8.A.86 The Chloroplast Trigalactosyldiacylglycerol-5 (TGD5) Protein Family.
TGD5 facilitates lipid transfer from the outer to the inner plastid envelope by bridging TGD4 (TC# 1.B.82.1.1) with the TGD1,2,3 transport complex (3.A.220.127.116.11). TGD5 is required for endoplasmic reticulum-to-plastid lipid trafficking as part of the eukaryotic pathway of thylakoid lipid assembly (Fan et al. 2015). Disruption of TGD5 results in phenotypic effects similar to those for tgd1,2,3,4 mutants, including deficiency of ER-derived thylakoid lipids, accumulation of oligogalactolipids, and triacylglycerol. Genetic analysis indicated that TGD4 is epistatic to TGD5 in ER-to-plastid lipid trafficking, whereas double mutants of a null tgd5 allele with tgd1-1 or tgd2-1 showed a synergistic embryo-lethal phenotype. TGD5 is a small glycine-rich protein that is localized in the envelope membranes of chloroplasts. Coimmunoprecipitation assays suggested that TGD5 physically interacts with TGD1, TGD2, TGD3, and TGD4. Collectively, these results suggest that TGD5 facilitates lipid transfer from the outer to the inner plastid envelope by bridging TGD4 with the TGD1,2,3 transporter complex (Fan et al. 2015).