Structural insights and membrane binding properties of MGD1, the major galactolipid synthase in plants

Abstract

). The structures of cdMGD1 and cdMGD1:UDP have been deposited in the Protein Data Bank under accession numbers 4WYI and 4X1T, respectively. SUMMARYMonogalactosyldiacylglycerol (MGDG) and digalactosyldiacylglycerol (DGDG) are the major lipid components of photosynthetic membranes, and hence the most abundant lipids in the biosphere. They are essential for assembly and function of the photosynthetic apparatus. In Arabidopsis, the first step of galactolipid synthesis is catalyzed by MGDG synthase 1 (MGD1), which transfers a galactosyl residue from UDP-galactose to diacylglycerol (DAG). MGD1 is a monotopic protein that is embedded in the inner envelope membrane of chloroplasts. Once produced, MGDG is transferred to the outer envelope membrane, where DGDG synthesis occurs, and to thylakoids. Here we present two crystal structures of MGD1: one unliganded and one complexed with UDP. MGD1 has a long and flexible region (approximately 50 amino acids) that is required for DAG binding. The structures reveal critical features of the MGD1 catalytic mechanism and its membrane binding mode, tested on biomimetic Langmuir monolayers, giving insights into chloroplast membrane biogenesis. The structural plasticity of MGD1, ensuring very rapid capture and utilization of DAG, and its interaction with anionic lipids, possibly driving the construction of lipoproteic clusters, are consistent with the role of this enzyme, not only in expansion of the inner envelope membrane, but also in supplying MGDG to the outer envelope and nascent thylakoid membranes.

Structural basis of the molecular ruler mechanism of a bacterial glycosyltransferase

“…The GalNAc unit is located in the cleft formed between the N- and C-terminal domains. Unlike in some structural studies of GT-B enzymes 19 , 21 , 22 , the saccharide moieties were clearly visible not only for the non-hydrolyzable UDP-CH 2 -GalNAc, but also for the functional UDP-GalNAc (Supplementary Fig. 7 ).…”

“…2b ). Whereas similar helices were observed in other membrane-associated GTs such as the phosphatidyl mannosyltransferase PimA 17 – 19 , α2 is considerably longer in PglH than in other GT-B family members of known structure (Supplementary Fig. 6 ).…”

“…Instead, it contains a ruler helix that also mediates its association to the plasma membrane. This helix and its interaction with membrane lipids has been proposed for the related GTs GumK 18 , MGD1 19 , and MurG 42 . For PimA, it was shown that mutations in a cluster of positive residues interacting with head groups of the lipid bilayer reduced the activity in vivo, despite the fact that in vitro binding of donor substrate was unaffected 17 , 25 .…”

Genome-wide analysis of UDP-glycosyltransferase super family in Brassica rapa and Brassica oleracea reveals its evolutionary history and functional characterization

“…Moreover, the members (UGT81A1: AT4G31780; UGT81A3: AT5G20410; UGT81A4: AT2G11810) of UGT81 gene family were not classified in A. thaliana due to the absent UGTs conserved domain in the three UGTs. Through the analysis of conserved domains of UGT81 gene family, the three members of UGT81 gene family contained mnogalactosyldiacylglycerol (MGDG) synthase (PF06925) and MGDG synthase type C (PF04101) conserved domains, which were involved in the galactolipid biosynthesis [52, 53]. So, we excluded the analysis of UGT81 gene family in UGT super gene family among above seven plant species.…”

A predicted plastid rhomboid protease affects phosphatidic acid metabolism in Arabidopsis thaliana

“…However, it should be noted that many plants do not use the plastid pathway and predominately rely on the ER pathway for thylakoid lipid biosynthesis. We know the genes that encode the PA biosynthetic enzymes in the chloroplast that first acylate glycerol‐P and then lysophosphatidic acid (Kunst et al ., ; Nishida et al ., ; Xu et al ., ) as well as the genes encoding the galactosyltransferase MGD1, which uses diacylglycerol (DAG) (Jarvis et al ., ; Awai et al ., ; Rocha et al ., ) derived from dephosphorylation of PA by a phosphatase. However, the gene encoding the PA phosphatase (PAP) responsible for creating the bulk plastid DAG pool in vegetative tissues has only been tentatively identified (Nakamura et al ., ) and its topology within the inner envelope membrane has yet to be determined, although its activity has been measured in the chloroplast envelopes (Block et al ., ; Malherbe et al ., ).…”