addition, it is demonstrated that a previously sequenced pea chloroplast cDNA of unknown function (IEP96) with a predicted molecular weight of 91 kDa encodes the alpha-CT subunit of the MS-ACCase. Antibodies raised against the first 404 amino acids of IEP96 protein detected a polypeptide with molecular weight of 91 kDa that co-eluted during gel permeation chromatography with plastidial CT and ACCase activities. These antibodies also immunoprecipitated the activities of both ACCase and CT with the concomitant precipitation of the beta-CT subunit. Furthermore, antibodies against beta-CT immunoprecipitated the IEP96 protein. Two-dimensional PAGE and DEAE purification of ACCase protein demonstrated that the beta-CT forms a tight association with the IEP96 protein. Pea leaf was fractionated into soluble and membrane fractions and the alpha-CT subunit was primarily associated with the membrane fraction. Together, these data demonstrate that IEP96 is the alpha-CT subunit of pea chloroplast ACCase.