Plastidic phosphatidic acid phosphatases identified in a distinct subfamily of lipid phosphate phosphatases with prokaryotic origin.

Plastidic phosphatidic acid phosphatase (PAP) dephosphorylates phosphatidic acid to yield diacylglycerol, which is a precursor for galactolipids, a primary and indispensable component of photosynthetic membranes. Despite its functional importance, the molecular characteristics and phylogenetic origin of plastidic PAP were unknown because no potential homologs have been found. Here, we report the isolation and characterization of plastidic PAPs in Arabidopsis that belong to a distinct lipid phosphate phosphatase (LPP) subfamily with prokaryotic origin. Because no homolog of mammalian LPP was found in cyanobacteria, we sought an LPP ortholog in a more primitive organism, Chlorobium tepidum, and its homologs in cyanobacteria. Arabidopsis had five homologs of cyanobacterial LPP, three of which (LPP gamma, LPP epsilon 1, and LPP epsilon 2) localized to chloroplasts. Complementation of yeast Delta dpp1 Delta lpp1 Delta pah1 by plastidic LPPs rescued the relevant phenotype in vitro and in vivo, suggesting that they function as PAPs. Of the three LPPs, LPP gamma activity best resembled the native activity. The three plastidic LPPs were differentially expressed both in green and nongreen tissues, with LPP gamma expressed the highest in shoots. A knock-out mutant for LPP gamma could not be obtained, although a lpp epsilon 1 lpp epsilon 2 double knock-out showed no significant changes in lipid composition. However, lpp gamma homozygous mutant was isolated only under ectopic overexpression of LPP gamma, suggesting that loss of LPP gamma may cause lethal effect on plant viability. Thus, in Arabidopsis, there are three isoforms of plastidic PAP that belong to a distinct subfamily of LPP, and LPP gamma may be the primary plastidic PAP.

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