Arabidopsis Sec14 proteins (SFH5 and SFH7) mediate interorganelle transport of phosphatidic acid and regulate chloroplast development

Abstract

Lipids establish the specialized thylakoid membrane of chloroplast in eukaryotic photosynthetic organisms, while the molecular basis of lipid transfer from other organelles to chloroplast remains further elucidation. Here we revealed the structural basis of Arabidopsis Sec14 homology proteins AtSFH5 and AtSFH7 in transferring phosphatidic acid (PA) from endoplasmic reticulum (ER) to chloroplast, and whose function in regulating the lipid composition of chloroplast and thylakoid development. AtSFH5 and AtSFH7 localize at both ER and chloroplast, whose deficiency resulted in an abnormal chloroplast structure and a decreased thickness of stacked thylakoid membranes. We demonstrated that AtSFH5, but not yeast and human Sec14 proteins, could specifically recognize and transfer PA in vitro. Crystal structures of the AtSFH5-Sec14 domain in complex with L-α-phosphatidic acid (L-α-PA) and 1,2-dipalmitoyl-sn-glycero-3-phosphate (DPPA) revealed that two PA ligands nestled in the central cavity with different configurations, elucidating the specific binding mode of PA to AtSFH5, different from the reported phosphatidylethanolamine (PE)/phosphatidylcholine (PC)/phosphatidylinositol (PI) binding modes. Quantitative lipidomic analysis of chloroplast lipids showed that PA and monogalactosyldiacylglycerol (MGDG), particularly the C18 fatty acids at sn -2 position in MGDG were significantly decreased, indicating a disrupted ER-to-plastid (chloroplast) lipid transfer, under deficiency of AtSFH5 and AtSFH7. Our studies identified the role and elucidated the structural basis of plant SFH proteins in transferring PA between organelles, and suggested a model for ER-chloroplast interorganelle phospholipid transport from inherent ER to chloroplast derived from endosymbiosis of a cyanobacteriumproviding a mechanism involved in the adaptive evolution of cellular plastids.

Role of SEC14-like phosphatidylinositol transfer proteins in membrane identity and dynamics

“…Interestingly, phosphatidic acid is the precursor to several lipids produced in chloroplasts and required for thylakoid assembly, and the sfh5 sfh7 double mutants had reduced amounts of such lipids. Hence, these findings strongly suggest that the SEC14-nodulin proteins SFH5 and SFH7 mediate the transport of phosphatidic acid from ER to chloroplast perhaps at interorganellar contact sites ( Yao et al., 2023 ). SEC14-nodulin proteins therefore play roles in different organs (roots and leaves) and are involved in different processes requiring lipid transfer, such as polarity control and organellar contacts.…”

“…The amphipathic gate helix keeps the lipid-binding site in an open or close conformation ( Sha et al., 1998 ; Schaaf et al., 2008 ; Sugiura et al., 2021 ; Hornbergs et al., 2022 ; Yao et al., 2023 ) ( Figure 1E ). Additional residues may favor orientation either towards the negatively charged membrane or the cytosol, and they may also steer specificity for the lipid substrate ( Sha et al., 1998 ; Schaaf et al., 2008 ; Sugiura et al., 2021 ; Hornbergs et al., 2022 ; Yao et al., 2023 ). Upon a heterotypic lipid exchange, an intermediate with two different lipophilic substrate-binding sites can form ( Schaaf et al., 2008 ).…”

“…A similar function is proposed for rice OsSNDP2 and OsSNDP3 during pollen tube elongation ( Moon et al., 2022 ). Two further SEC14-nodulin proteins from Arabidopsis have been recently studied in physiological contexts, which are SFH5 and SFH7 ( Yao et al., 2023 ). These two proteins were found to localize at ER and chloroplast membranes.…”

Role of SEC14-like phosphatidylinositol transfer proteins in membrane identity and dynamics

“…OsSNDP3 during pollen tube elongation (Moon et al, 2022). Two further SEC14-nodulin proteins from Arabidopsis have been recently studied in physiological contexts, which are SFH5 and SFH7 (Yao et al, 2023). These two proteins were found to localize at ER and chloroplast membranes.…”

Powdery mildew infection induces a non-canonical route to storage lipid formation at the expense of host thylakoid lipids to fuel its spore production

“…In Arabidopsis leaves, approximately 38% of newly synthesized FAs are utilized in the prokaryotic lipid-synthesis pathway, whereas the remaining 62% are directed towards the eukaryotic pathway (Browse et al 1986). A portion of acyl-chains from ER-assembled lipids are subsequently transported – likely as PA and/or DAG – back to the plastid to serve as substrates for thylakoid lipid synthesis (Yao et al 2023; Hölzl and Dörmann 2019). Triacylglycerols (TAGs), neutral storage lipids with three fatty acids attached to a glycerol backbone, are packaged into lipid bodies.…”