Acyl Editing and Headgroup Exchange Are the Major Mechanisms That Direct Polyunsaturated Fatty Acid Flux into Triacylglycerols    

Abstract

Triacylglycerols (TAG) in seeds of Arabidopsis (Arabidopsis thaliana) and many plant species contain large amounts of polyunsaturated fatty acids (PUFA). These PUFA are synthesized on the membrane lipid phosphatidylcholine (PC). However, the exact mechanisms of how fatty acids enter PC and how they are removed from PC after being modified to participate in the TAG assembly are unclear, nor are the identities of the key enzymes/genes that control these fluxes known. By reverse genetics and metabolic labeling experiments, we demonstrate that two genes encoding the lysophosphatidylcholine acyltransferases LPCAT1 and LPCAT2 in Arabidopsis control the previously identified "acyl-editing" process, the main entry of fatty acids into PC. The lpcat1/lpcat2 mutant showed increased contents of very-long-chain fatty acids and decreased PUFA in TAG and the accumulation of small amounts of lysophosphatidylcholine in developing seeds revealed by [ 14 C]acetate-labeling experiments. We also showed that mutations in LPCATs and the PC diacylglycerol cholinephosphotransferase in the reduced oleate desaturation1 (rod1)/lpcat1/lpcat2 mutant resulted in a drastic reduction of PUFA content in seed TAG, accumulating only one-third of the wild-type level. These results indicate that PC acyl editing and phosphocholine headgroup exchange between PC and diacylglycerols control the majority of acyl fluxes through PC to provide PUFA for TAG synthesis.

Carbon and Acyl Chain Flux during Stress-induced Triglyceride Accumulation by Stable Isotopic Labeling of the Polar Microalga Coccomyxa subellipsoidea C169

“…The trafficking of labeled acyl chains from the PL is higher than can be accounted for by efflux of whole lipids, whereas in the GL, it is approximately equal. Membrane Lipid Fatty Acid Moieties Are Incorporated into TG-Membrane lipid acyl chain flux is currently poorly defined both spatially and temporally, yet relies on distinct enzymatic reactions that collectively provide flexibility in lipid metabolic homeostasis (43)(44)(45)(46). The insertion and removal of fatty acids that differ by chain length and number of double bonds defines membrane responses to temperature, salinity, and a variety of other abiotic conditions (47,48).…”

Role of Aminoalcoholphosphotransferases 1 and 2 in Phospholipid Homeostasis in Arabidopsis

“…Radioisotope labeling of seedlings was adapted from a method reported previously (Bates et al, 2012). Twenty seedlings were used for each sample and grown in media (0.53 MS media with 0.5% sucrose, pH 5.8) in 24-well cell culture plates under 24 h light at 22°C for 10 d. At 3, 6, 10, 30, and 60 min, labeled seedlings were quickly transferred to a tube containing preheated (>75°C) isopropanol with 0.01% butylated hydroxytoluene to stop labeling and lipids were extracted as described previously.…”

In Vivo and in Vitro Evidence for Biochemical Coupling of Reactions Catalyzed by Lysophosphatidylcholine Acyltransferase and Diacylglycerol Acyltransferase

“…In the cytoplasm, the 18:1 from the acylcoenzyme A (CoA) pool is quickly esterified to the phosphatidylcholine (PC) of the endoplasmic reticulum. Desaturation of 18:1 to linoleic acid (LA; 18:2 cis⌬9, 12 ) and then ␣-linolenic acid (ALA; 18:3 cis⌬9, 12,15 ) occurs through the catalytic action of endoplasmic reticulum-localized fatty acid desaturases, the oleate desaturase (FAD2), and the linoleate desaturase (FAD3), producing PUFA-enriched PC (2). It has been shown that PC also is the major site that synthesizes a number of unusual fatty acids, such as hydroxy and conjugated fatty acids (3).…”