Abstract
In all seed plants, the assembly of galactolipids in the thylakoid membrane involves the endoplasmic reticulum (ER), where the glycerolipid precursor phosphatidic acid is synthesized. We have isolated mutants of Arabidopsis thaliana that are disrupted in the biosynthesis of galactolipids from ER-derived precursors. These mutants rely on the de novo assembly of galactolipids from newly synthesized fatty acids in the plastid. The mutants accumulate triacylglycerols and oligogalactolipids, trigalactosyldiacylglycerol, in the leaves leading to their trigalactosyldiacylglycerol (tgd) designation. One of the mutant allele proteins, TGD2 is similar to a substrate binding protein of multipartite ABC transporters as found in bacteria. It is localized in the inner chloroplast envelope membrane and specifically binds phosphatidic acid. Based on this evidence, we hypothesize that the TGD2 is involved in transport of phosphatidic acid from the outer envelope membrane to the inner envelope membrane, thereby providing the ER-derived DAG precursor for galactolipid biosynthesis.
