Chloroplast lipid transfer processes in Chlamydomonas reinhardtii involving a TRIGALACTOSYLDIACYLGLYCEROL 2 (TGD2) orthologue.
In plants, lipids of the photosynthetic membrane are synthesized by parallel pathways associated with the endoplasmic reticulum (ER) and the chloroplast envelope membranes. Lipids derived from the two pathways are distinguished by their acyl-constituents. Following this plant paradigm, the prevalent acyl composition of chloroplast lipids suggests that Chlamydomonas reinhardtii (Chlamydomonas) does not use the ER pathway; however, the Chlamydomonas genome encodes presumed plant orthologues of a chloroplast lipid transporter consisting of TGD (TRIGALACTOSYLDIACYLGLYCEROL) proteins that are required for ER-to-chloroplast lipid trafficking in plants. To resolve this conundrum, we identified a mutant of Chlamydomonas deleted in the TGD2 gene and characterized the respective protein, CrTGD2. Notably, the viability of the mutant was reduced, showing the importance of CrTGD2. Galactoglycerolipid metabolism was altered in the tgd2 mutant with monogalactosyldiacylglycerol (MGDG) synthase activity being strongly stimulated. We hypothesize this to be a result of phosphatidic acid accumulation in the chloroplast outer envelope membrane, the location of MGDG synthase in Chlamydomonas. Concomitantly, increased conversion of MGDG into triacylglycerol (TAG) was observed. This TAG accumulated in lipid droplets in the tgd2 mutant under normal growth conditions. Labeling kinetics indicate that Chlamydomonas can import lipid precursors from the ER, a process that is impaired in the tgd2 mutant.
Department of Plant Biology, Michigan State University, East Lansing, Michigan, 48824, USA.|MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan, 48824, USA.
Jaruswan
J
Warakanont
Department of Plant Biology, Michigan State University, East Lansing, Michigan, 48824, USA.|MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan, 48824, USA.
Chia-Hong
CH
Tsai
Department of Plant Biology, Michigan State University, East Lansing, Michigan, 48824, USA.
Elena J S
EJ
Michel
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan, 48824, USA.
George R
GR
Murphy
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan, 48824, USA.
Peter Y
PY
Hsueh
Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan, 48824, USA.
Rebecca L
RL
Roston
Department of Plant Biology, Michigan State University, East Lansing, Michigan, 48824, USA.|MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan, 48824, USA.
Barbara B
BB
Sears
MSU-DOE Plant Research Laboratory, Michigan State University, East Lansing, Michigan, 48824, USA.|Department of Biochemistry and Molecular Biology, Michigan State University, East Lansing, Michigan, 48824, USA.
Christoph
C
Benning
D001692:Biological Transport; D016825:Chlamydomonas reinhardtii / Q000235:genetics / Q000378:metabolism*; D002736:Chloroplasts / Q000378:metabolism; D004721:Endoplasmic Reticulum / Q000378:metabolism; D018745:Genome, Plant; D050356:Lipid Metabolism / Q000235:genetics*; D009154:Mutation; D010788:Photosynthesis; D010940:Plant Proteins / Q000235:genetics / Q000378:metabolism / Q000502:physiology*
D010940:Plant Proteins
Chlamydomonas reinhardtii; chloroplast lipids; lipid metabolism; lipid transport; membranes