Lipid Transport Mediated by Arabidopsis TGD Proteins is Unidirectional from the Endoplasmic Reticulum to the Plastid

Abstract

The transfer of lipids between the endoplasmic reticulum (ER) and the plastid in Arabidopsis involves the TRIGALACTOSYLDIACYLGLYCEROL (TGD) proteins. Lipid exchange is thought to be bidirectional based on the presence of specific lipid molecular species in Arabidopsis mutants impaired in the desaturation of fatty acids of membrane lipids in the ER and plastid. However, it was unclear whether TGD proteins were required for lipid trafficking in both directions. This question was addressed through the analysis of double mutants of tgd1-1 or tgd4-3 in genetic mutant backgrounds leading to a defect in lipid fatty acid desaturation either in the ER (fad2) or the plastid (fad6). The fad6 tgd1-1 and fad6 tgd4-3 double mutants showed drastic reductions in the relative levels of polyunsaturated fatty acids and of galactolipids. The growth of these plants and the development of photosynthetic membrane systems were severely compromised, suggesting a disruption in the import of polyunsaturated fatty acid-containing lipid species from the ER. Furthermore, a forward-genetic screen in the tgd1-2 dgd1 mutant background led to the isolation of a new fad6-2 allele with a marked reduction in the amount of digalactosyldiacylglycerol. In contrast, the introduction of fad2, affecting fatty acid desaturation of lipids in the ER, into the two tgd mutant backgrounds did not further decrease the level of fatty acid desaturation in lipids of extraplastidic membranes. These results suggest that the role of TGD proteins is limited to plastid lipid import, but does not extend to lipid export from the plastid to extraplastidic membranes.

Arabidopsis TRIGALACTOSYLDIACYLGLYCEROL5 Interacts with TGD1, TGD2, and TGD4 to Facilitate Lipid Transfer from the Endoplasmic Reticulum to Plastids

“…The rationale for the screen is based on extensive genetic evidence showing that the fad6 mutant is dependent on import of polyunsaturated lipid precursors from the ER for thylakoid lipid assembly (Browse et al, 1989), a process involving TGD proteins (Xu et al, 2010). Whereas the fad6 mutant itself is similar to the wild type in terms of growth and development, the fad2 fad6 double mutant, which is deficient in the conversion of 18:1 to 18:2 in both the ER and plastids, is chlorotic because polyunsaturated fatty acids that are necessary for a competent photosynthetic membrane system are lacking (McConn and Browse, 1998).…”

Chloroplast lipid synthesis and lipid trafficking through ER–plastid membrane contact sites

“…The TGD1, TGD2 and TGD3 proteins form a bacterial-type ABC transporter (ATP-binding-cassette transporter) in the inner envelope of chloroplasts touching the outer envelope [17]. TGD1 is a permease and TGD3 is an ATPase providing the energy for lipid transport [11,25,56], probably making the lipid transfer from the ER to the chloroplast unidirectional [57]. TGD2 has an N-terminal transmembrane domain and a C-terminal soluble domain facing the intermembrane space [58].…”

Genetic analysis of Arabidopsis mutants impaired in plastid lipid import reveals a role of membrane lipids in chloroplast division

“…increased, 14 suggesting a decreased ratio of plastid-to-extraplastidic membranes in the double mutants. In support, ultrastructural analysis revealed that the thylakoid membrane systems in double mutants were severely underdeveloped with almost no grana stacking, particularly in the tgd4-3 fad6-1 double mutant.…”

“…In both tgds fad6-1 and tgd4-1 ats1-1 double mutants, there were more than 50% reductions in levels of MGDG with concomitant increases in phospholipids, particularly PC. 9,14 Whether these lipid changes are the cause of the chloroplast division defects in the double mutants can not be ascertained at this time. However, it has been shown that a similar extent of reduction in the level of MGDG in the pgp1 mutant does not compromise chloroplast division.…”