A Prokaryotic‐like Lysophosphatidic Acid Acyltransferase Reveals Unique Features of Triacylglycerol Biosynthesis in Microalgae

Abstract

Understanding algal triacylglycerol (TAG) metabolism has strong implications for the biology of storage lipids as well as for biofuel production. Higher plants have prokaryotic (chloroplast located) and eukaryotic (endoplasmic reticulum located) pathways to synthesize specific glycerolipids. Lysophosphatidic acid acyltransferases (LPAATs) in the chloroplast prefer adding C16 fatty acids at the sn‐2 position of the glycerol backbone, generating ‘prokaryotic’ lipid precursors. In contrast, LPAATs in the endoplasmic reticulum (ER) direct the preferential esterification of C18 fatty acids at the sn‐2 position, synthesizing ‘eukaryotic’ lipid precursors. In higher plants, triacylglycerols are considered eukaryotic lipids, as they have predominantly C18 esterified at the sn‐2 position. Moreover, TAG assembly enzymes have been found to localize to the ER in diverse eukaryotes.Chlamydomonas reinhardtii, a unicellular green alga, has primarily C16 fatty acids esterified at the sn‐2 position of TAGs. This observation led to the proposal that Chlamydomonas mainly employs the prokaryotic (chloroplastic) pathway for TAG precursor biosynthesis. However, an alternative hypothesis is suggested by our characterization of a unique C. reinhardtii LPAAT, termed CrLPAAT2. In phylogenetic analyses this enzyme belongs within a Chlorophyte‐specific clade (limited to the Chlorophyceae and Trebouxiophyceae classes of green algae) and it is absent from all other eukaryotes examined. Interestingly, CrLPAAT2 displays a chimeric structure between known eukaryotic and prokaryotic LPAATs and it was found to localize to the ER. RNA‐mediated silencing of CrLPAAT2 indicated that the enzyme is required for TAG accumulation under nitrogen deprivation. We also demonstrated that CrLPAAT2 prefers as substrate C16:0‐CoA over C18:1‐CoA in vitro, by substrate competition assays, and in vivo, by complementation of a LPAAT‐defective yeast strain. Thus, Chlamydomonas (and related green algae) possesses a distinct glycerolipid synthesis pathway that relies on CrLPAAT2 to generate prokaryotic‐like TAG species (with C16 at the sn‐2 position) in the endoplasmic reticulum.Support or Funding InformationThis research is funded by EPSCoR (1004094).