The plastid outer membrane localized LPTD1 is important for glycerolipid remodelling under phosphate starvation

Abstract

Glycerolipid synthesis in plants is coordinated between plastids and the endoplasmic reticulum (ER). A central step within the glycerolipid synthesis is the transport of phosphatidic acid from ER to chloroplasts. The chloroplast outer envelope protein TGD4 belongs to the LptD family conserved in bacteria and plants and selectively binds and may transport phosphatidic acid. We describe a second LptD-family protein in A. thaliana (atLPTD1; At2g44640) characterized by a barrel domain with an amino-acid signature typical for cyanobacterial LptDs. It forms a cation selective channel in vitro with a diameter of about 9 Å. atLPTD1 levels are induced under phosphate starvation. Plants expressing an RNAi construct against atLPTD1 show a growth phenotype under normal conditions. Expressing the RNAi against atLPTD1 in the tgd4-1 background renders the plants more sensitive to light stress or phosphate limitation than the individual mutants. Moreover, lipid analysis revealed that digalactosyldiacylglycerol and sulfoquinovosyldiacylglycerol levels remain constant in the RNAi mutants under phosphate starvation, while these two lipids are enhanced in wild-type. Based on our results, we propose a function of atLPTD1 in the transport of lipids from ER to chloroplast under phosphate starvation, which is combinatory with the function of TGD4.

Unraveling Hidden Components of the Chloroplast Envelope Proteome: Opportunities and Limits of Better MS Sensitivity

“…This short list is composed of proteins from the chloroplast IEM or OEM, the cytosol, and the ER. During this work, we detected members from all involved cell compartments: i ) the Phospholipase D PLDα1 (At3g15730, EF = 0.04) from the cytosol, ii ) the two long chain acyl-CoA synthetases LACS4 (At4g23850, EF = 0.8) and LACS8 (At2g04350, EF = 0.1) from the ER, iii ) the fatty acid translocator FAX1 (At3g57280, EF = 4.3), lipid translocators TGD1 (At1g19800, Env_only, EF = 21.8), TGD2 (At3g20320, EF = 7.8) and TGD3 (At1g65410, EF = 28.5), the MGDG synthase, MGD1 (At4g31780, Env_only, EF = 37.8), and the three lipid desaturases that were only detected in the envelope, FAD6 (At4g30950, EF = 68.3), FAD7 (At3g11170, EF = 43.2) and FAD8 (At5g05580, EF = 5.3) from the chloroplast IEM (but not FAD2 and FAD3 from the ER), and the lipid translocator LPTD1 or TGD4 (At3g06960, EF = 6.7) from the chloroplast OEM (whose subcellular localization was unclear, chloroplast-associated or ER in (78)) but which was classified as an OEM protein during this work thanks to its EF value of 6.7, which agrees with recently published data (80). In other words, the correlation between EF values of the known or predicted locations of the main actors in the ER-to-chloroplast lipid trafficking process was robust.…”

Lipid Trafficking at Membrane Contact Sites During Plant Development and Stress Response

“…In addition, the phospholipase C NPC5 is localized in the cytosol and might associate with different organelle membranes to form the DAG backbones required for DGDG synthesis during Pi starvation (Gaude et al, 2008). Recently, the protein LPTD1 was shown to play an important role in the synthesis of galactoglycerolipids during Pi starvation in A. thaliana (Hsueh et al, 2017). LPTD1 is located in the OE of plastids and is highly induced in the absence of Pi (Hsueh et al, 2017).…”

“…Recently, the protein LPTD1 was shown to play an important role in the synthesis of galactoglycerolipids during Pi starvation in A. thaliana (Hsueh et al, 2017). LPTD1 is located in the OE of plastids and is highly induced in the absence of Pi (Hsueh et al, 2017). Lptd1 mutant plants show an alteration of the lipid remodeling during Pi starvation, in particular, no increase of the DGDG content was observed in this mutant (Hsueh et al, 2017).…”

“…LPTD1 is located in the OE of plastids and is highly induced in the absence of Pi (Hsueh et al, 2017). Lptd1 mutant plants show an alteration of the lipid remodeling during Pi starvation, in particular, no increase of the DGDG content was observed in this mutant (Hsueh et al, 2017). Thus, LPTD1 could play a role in the recycling of phospholipid backbones in response to Pi starvation.…”

“…Thus, LPTD1 could play a role in the recycling of phospholipid backbones in response to Pi starvation. As LPTD1 is a homolog of the TGD4 protein (Hsueh et al, 2017), it would be interesting to know whether LPTD1 is also part of the TGD complex and whether it is involved in lipid transfer from the ER and/or from other organelles to plastids to sustain phospholipids recycling in this situation.…”