[Separation and role of diacylglycerols in the envelope of spinach chloroplasts (author's transl)]

Lipid requirement and kinetic studies of solubilized UDP-galactose:diacylglycerol galactosyltransferase activity from spinach chloroplast envelope membranes

We have demonstrated a lipid requirement for the UDPgalactose:1,2-diacylglycerol 3-beta-D-galactosyl-transferase (or monogalactosyldiacylglycerol synthase; EC 2.4.1.46), an enzyme involved in the biosynthesis of monogalactosyldiacylglycerol, solubilized from chloroplast envelope membranes and partially purified by hydroxyapatite chromatography. The enzyme fraction was highly delipidated (<0.1 mg of lipid per mg of protein), and addition of lipids extracted from chloroplast membranes was necessary to reveal the activity. Acidic glycerolipids, and especially phosphatidylglycerol, were the best activators of the enzyme. The preparation of a delipidated enzyme fraction and the development of optimal assay conditions were prerequisites for the determination of the kinetic parameters for the hydrophobic substrate of the enzyme, diacylglycerol. In addition, we have demonstrated the existence of two substrate-binding sites: a hydrophobic one for diacylglycerol and a hydrophilic one for UDP-galactose.

Biochemical and topological properties of type A MGDG synthase, a spinach chloroplast envelope enzyme catalyzing the synthesis of both prokaryotic and eukaryotic MGDG

MGDG synthase, the enzyme that catalyzes the synthesis of the major chloroplast membrane lipid monogalactosyldiacylglycerol (MGDG), is encoded by a multigenic family. We have analyzed the biochemical properties, subcellular localization and membrane topology of a spinach chloroplast MGDG synthase, a representative member of the type A family from Spinacia oleracea (soMGD A), using a recombinant protein that was functionally overexpressed in Escherichia coli and specific polyclonal antibodies. We demonstrated that soMGD A could catalyze the synthesis of both 'prokaryotic' and 'eukaryotic' MGDG molecular species in vitro, with a selectivity for diacylglycerol similar to that of purified chloroplast envelope MGDG synthase activity. Furthermore, soMGD A was shown to be sensitive to chemical reagents (dithiothreitol, N-ethylmaleimide and o-phenanthroline) known to affect MGDG synthesis by the partially purified enzyme, as well as in isolated chloroplast envelope membranes. In spinach chloroplasts, soMGD A was localized by Western blot analysis in the inner envelope membrane. Topological studies demonstrated that soMGD A is a monotopic enzyme, embedded within one leaflet of the inner envelope membrane from spinach chloroplasts, a structure which may involve amphipathic alpha helices. We further demonstrated that in vitro, soMGD A precursor is imported and processed to its correct mature form in intact chloroplasts. These results show that soMGD A corresponds to a mature polypeptide of approximately 45 kDa. In addition, inactivation kinetics after gamma-ray irradiation strongly suggest that both native chloroplast envelope MGDG synthase and recombinant soMGD A have a functional molecular mass of 95-100 kDa, indicating that they are probably active as homodimers made of two 45-kDa subunits. This study suggests that, in spite of the growing evidence that MGDG synthesis is catalyzed by a multigenic family of enzymes, in spinach leaves both prokaryotic and eukaryotic MGDG syntheses could be attributable to a unique dimeric enzyme, provided that diacylglycerol is transported from the outer membrane to the inner membrane of the chloroplast envelope.

Chloroplast biogenesis. Cell-free transfer of envelope monogalactosylglycerides to thylakoids

