Biosynthesis of sulfoquinovosyldiacylglycerol in higher plants: the incorporation of 35SO4 by intact chloroplasts

Biosynthesis and functions of the plant sulfolipid

Higher-plant chloroplast membranes are composed primarily of four characteristic lipids, namely monogalactosyldiacylglycerol, digalactosyldiacylglycerol, sulfoquinovosyldiacylglycerol (SQDG), and phosphatidylglycerol. Among them, SQDG is the only sulfur-containing anionic glycerolipid and is the least prevalent component of photosynthetic membrane lipids. SQDG biosynthesis is mostly mediated by UDP-sulfoquinovose synthase (SQD1) and SQDG synthase (SQD2). Recently, another essential gene for SQDG synthesis, UGP3, was identified using transcriptome coexpression analysis and reverse genetics. UGP3 is a novel plastid UDP-glucose pyrophosphorylase that supplies UDP-glucose to SQD1 in plastids. In Arabidopsis, SQDG is dispensable under normal growth conditions but important in certain environments, particularly phosphate-depleted conditions. The function of SQDG under phosphate-limited growth conditions is highly correlated with the regulation of other plant glycerolipid biosyntheses. This review summarizes recent research defining the mechanism for SQDG biosynthesis and its biological function in higher plants, particularly under phosphate-starved conditions.

A chloroplastic UDP-glucose pyrophosphorylase from Arabidopsis is the committed enzyme for the first step of sulfolipid biosynthesis

Plants synthesize a sulfur-containing lipid, sulfoquinovosyldiacylglycerol, which is one of three nonphosphorus glycerolipids that provide the bulk of the structural lipids in photosynthetic membranes. Here, the identification of a novel gene, UDP-glucose pyrophosphorylase3 (UGP3), required for sulfolipid biosynthesis is described. Transcriptome coexpression analysis demonstrated highly correlated expression of UGP3 with known genes for sulfolipid biosynthesis in Arabidopsis thaliana. Liquid chromatography-mass spectrometry analysis of leaf lipids in two Arabidopsis ugp3 mutants revealed that no sulfolipid was accumulated in these mutants, indicating the participation of UGP3 in sulfolipid biosynthesis. From the deduced amino acid sequence, UGP3 was presumed to be a UDP-glucose pyrophosphorylase (UGPase) involved in the generation of UDP-glucose, serving as the precursor of the polar head of sulfolipid. Recombinant UGP3 was able to catalyze the formation of UDP-glucose from glucose-1-phosphate and UTP. A transient assay using fluorescence fusion proteins and UGPase activity in isolated chloroplasts indicated chloroplastic localization of UGP3. The transcription level of UGP3 was increased by phosphate starvation. A comparative genomics study on UGP3 homologs across different plant species suggested the structural and functional conservation of the proteins and, thus, a committing role for UGP3 in sulfolipid synthesis.

Questions remaining in sulfolipid biosynthesis: a historical perspective

The plant sulfolipid sulfoquinovosyldiacylglycerol was discovered by A.A. Benson in the late 1950s. The increasing availability of radioisotope-containing biological substrates such as (35)S-sulfate provided the means to discover novel biological compounds and to sketch out their biosynthetic pathways. During this time the structure of sulfolipid with its 6-deoxy-6-sulfo-alpha-D: -glucose (sulfoquinovose) headgroup was determined. Immediately, the origin of this unusual biological sulfonic acid mystified the scientific community and several proposals for its biosynthesis were developed and tested. Strong supportive evidence for the nucleotide pathway of sulfolipid biosynthesis became available with the discovery of the bacterial and plant genes encoding the enzymes of sulfolipid biosynthesis during the 1990s. This latter work was based on the foundations laid by A.A. Benson and confirmed one initial hypothesis on sulfolipid biosynthesis. An abbreviated summary of the turning points in defining the mechanism for sulfolipid biosynthesis and remaining issues in sulfolipid biochemistry are provided.

