Date Published: January 30, 2019
Publisher: Public Library of Science
Author(s): Eri Nutahara, Eriko Abe, Shinya Uno, Yohei Ishibashi, Takashi Watanabe, Masahiro Hayashi, Nozomu Okino, Makoto Ito, Juan J. Loor.
Thraustochytrids possess docosahexaenoic acid (DHA, 22:6n-3) as acyl chain(s) of triacylglycerol (TG) and phosphatidylcholine (PC), some of which contain multiple DHAs. However, little is known about how these DHA-rich glycerolipids are produced in thraustochytrids. In this study, we identified PLAT2 in Aurantiochytrium limacinum F26-b as a glycerol-3-phosphate (G3P) acyltransferase (GPAT) by heterologous expression of the gene in budding yeast. Subsequently, we found that GPAT activity was reduced by disruption of the PLAT2 gene in A. limacinum, resulting in a decrease in DHA-containing lysophosphatidic acid (LPA 22:6). Conversely, overexpression of PLAT2 increased both GPAT activity and LPA 22:6. These results indicate that PLAT2 is a GPAT that transfers DHA to G3P in vivo as well as in vitro. Overexpression of the PLAT2 gene increased the production of a two DHA-containing diacylglycerol (DG 44:12), followed by an increase in the three DHA-containing TG (TG 66:18), two-DHA-containing TG (TG 60:12), and two DHA-containing PC (PC 44:12). However, overexpression of PLAT2 did not increase DHA-free DG (DG32:0), which was preferentially converted to three 16:0-containing TG (TG 48:0) but not two 16:0-containing PC (PC 32:0). Collectively, we revealed that DHA-rich glycerolipids are produced from a precursor, LPA 22:6, which is generated by incorporating DHA to G3P by PLAT2 in the A. limacinum.
N-3 polyunsaturated fatty acids (n-3 PUFAs), such as docosahexaenoic acid (DHA, 22:6n-3), decrease the level of blood neutral fat and function in preventing arteriosclerosis and thrombus formation, thereby reducing the risk of cardiovascular diseases [1, 2]. DHA, which is abundant in the brain and retina, is important for the development of the nervous system, the transmission of optical information, and the regeneration of rhodopsin [3, 4]. Recently, DHA was found to alleviate the progression of Alzheimer’s disease and suppress neural cell death . In addition, protectin D1 and resolvin D1, metabolites of DHA, have been reported to inhibit the replication of influenza virus  and to exhibit anti-inflammatory effects . Based on these health benefits, DHA is used in supplements and medicine, and its demand is rapidly increasing. DHA is industrially produced mainly from fish oils. However, due to the declining fish stocks and yearly fluctuation, an alternative resource is strongly desired for DHA production .
PLAT2 is a rate-limiting enzyme in DHA-rich glycerolipid synthesis in A. limacinum. The mechanism by which DHA is incorporated into the glycerol backbone at the early step of glycerolipid synthesis by PLAT2 is reasonable for thraustochytrids, which produce DHA-rich glycerolipids.