An ATP- and temperature-dependent transfer of monogalactosylglycerides from the chloroplast envelope to the chloroplast thylakoids was reconstituted in a cell-free system prepared from isolated chloroplasts of garden pea (Pisum sativum) or spinach (Spinacia oleracea). Isolated envelope membranes, in which the label was present exclusively in monogalactosylglycerides, were prepared radiolabeled in vitro with [14C]galactose from UDP-[14C]galactose to label galactolipids as the donor. ATP-dependent transfer of radioactivity from donor to unlabeled acceptor thylakoids, immobilized on nitrocellulose strips, was observed. In some experiments linear transfer for longer than 30 min of incubation was facilitated by the addition of stroma proteins but in other experiments stroma was without effect or inhibitory suggesting no absolute requirements for a soluble protein carrier. Transfer was donor specific. No membrane fraction tested (plasma membrane, tonoplast, endoplasmic reticulum, nuclei, Golgi apparatus, mitochondria or thylakoids) (isolated from tissue radiolabeled in vivo with [14C]acetate) other than chloroplast envelopes demonstrated any significant ability to transfer labeled membrane lipids to immobilized thylakoids. Acceptor specificity, while not absolute, showed a 3-10-fold greater ATP-dependent transfer of labeled galactolipids from chloroplast envelopes to immobilized thylakoids than to other leaf membranes. The results provide independent confirmation of the potential for transfer of galactolipids between chloroplast envelopes and thylakoids suggested previously from ultrastructural studies and of the known location of thylakoid galactolipid biosynthetic activities in the chloroplast envelope.

Separation of chloroplast polar lipids and measurement of galactolipid metabolism by high-performance liquid chromatography

Procedures are described for the separation of polar lipids from plant chloroplasts by high-performance liquid chromatography, using a polar-modified silica column. Glycolipids and phospholipids were eluted with a gradient of 2-propanol/n-hexane (80:55, v/v) and 2-propanol/n-hexane/water/methanol (80:55:15:10, v/v). The lipids were detected by uv absorbance at 202 nm. Diacylglycerol and mono-, di-, and trigalactosyldiacylglycerol and phosphatidylcholine were separated on a LiChrosorb NH2 column (7-microns particles, Merck, FRG), but acidic lipids were retained. These lipids could be quantified from their 202-nm absorbance recording. The absorption coefficients obtained depended on the mean number of double bonds in the different lipid classes. The separation was applied for a rapid monitoring of the lipid composition in thylakoids and in fractionated inner and outer envelopes. The activities of galactosyltransferases involved in galactolipid metabolism, UDPGal:diacylglycerol galactosyltransferase and galactolipid:galactolipid galactosyltransferase, could be measured quantitatively in specific assays for both enzymes.

Membrane lipid metabolism in Chlamydomonas reinhardtii 137+ and y-1: II. Cytochemical localization of acyltransferase activities

We have cytochemically localized the acyltransferase activities in the alga Chlamydomonas reinhardtii. Glycerol 3-phosphate (G3P) acyltransferase and lysophosphatidate (LPA) acyltransferase activities were cytochemically assayed utilizing biochemically optimized reaction conditions (Jelsema et al., 1982). The cytochemical assays clearly demonstrate that in the wild-type cells (137+) and the y-1 mutant, both enzymes were present in the photosynthetic membranes, envelope, and pyrenoid tubules of the chloroplast. Activity was also localized to the Golgi apparatus and its associated vesicles. Additionally, LPA acyltransferase activity was found associated with the outer mitochondrial membranes. The cytochemical data from this study confirm the biochemical data obtained using purified chloroplast membranes (Jelsema et al., 1982) and establishes the presence of these glycerolipid-synthesizing enzymes in the photosynthetic membranes of the chloroplast. These findings support the earlier reports of the presence and activity of lipid-synthesizing enzymes in the chloroplast, and also is in agreement with the postulated role of the pyrenoid tubules in the synthesis of the thylakoid membranes. This report differs in that these lipid-synthesizing enzymes have been localized to the chloroplast photosynthetic membranes themselves as well as exhibiting significant levels of activity for both enzymes in the Golgi apparatus. During light-induced chloroplast biogenesis in the yellow mutant of C. reinhardtii (y-1), the photosynthetic membranes appear to be the primary site of acyltransferase activity, suggesting that in situ synthesis of the membrane lipids during this period of rapid membrane formation is the primary mode for the synthesis of the thylakoid lipids. That these intrinsic thylakoid enzyme activities are not exclusively a function of the growth phase of the cell, but are found in mature chloroplast of the 137+ cells as well, further supports the conclusion that the photosynthetic membranes of the chloroplast have the capacity for synthesis of their own membrane lipids.