Native uridine 5'-diphosphate-sulfoquinovose synthase, SQD1, from spinach purifies as a 250-kDa complex

Sulfoquinovosyldiacylglycerol is a polar lipid present in photosynthetic membranes. It contributes to the negative surface charge of the membrane and plays a pivotal role under phosphate stress. The SQD1 protein is the key enzyme involved in the formation of the sulfolipid head group precursor, uridine 5(')-diphosphate (UDP)-sulfoquinovose, from UDP-glucose and sulfite. A cDNA encoding the spinach SQD1 protein was isolated and functionally expressed in Escherichia coli. The recombinant enzyme was compared to the native enzyme purified from isolated spinach chloroplasts. While the K(m) for UDP-glucose was indistinguishable for the two forms, the K(m) for sulfite was more than fourfold lower (< microM) for the native enzyme. Sizing by gel filtration indicated that the native form purified as a large complex of approximately 250 kDa, which is more than twice as large as the calculated size for the homodimer. It is proposed that in vivo SQD1 forms a complex with accessory proteins.

Recombinant Arabidopsis SQD1 converts udp-glucose and sulfite to the sulfolipid head group precursor UDP-sulfoquinovose in vitro

The sulfolipid sulfoquinovosyldiacylglycerol is a component of plant photosynthetic membranes and represents one of the few naturally occurring sulfonic acids with detergent properties. Sulfolipid biosynthesis involves the transfer of sulfoquinovose, a 6-deoxy-6-sulfoglucose, from UDP-sulfoquinovose to diacylglycerol. The formation of the sulfonic acid precursor, UDP-sulfoquinovose, from UDP-glucose and a sulfur donor is proposed to be catalyzed by the bacterial SQDB proteins or the orthologous plant SQD1 proteins. To investigate the underlying enzymatic mechanism and to elucidate the de novo synthesis of sulfonic acids in biological systems, we developed an in vitro assay for the recombinant SQD1 protein from Arabidopsis thaliana. Among different possible sulfur donors tested, sulfite led to the formation of UDP-sulfoquinovose in the presence of UDP-glucose and SQD1. An SQD1 T145A mutant showed greatly reduced activity. The UDP-sulfoquinovose formed in this assay was identified by co-chromatography with standards and served as substrate for the sulfolipid synthase associated with spinach chloroplast membranes. Approximate K(m) values of 150 microm for UDP-glucose and 10 microm for sulfite were established for SQD1. Based on our results, we propose that SQD1 catalyzes the formation of UDP-sulfoquinovose from UDP-glucose and sulfite, derived from the sulfate reduction pathway in the chloroplast.

BIOSYNTHESIS AND FUNCTION OF THE SULFOLIPID SULFOQUINOVOSYL DIACYLGLYCEROL

The sulfolipid sulfoquinovosyl diacylglycerol is an abundant sulfur-containing nonphosphorous glycerolipid that is specifically associated with photosynthetic membranes of higher plants, mosses, ferns, algae, and most photosynthetic bacteria. The characteristic structural feature of sulfoquinovosyl diacylglycerol is the unique head group constituent sulfoquinovose, a derivative of glucose in which the 6-hydroxyl is replaced by a sulfonate group. While there is growing evidence for the final assembly of the sulfolipid by the transfer of the sulfoquinovosyl moiety from UDP-sulfoquinovose to the sn-3 position of diacylglycerol, very little is known about the biosynthesis of the precursor UDP-sulfoquinovose. Recently, a number of mutants deficient in sulfolipid biosynthesis and the corresponding sqd genes have become available from different organisms. These provide novel tools to analyze sulfolipid biosynthesis by a combination of molecular and biochemical approaches. Furthermore, the analysis of sulfolipid-deficient mutants has provided novel insights into the function of sulfoquinovosyl diacylglycerol in photosynthetic membranes.

A new pathway for the synthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol

A new pathway is proposed for the biosynthesis of the plant sulpholipid, sulphoquinovosyldiacylglycerol. The pathway begins at UDP-glucose and involves the formation therefrom of UDP-4-ketoglucose-5-ene to which is subsequently added sulphite (or its metabolic equivalent). Evidence consistent with this pathway, rather than with the previously proposed 'sulphoglycolytic' route, was obtained from experiments with pea chloroplast preparations. The evidence included the failure of potential inhibitors of the sulphoglycolytic pathway to alter the rate of synthesis of sulpholipid and the stimulation of the incorporation of 35SO4(2-) into the latter by UTP. Radioactivity was effectively incorporated into sulpholipid from UDP-[14C]glucose and this radiolabelling was stimulated by the addition of methyl alpha-glucose-enide or of an enzyme system known to be forming (although not accumulating) UDP-4-ketoglucose-5-ene. This new pathway is also consistent with other data in the literature.

