Left panel: Light microscopic image of Nannochloropsis. Lipid droplets look slightly bluish by light refraction. Right panel: Fluorescence micrograph of that accumulated a large amount of oils. With color-coding, green indicates chloroplasts, and yellow oil droplets. (Credit: Tokyo Institute of Technology)
Modern energy generation is heavily dependent on fossil fuels, which lead to a depletion of non-renewable resources and cause environmental pollution.
Atmospheric carbon dioxide is transformed into carbon storage molecules, especially oils such as triacylglycerols (TAGs), by photosynthetic organisms like green algae and plants. The TAGs can be used as biofuels. Microalgae have high oil content, and can grow even in extreme environments, including high salinity, pH, or temperature.
A genus of microalgae called nannochloropsis can accumulate TAGs up to 50% of dry weight. The mechanisms underlying their oleaginous characteristics, however, are largely unknown.
Professor Hiroyuki Ohta and other scientists from the
Tokyo Institute of Technology have addressed this problem by studying the lipid metabolism in Nannochloropsis oceanica. The sequential addition of three fatty acyl moieties to the glycerol backbone results in synthesizing of TAGs in the extraplastidic Kennedy pathway.
Among the participating enzymes the focus of the scientists was on four lysophosphatidic acid acyltransferases (LPATs 1-4) responsible for the addition of fatty acids at position 2.
It was discovered that LPAT1 and LPAT2 belong to varied subfamilies phylogenetically, whereas LPAT3 and LPAT4 are evolutionarily closer. Using mutant strains of N. oceanica lacking either one or two of the four LPATs, it appeared that these enzymes have distinct functional activities.
It was found that the main role of LPAT1 was to participate in the synthesis of membrane lipids, while LPAT4 was involved in TAG biosynthesis, and LPAT3 and LPAT2 contributed to both processes.
Fluorescent tags were used to label LPATs, and confocal microscopy was employed to examine their intracellular location. A typical ER localization pattern was shown by LPAT1 and LPAT2, and LPAT3 and LPAT4 were specifically observed at the perimeter of lipid droplets (LDs).
This could be because of the presence of long (30 - 40 residues) hydrophobic domains in their structures that help in anchoring to the LD surface.
On the basis of their results, the scientists suggest that for LD formation, the initial TAG synthesis in the ER is through LPAT2 mainly, and LPAT3 and LPAT4 contribute to further growth of LDs by localizing on LD surface in the periphery.
The fact that the oleaginous trait of Nannochloropsis is supported by LPATs in the perimeter of LDs is, thus, directly evidenced by the study of Professor Ohta and his colleagues.