Labelling in vivo and in vitro of molecular species of lipids from chloroplast envelopes and thylakoids

Lipid mixtures from chloroplast envelope and thylakoid membrane were isolated after different labelling times in vivo and in vitro and separated into major components. The isolated compounds were subjected to analyses such as separation of molecular species, determination of radioactivity in fatty acids and water-soluble hydrolysis products and radio gas-liquid chromatography of fatty acid mixtures. In the case of monogalactosyl and digalactosyl diacylglycerol these procedures were also applied to several individual molecular species. To investigate the extent of de novo synthesis these species were also used for methylation studies and their fatty acids subjected to alpha-oxidation. In envelope membranes diacylglycerols and monogalactosyl diacylglycerols may each be separated into several distinct and non-mixing pools. Molecules made de novo with oligoene fatty acids are very efficient substrates for galactosylation in vivo. The time-dependent changes in patterns of galactolipid molecular species may indicate a desaturation of acyl chains operating in close contact to intact lipids. After isolation, envelopes incorporated UDP-[14C]galactose into completely different patterns of galactolipids and molecular species pointing to changed properties of this membrane system or to a loss of regulatory factors.

Galactolipid synthesis in Vicia faba leaves. V. Redistribution of 14C-labelling in the polar moieties and the 14C-labelling kinetics of the fatty acids of the molecular species of mono- and digalactosyl diacylglycerols

The redistribution of 14C in the galactosyl glycerol and digalactosyl glycerol and the labelling kinetics of the fatty acids of mono- and digalactosyl diacylglycerol molecular species were examined over a period of 24 h following 14CO2-feeding of Vicia faba leaves. The results indicate that monogalactosyl diacylglycerol is formed from highly unsaturated diacylglycerols and that further desaturation of its C18 fatty acids occurs after formation of this galactolipid. There appear to be at least two pools of monogalactosyl diacylglycerol, differing in the degree of unsaturation of the fatty acids, from which digalactosyl diacyglycerol is formed. Digalactosyl diacylglycerol fatty acids did not appear to undergo further desaturation. A model for galactolipid biosynthesis and fatty acid desaturation is presented. There appear to be two distinct groups of diacyglycerol precursors, one consisting of C18/C18 molecular species derived from phosphatidylcholine, the other of C16/C18 molecular species derived from phosphatidylglycerol. The rapid turnover of monogalactosyl diacylglycerol to digalactosyl diacylglycerol from the C16/C18 group leads to one of the major differences in fatty acid composition between the two galactolipids observed in most plants.

Linoleate and alpha-linolenate synthesis by isolated spinach (Spinacia oleracea) chloroplasts

Diacylgalactosylglycerol synthesis was a prerequisite for the incorporation of [1-14C]-acetate into linoleate and alpha-linolenate of isolated spinach (Spinacia oleracea) chloroplasts. Oleate at position 1 of diacylgalactosylglycerol was desaturated to linoleate and alpha-linolenate both in the light and in the dark. Some desaturation of palmitate was also observed after prolonged incubations.

Characterization of lipids from chloroplast envelopes

The major neutral, glycolipids and phospholipids from envelopes of spinach chloroplasts were analyzed with respect to proportions, positional distribution and pairing of fatty acids. All specificities in the diacylglycerol portions of lipids known from previous analyses of lipids from whole leaves were also found in envelope lipids. Diacylglycerols and galactolipids share a common diacylglycerol portion. The only exception is digalactosyl diacylglycerol, which contains 18:3/16:0 but lacks 18:3/16:3 species reverting the distribution in other galactolipids. Phosphatidylcholine, phosphatidylglycerol and sulfoquinovosyl diacylglycerol are distinct from the galactolipids, because each one has a unique diacylglycerol profile. The diacylglycerol species present in phosphatidylcholine and galactolipids or free diacylglycerols do not provide evidence for a biogenetic relation between phosphatidylcholine and galactolipids at the level of envelopes.

