Arabidopsis chloroplast lipid transport protein TGD2 disrupts membranes and is part of a large complex

In most plants the assembly of the photosynthetic thylakoid membrane requires lipid precursors synthesized at the endoplasmic reticulum (ER). Thus, the transport of lipids from the ER to the chloroplast is essential for biogenesis of the thylakoids. TGD2 is one of four proteins in Arabidopsis required for lipid import into the chloroplast, and was found to bind phosphatidic acid in vitro. However, the significance of phosphatidic acid binding for the function of TGD2 in vivo and TGD2 interaction with membranes remained unclear. Developing three functional assays probing how TGD2 affects lipid bilayers in vitro, we show that it perturbs membranes to the point of fusion, causes liposome leakage and redistributes lipids in the bilayer. By identifying and characterizing five new mutant alleles, we demonstrate that these functions are impaired in specific mutants with lipid phenotypes in vivo. At the structural level, we show that TGD2 is part of a protein complex larger than 500 kDa, the formation of which is disrupted in two mutant alleles, indicative of the biological relevance of this TGD2-containing complex. Based on the data presented, we propose that TGD2, as part of a larger complex, forms a lipid transport conduit between the inner and outer chloroplast envelope membranes, with its N terminus anchored in the inner membrane and its C terminus binding phosphatidic acid in the outer membrane.