Abstract
BackgroundOleaginous microalgae are promising sources of energy-rich triacylglycerols (TAGs) for direct use for food, feed and industrial applications. Lobosphaera incisa is a fresh water unicellular alga, which in response to nutrient stress accumulates a high amount of TAGs with a high proportion of arachidonic acid (ARA). The final committed step of de novo TAG biosynthesis is catalyzed by acyl-CoA:diacylglycerol acyltransferases (DGATs), which add a fatty acid (FA) to the final sn-3 position of diacylglycerol (DAG).ResultsGenome analysis revealed the presence of five putative DGAT isoforms in L. incisa, including one DGAT of type 1, three DGATs of type 2 and a single isoform of a type 3 DGAT. For LiDGAT1, LiDGAT2.1, LiDGAT2.2 and LiDGAT2.3 enzyme activity was confirmed by expressing them in the TAG-deficient yeast strain H1246. Feeding experiments of yeast transformants with fatty acids suggest a broad substrate specificity spectrum for LiDGAT1. A significant TAG production in response to exogenous ARA was found for LiDGAT2.2. Cellular localization of the four type 1 and type 2 DGATs expressed in yeast revealed that they all localize to distinct ER domains. A prominent association of LiDGAT1 with ER domains in close proximity to forming lipid droplets (LDs) was also observed.ConclusionsThe data revealed a distinct molecular, functional and cellular nature of type 1 and type 2 DGATs from L. incisa, with LiDGAT1 being a major contributor to the TAG pool. LiDGATs of type 2 might be in turn involved in the incorporation of unusual fatty acids into TAG and thus regulate the composition of TAG. This report provides a valuable resource for the further research of microalgae DGATs oriented towards production of fresh-water strains with higher oil content of valuable composition, not only for oil industry but also for human and animal nutrition.
Highlights
Oleaginous microalgae are promising sources of energy-rich triacylglycerols (TAGs) for direct use for food, feed and industrial applications
We identified and characterized here for the first time the third member of this family - LiDGAT2.3 and provide novel data on the molecular, functional and cellular nature of membrane-bound diacylglycerol acyltransferases (DGATs) encoded by the L. incisa genome
We propose for L. incisa that DGAT1 isoforms may contribute mainly to net TAG synthesis and DGAT2 isoforms may play a role in regulating the TAG composition
Summary
Oleaginous microalgae are promising sources of energy-rich triacylglycerols (TAGs) for direct use for food, feed and industrial applications. Lobosphaera incisa is a fresh water unicellular alga, which in response to nutrient stress accumulates a high amount of TAGs with a high proportion of arachidonic acid (ARA). Oleaginous microalgae can synthesize high-energy compounds using CO2 as carbon source, water as electron donor and light as primary energy source. These organisms gain considerable attention for a renewable production of biofuels, chemical feedstock or bioplastics [1,2,3]. Under environmental stress microalgae accumulate substantial amount of lipids, mainly in the form of triacylglyerols (TAGs) [5]. The presence of three fatty acids (FAs) in one TAG molecule results in an extremely high potential energy. Most of the current models rely mostly on the homology between the genetic equipment governing the core pathways of lipid metabolism revealed by recent
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