Galactolipid synthesis in chromoplasts in vitro : Specificities of acyl-and galactosyltransferases

Isolated chromoplasts from Narcissus pseudonarcissus flowers contain: a fatty acid synthesizing system; acyl-CoA synthetase (EC 6.2.1.3); glycero-phosphate acyltransferase (EC 2.3.1.15); acylglycero-phosphate acyltransferase; phosphatidate phosphatase (EC 3.1.3.4); diacylglycerol galactosyltransferase (EC 2.4.1.46); and diacylgalactosylglycerol galactosyltransferase, i.e. all enzymatic activities necessary for the synthesis of diacylgalactosylglycerol and diacylgalabiosylglycerol from acetate, HCO - (3) , sn-glycerol 3-phosphate, and UDP-D-galactose. Diacylgalactosylglycerol and diacylgalabiosylglycerol, however, are synthesized from these precursors to only a very low extent in an in vitro system. This is attributed to a specificity of diacylglycerol galactosyltransferase for highly unsaturated diacylglycerols. Specificities of acyltransferase reactions were also found.

Fatty acid biosynthesis in the leaves of barley, wheat and pea

1. The incorporation of radioactivity from [1-14C]acetate into the leaf lipids of barley, pea and wheat has been studied in pulse-labelling experiments. 2. There was little increase in the total labelling of lipids after the leaves were transferred to non-radioactive medium. However, there was an increase in the relative labelling of unsaturated fatty acids. In addition, there was an increase in the relative labelling of diacylgalactosylglycerol. 3. The principal radioactively labelled acyl lipids were diacylgalactosylglycerol and phosphatidylcholine. Phosphatidylcholine showed a decreasing proportion of [14C]oleate and an increasing amount of [14C]linoleate with time. Diacylgalactosylglycerol also had decreasing amounts of [14C]oleate but, in addition, had an increasing proportion of [14C]linolenate with time. 4. The absence of significant amounts of [14C]linolenate in phosphatidylcholine appeared to exclude a role for this phospholipid in linoleate desaturation. 5. The specific radioactivities of oleate and linoleate in phosphatidylcholine, diacylgalactosylglycerol and diacylgalabiosylglycerol were very similar in any single experiment. It was concluded that these fatty acids can rapidly exchange between the three intact lipids.

Fatty acid synthesis by isolated chromoplasts from the daffodil. [14C]Acetate incorporation and distribution of labelled acids

Isolated daffodil (Narcissus pseudonarcissus) chromoplasts showed high rates of [14C]acetate incorporation into lipids. The fatty acids synthesized were predominantly palmitic acid (93%). The radioactivity incorporated was shared mainly between long-chain acyl-CoA (25%), free fatty acids (24%), phosphatidic acid (17%), diacylglycerol (15%), and phosphatidycholine (11%). Galactolipids were not labelled. ATP, NaHCO3, and also the structural integrity of the organelles were essential. Omission of exogenous CoA led to a decreased incorporation (49%); under these conditions the label was distributed mainly between free fatty acids (66%) and diacylglycerol (19%). Addition of lysophosphatidylcholine increased the labelling of phosphatidylcholine, whereas addition of glycerol 1-phosphate increased the labelling of phosphatidic acid and diacylglycerol. Acyl-CoA synthetase and acyl thioesterase (acyl-Coa) activities could be demonstrated. The results are discussed in terms of chromoplasts as non-photosynthetic organelles exhibiting high lipid-synthesizing capabilities.

Site of synthesis of phosphatidic acid and diacyglycerol in spinach chloroplasts

The enzymatic synthesis of lysophosphatidic acid, phosphatidic acid, monoacylglycerol and diacylglycerol from sn-[14C]glycerol 3-phosphate occurs in purified chloroplasts. The results indicate that: (1) the chloroplast extract contains a soluble acylase (acyl-CoA: sn-glycerol 3-phosphate acyltransferase); (2) the envelope fraction contains an acyl-CoA synthetase, a bound acylase (acyl-CoA: acyl-sn glycerol 3-phosphate acyltransferase) and a phosphatidic acid phosphatase; without chloroplast extract in the incubation medium, the envelope is unable to incorporate sn-glycerol 3-phosphate into phosphatidic acid and diacylglycerol; addition of chloroplast extract to the incubation medium induced a fast increase of the incorporation of sn-glycerol 3-phosphate into phosphatidic acid and diacylglycerol; thylakoids being unable to incorporate sn-glycerol 3-phosphate (in presence or absence of soluble chloroplast extract in the incubation medium) our results indicate that the envelope of spinach chloroplast is the site of phosphatidic acid and diacylglycerol synthesis; (3) diacylglycerol actively synthesized by the envelope is also the substrate for the first galactosylation enzyme.

