1.B.82 The Chloroplast Trigalactosyldiacylglycerol-4 Porin (TGD4) Family.
Chloroplast membrane lipid synthesis relies on the import of glycerolipids from the ER. The TGD (TriGalactosylDiacylglycerol) proteins are required for this lipid transfer process. The TGD1, 2, and 3 proteins form an ABC (ATP-binding cassette) transporter transporting ER-derived lipids through the inner envelope membrane of the chloroplast, while TGD4 binds phosphatidic acid (PtdOH) and resides in the outer chloroplast envelope. Wang et al. 2013 identified two sequences in TGD4, amino acids 1-80 and 110-145, which are necessary and sufficient for PtdOH binding. Deletion of both sequences abolished PtdOH binding activity. They also found that TGD4 from 18:3 plants bound specifically and with increased affinity PtdOH. TGD4 did not interact with other proteins and formed a homodimer both in vitro and in vivo. Their results suggested that TGD4 is an integral dimeric β-barrel lipid transfer protein that binds PtdOH with its N terminus and contains dimerization domains at its C terminus (Wang et al. 2013).
TGD4 of 479 aas and 19 - 21 putative beta strands in a β-barrel structure (Wang et al. 2013) that may form a porin to transfer lipids from the ER across the outer chloroplast membrane. TGD5 (TC#8.A.86; 91 aas and 2 TMSs) may function to transfer lipids from the ER to TGD4, and then to the TGD123 ABC complex (TC# 3.A.126.96.36.199) in the inner envelope membrane for import into the chloroplast inner membrane or matrix (Fan et al. 2015).
TGD4 of Arabidopsis thaliana (Mouse-ear cress)
TDG4 of Ostreococcus lucimarinus
Uncharacterized protein of 701 aas.
UP of Vitis vinifera (Grape)