Differently localized lysophosphatidic acid acyltransferases crucial for triacylglycerol biosynthesis in the oleaginous alga Nannochloropsis

Abstract

SUMMARYThe production of renewable bioenergy will be necessary to meet rising global fossil fuel demands. Members of the marine microalgae genus Nannochloropsis produce large quantities of oils (triacylglycerols; TAGs), and this genus is regarded as one of the most promising for biodiesel production. Recent genome sequencing and transcriptomic studies on Nannochloropsis have provided a foundation for understanding its oleaginous trait, but the mechanism underlying oil accumulation remains to be clarified. Here we report Nannochloropsis knock-out strains of four extraplastidic lysophosphatidic acid acyltransferases (LPAT1-LPAT4) that catalyze a major de novo biosynthetic step of TAGs and membrane lipids. We found that the four LPATs are differently involved in lipid metabolic flow in Nannochloropsis. Double knock-outs among the LPATs revealed the pivotal LPATs for TAG biosynthesis, and localization analysis indicated that the stramenopile-specific LPATs (LPAT3 and LPAT4) associated with TAG synthesis reside at the perimeter of lipid droplets. No homologous region has been found with other lipid droplet-associated proteins, however. Lipid droplets are an organelle found in nearly all organisms, and recently they were shown to play important roles in cellular metabolism and signaling. Our results provide direct evidence for the importance of the perimeter of lipid droplet in TAG synthesis in addition to its known role in maintaining TAG stability, and these findings suggest that the oleaginous trait of Nannochloropsis is enabled by the acquisition of LPATs at the perimeter of lipid droplets.

Principles of plastid reductive evolution illuminated by nonphotosynthetic chrysophytes

“…4 and SI Appendix, Figs. S5, S13, and S16) and previous studies (5,(60)(61)(62)(63)(64). The threshold values were manipulated for each predictor to define optimal ones for the detection of experimental localizations, with the best trade-offs between sensitivity and specificity identified for the following: the default conditions for SignalP and ASAFind; modified mitochondria targeting thresholds of 0.7 and 0.35 for TargetP and MitoFates, respectively, as previously published (5,9); and a substantially lower chloroplast targeting threshold (0.0855, defined as the midpoint between the 90% sensitivity and 90% stringency; with the additional stipulation that the chloroplast targeting value be greater than the signal anchor targeting value) for HECTAR (SI Appendix, Fig.…”

LYSOPHOSPHATIDIC ACID ACYLTRANSFERASES 4 and 5 are involved in glycerolipid metabolism and nitrogen starvation response in Arabidopsis

“…Besides an involvement in the biosynthesis of polar glycerolipids, LPAT activity has strong flux control in the biosynthesis of TAG (Zou et al, 1997;Taylor et al, 2002;Maisonneuve et al, 2010). Indeed, a recent report in an oleaginous alga, Nannochloropsis, revealed a critical commitment of the LPATs in TAG biosynthesis (Nobusawa et al, 2017). Given that stimulation of de novo glycerolipid biosynthesis pathways is a prerequisite for TAG accumulation under N starvation, some of these uncharacterized LPAT isoforms may play a role under N starvation in Arabidopsis.…”

Betaine Lipid Is Crucial for Adapting to Low Temperature and Phosphate Deficiency in Nannochloropsis

“…The complementary association between BLs and phospholipids has long been accepted as an adaptive strategy to cope with periodic Pi deficiency in the natural environment. In certain algae species, however, BLs seem to have a distinct role aside from substituting for phospholipids since the fatty acid profile of BLs is dissimilar to that of any of the phospholipids, since it contains higher unsaturated groups (Iwai et al, 2015;Cañavate et al, 2016;Dolch et al, 2017;Nobusawa et al, 2017). Moreover, the degree of change in cellular BLs and PC contents upon fluctuations in external Pi availability is quite different among species (Cañavate et al, 2017).…”

“…Moreover, the development of molecular biology tools that enable the generation of Nannochloropsis transformants is increasing (Kilian et al, 2011;Wang et al, 2016;Wei et al, 2017). The coordination of omics data and functional analyses of key genes related to lipid biosynthesis would yield important information on algal lipid metabolism that is expected to form the basis for future metabolic engineering techniques (Dolch et al, 2017;Nobusawa et al, 2017;Zienkiewicz et al, 2017).…”