Synthesis of different nucleoside 5'-diphospho-sulfoquinovoses and their use for studies on sulfolipid biosynthesis in chloroplasts

6-Sulfo-alpha-D-quinovopyranosyl phosphate was reacted with different nucleoside monophosphate morpholidates to form ADP-, CDP-, GDP- and UDP-sulfoquinovose. Analytical and preparative HPLC of these nucleotides was performed on reversed-phase columns using volatile buffer systems as eluant. The isolated compounds were characterized by NMR spectroscopy (except the CDP derivative) and used for an investigation of sulfolipid biosynthesis by chloroplasts. For this purpose intact spinach chloroplasts were biosynthetically preloaded with radioactive diacylglycerol to provide a sulfoquinovosyl acceptor. When sulfosugar nucleotides were added to such prelabelled intact organelles, the background levels of sulfolipid biosynthesis did not rise. On the other hand, after osmotic shock of prelabelled chloroplasts sulfolipid labelling was significantly increased by the addition of UDP- or GDP-sulfoquinovose. The same stimulation was observed with isolated envelope membranes, and UDP-sulfoquinovose proved to be twice as active as the GDP derivative. From these results it was concluded that the final step in sulfolipid biosynthesis is catalyzed by a UDP-sulfoquinovose: 1,2-diacylglycerol 3-O-alpha-D-sulfoquinovosyltransferase. This chloroplast enzyme cannot use exogenously supplied sulfosugar nucleotides, which as membrane-impermeable compounds are expected to be formed in vivo within chloroplasts.

The biosynthesis of sulfoquinovosyldiacylglycerol: studies with groundnut (Arachis hypogaea) leaves

The biosynthetic pathway of sulfoquinovosyldiacylglycerol (SQDG) was investigated using groundnut (Arachis hypogaea) leaf discs and 35S-labeled precursors. [35S]SO4(2-) was actively taken up by the leaf discs and rapidly incorporated into SQDG. After 2 h, 1.5% of the [35S]SO4(2-) added to the incubation medium was taken up, of which 28% was incorporated into SQDG. The methanol-water phases of the lipid extracts of the leaf discs were analyzed for the 35S-labeled intermediates. Up to 2 h of incubation, cysteic acid, 3-sulfopyruvate, 3-sulfolactate, 3-sulfolactaldehyde, and sulfoquinovose (SQ) which have been proposed as intermediates [Davies et al. (1966) Biochem. J. 98, 369-373] were not labeled. Only a negligible amount of radioactivity was observed in these compounds after incubation for 4 h and more. Addition of sodium molybdate inhibited the uptake of [35S]SO4(2-) as well as its incorporation into SQDG by the leaf discs, suggesting that 3'-phosphoadenosine-5'-phosphosulfate may be involved in the biosynthesis of SQDG. Addition of unlabeled cysteic acid to the incubation medium enhanced the uptake of [35S]SO4(2-) but did not affect its incorporation into SQDG. 35S-labeled cysteic acid was taken up by the leaf discs and metabolized to sulfoacetic acid but not incorporated into SQ or SQDG. These results show that cysteic acid is not an intermediate in SQDG biosynthesis. [35S]SQ was taken up by the leaf discs and incorporated into SQDG in a time-dependent manner. [35S]Sulfoquinovosylglycerol was also taken up by the leaf discs but not incorporated into SQDG. It is concluded that SQDG is not biosynthesized by the proposed sulfoglycolytic pathway in higher plants. Though [35S]SQ was converted to SQDG, the rates are much lower compared to [35S]SO4(2-) incorporation, which suggests that a more direct pathway involving sulfonation of a lipid precursor may exist in higher plants.

Biosynthesis of sulfoquinovosyldiacylglycerol in higher plants: the origin of the diacylglycerol moiety

The positional distribution of fatty acids in chloroplast polar lipids and phosphatidylcholine from leaves of four plants has been measured in order to determine the origin of the diacylglycerol (DAG) moieties of each lipid. In spinach and tobacco, the DAG of sulfoquinovosyldiacylglycerol (SQDG), monogalactosyldiacylglycerol (MGDG) and digalactosylglycerol (DGDG) were derived partly from the chloroplast and partly from the cytoplasm. The contribution of the chloroplast pathway differed for each lipid, but in both plants the proportion of a lipid derived from that pathway was in the order SQDG greater than MGDG greater than DGDG. In contrast, all the DAG moieties of the three glycolipids of wheat and cucumber were derived from the cytoplasm. The DAG moiety of chloroplast phosphatidylglycerol was synthesized in the chloroplast in all four plants.