Labelling studies in vivo on the metabolism of the acyl and glycerol moieties of the glycerolipids in the developing maize leaf

1. When [2-3H]glycerol was supplied to developing maize-leaf laminae, label entered 3-sn-phosphatidycholine at a linear rate essentially from zero time, whereas other lipids were labelled at accelerating rates. On transfer of laminae from [3H]glycerol to unlabelled glycerol, radioactivity was rapidly lost from 3-sn-phosphatidylcholine and accumulated in other lipids, principally monogalactosyl diacyglycerol. 2. Degradation of these lipids showed that 3H was present only in the glycerol moiety of the lipids. 3. In double-labelling pulse-chase experiments with [14C]acetate, which labelled essentially only fatty acids and [3H]glycerol similar amounts of 14C and 3H radioactivity were lost from 3-sn-phosphatidylcholine and accumulated by monogalactosyl diacylglycerol. 4. The different molecular species of both lipids isolated from laminae during a double-labelled pulse-chase study were separated by argentation t.l.c., and the changes in the amount of radioactivity and the 14C/3H ratio in different species were compared. The greatest loss of radioactivity during the period in unlabelled substrates occurred from the 3-sn-phosphatidylcholine species containing oleate and from the dilinoleate species, and radioactivity accumulated by monogalactosyl diacyglycerol was mainly in the dilinolenate species. However, despite the considerable change in the radioactivity in these species during the chase, the 14C/3H ratio in each of them remained relatively unchanged. 5. It is proposed that 3-sn-phosphatidylcholine in the developing leaf may serve as a donor or linoleate-containing diacyl-glycerols which are incorporated into other lipids, principally monogalactosyl diacylglycerol.

Long-chain acyl-coenzyme A synthetase activity of spinach chloroplasts is concentrated in the envelope

Purified chloroplasts were disrupted and then fractionated by discontinuous sucrose-density-gradient centrifugation. Envelopes contained long-chain acyl-CoA synthetase at a specific activity 80 times the activity in the lamellae or the stroma. Acetyl-CoA synthetase was concentrated in the stroma, and chlorophyll was confined to the lamellae membranes. Phospholipase D was not detected in any fraction.

Galactosylation of different monogalactosyldiacylglycerols by cell-free preparations from pea leaves

The formation of digalactosyldiglycerol (DGD) from monogalactosyldiglycerol (MDG) was studied in cell-free preparations from pea leaves. Under optimized conditions UDP-[U-14C]galactose was incorporated into a single lipid which was shown to be DGD by cochromatography before and after deacylation and by cocrystallization of the acetyl derivative. Seven MGD species were prepared which had different fatty acids and from zero to eight double bonds. These substrates were dispersed at identical overall concentrations by sonication in Triton X-100 solution and galactosylation rates were measured under optimized conditions. Galactosylation rates increased with the number of double bonds up to a total of six, beyond which the rate decreased again. These galactosylation rates cannot be related to difference in the diglyceride moieties of natural MGD and DGD.

Lipid metabolism in chromoplast membranes from the daffodil: Glycosylation and acylation

The non-photosynthetic chromoplast membranes from the corona ofNarcissus pseudonarcissus L. were investigated for their lipid synthetic capabilities. The following activities were detected: galactosylation of diacylglycerol and galactosydiacylglycerols, glycosylation of sterols, acylation of monogalactosyldiacylglycerol and steryl glycosides from an unknown endogenous donor, acylation of phospholipids from acyl-CoA, and acylation of phosphatidyl inositol from phosphatidyl choline. Furthermore, activities of an acyl thioesterase, a sugar epimerase, and a phospholipase A2 were measured.