Abstract
Galactoglycerolipids are major constituents of photosynthetic membranes in chloroplasts. At least three parallel sets of enzymes are involved in their biosynthesis that must be coordinated in response to changing growth conditions. A potential candidate for a protein affecting the activity of different galactoglycerolipid pathways is the recently described digalactosyldiacylglycerol1 (dgd1) SUPPRESSOR1 (DGS1) protein of Arabidopsis (Arabidopsis thaliana) localized in the mitochondrial outer membrane. It was discovered based on a specific gain-of-function point mutation allele, dgs1-1, that causes a partial restoration of chloroplast galactoglycerolipid deficiency in the dgd1 mutant, which is defective in the lipid galactosyltransferase, DGD1. The dgs1-1 allele causes the accumulation of hydrogen peroxide that leads to an activation of an alternative, DGD1-independent galactoglycerolipid biosynthesis pathway in chloroplasts. Analysis presented here shows that the DGS1 protein is a component of a large protein complex, which explains the previously observed dominant negative phenotype following the expression of the dgs1-1 allele. The dgs1-1 allele causes the loss of mitochondrial alternative oxidase (AOX) protein that might be related to the accumulation of hydrogen peroxide in the dgs1-1 mutant background. This effect was posttranscriptional because mRNA levels for the major form of AOX were not affected in dgs1-1 mutant seedlings. Unlike dgs1-1, a loss-of-function allele, dgs1-2, had no effect on plant growth, AOX, and lipid composition to the extent tested, leaving the quest for a possible molecular function of DGS1 open. Apparently, the DGS1 wild-type protein does not directly affect lipid metabolism in mitochondria or chloroplasts.
Highlights
Galactoglycerolipids are major constituents of photosynthetic membranes in chloroplasts
To query this mechanism for the possible action of the dgs1-1 allele, we explored whether the DGS1 protein is present in a higher Mr complex
While we have no evidence at this time to conclude whether DGS1 forms a complex with other proteins, our data consistently suggest that wild-type DGS1 and dgs1-1 mutant proteins are present in a higher Mr complex that could provide a mechanism for the function of the dominant negative dgs1-1 allele
Summary
Galactoglycerolipids are major constituents of photosynthetic membranes in chloroplasts. A potential candidate for a protein affecting the activity of different galactoglycerolipid pathways is the recently described digalactosyldiacylglycerol (dgd1) SUPPRESSOR1 (DGS1) protein of Arabidopsis (Arabidopsis thaliana) localized in the mitochondrial outer membrane It was discovered based on a specific gain-of-function point mutation allele, dgs, that causes a partial restoration of chloroplast galactoglycerolipid deficiency in the dgd mutant, which is defective in the lipid galactosyltransferase, DGD1. We critically test our hypothesis that the DGS1 wild-type protein is directly involved in the regulation of the alternative galactoglycerolipid biosynthetic pathway and that hydrogen peroxide formation is a component in the signal transduction pathway connecting phosphate deprivation and the expression of the DGD1-independent galactoglycerolipid biosynthetic pathway If this hypothesis were correct, a DGS1 loss-of-function mutant should be impaired in the activation of the alternative galactoglycerolipid biosynthetic pathway following phosphate deprivation. We provide a comparative analysis of a loss-offunction dgs T-DNA insertion allele and the apparent dgs gain-of-function point mutant allele that led to the originally described dgd suppressor phenotype
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