Subcellular Localization of Monoglucosyldiacylglycerol Synthase in Synechocystis sp. PCC6803 and Its Unique Regulation by Lipid Environment

Synthesis of monogalactosyldiacylglycerol (GalDAG) and digalactosyldiacylglycerol (GalGalDAG), the major membrane lipids in cyanobacteria, begins with production of the intermediate precursor monoglucosyldiacylglycerol (GlcDAG), by monoglucosyldiacylglycerol synthase (MGS). In Synechocystis sp. PCC6803 (Synechocystis) this activity is catalyzed by an integral membrane protein, Sll1377 or MgdA. In silico sequence analysis revealed that cyanobacterial homologues of MgdA are highly conserved and comprise a distinct group of lipid glycosyltransferases. Global regulation of lipid synthesis in Synechocystis and, more specifically, the influence of the lipid environment on MgdA activity have not yet been fully elucidated. Therefore, we purified membrane subfractions from this organism and assayed MGS activity in vitro, with and without different lipids and other potential effectors. Sulfoquinovosyldiacylglycerol (SQDAG) potently stimulates MgdA activity, in contrast to other enzymes of a similar nature, which are activated by phosphatidylglycerol instead. Moreover, the final products of galactolipid synthesis, GalDAG and GalGalDAG, inhibited this activity. Western blotting revealed the presence of MgdA both in plasma and thylakoid membranes, with a high specific level of the MgdA protein in the plasma membrane but highest MGS activity in the thylakoid membrane. This discrepancy in the subcellular localization of enzyme activity and protein may indicate the presence of either an unknown regulator and/or an as yet unidentified MGS-type enzyme. Furthermore, the stimulation of MgdA activity by SQDAG observed here provides a new insight into regulation of the biogenesis of both sulfolipids and galactolipids in cyanobacteria.


Introduction
Proposed by the endosymbiotic theory to be the ancestors of plant chloroplasts, cyanobacteria are commonly used to investigate the biogenesis of the thylakoid membrane (TM). Two major ideas currently prevail, one considering cyanobacterial TM to be a physically separate entity and the other stating that this membrane has either permanent or transient contact with the plasma membrane (PM) [1]. Although much work on the synthesis of both proteins and pigments of the photosystems and their regulation has been performed with the unicellular cyanobacterium Synechocystis sp. PCC6803 (hereafter referred to as Synechocystis) [2][3][4], much less is known about the lipid components. If the TM and PM have no connection, both membranes might synthesize their own lipids. Alternatively, vesicular lipid transport, such as in chloroplasts [5], could occur between these membranes, although there is at present no direct evidence for this theory. On the other hand, if the membranes are connected, lipids synthesized by either or both could flow between them. Accordingly, a better understanding of lipid synthesis and its regulation in cyanobacteria is central to understanding the process of thylakoid biogenesis.
Although the major lipid constituents of membranes in most non-photosynthetic organisms are phospholipids, the major membrane lipids of chloroplasts and cyanobacteria do not contain a phosphorus (P) atom, possibly because both plants and many bacterial species have limited access to in-organic phosphorus (P i ) [6][7][8][9]. Monogalactosyldiacylglycerol (GalDAG: 1,2-diacyl-3-O-(b-D-galactopyranosyl)-sn-glycerol), one of the most abundant membrane lipids in the biosphere, often constitutes 50 mol% or even more of the total lipid content of cellular (or chloroplast) membranes in photosynthetic organisms [10,11]. Not only does GalDAG play an important structural role in membranes but it is also vital for the correct functioning of the photosynthetic complexes and pathways [12].
The various homologues of MGS can potentially be regulated both transcriptionally and posttranslationally, but the lipid environment is a major regulatory factor of this enzymatic activity. In organisms such as Acholeplasma laidlawii, Streptococcus pneumoniae, Mycoplasma pneumoniae and M. genitalium GlcDAG synthesis is dependent on the presence of an activator lipid, the anionic phosphatidylglycerol (PG) [18][19][20][21] Enzymatic activity is stimulated further by the presence of small amounts (up to 10 mol%) of phosphatidylethanolamine (PE), possibly due to increase of the spontaneous curvature of the bilayer when this zwitterionic lipid, which itself is prone not to form a bilayer, is present [18].
The MGS enzymes of A. laidlawii and S. pneumoniae are monotopic membrane proteins, whereas their counterparts in Synechocystis and M. pneumoniae [22] are predicted to be integral membrane proteins containing several transmembrane helices. Therefore, regulation of MgdA may differ from that for other enzymes with similar activities. To further understand how sugar lipid synthesis is regulated in Synechocystis, we have examined the localization of MgdA and its response to different lipid species in this organism. Both in vitro and in vivo measurements indicated that the regulation of Synechocystis MgdA by its lipid environment differs from that of other types of MGS enzymes. The steps involved in the synthesis of GlcDAG and GalDAG were found to occur in all internal membranes. Moreover, in silico sequence analysis revealed that homologues of MgdA in cyanobacteria are highly conserved. Based on our findings, a model for the biogenesis of cyanobacterial membranes is presented.

Media and Cell Growth
Synechocystis sp. 6803 was grown in glass bottles with 1 L BG11 medium [23] at 30uC, bubbling with sterile air and at a light intensity of 50 mE sec 21 m 22 . Cell cultures with an OD 730 1-1.5 were harvested by centrifugation (8,000 g, 4uC, 15 minutes), washed once in cold 20 mM potassium phosphate buffer (pH 7.5) and cell pellets were stored at 280uC. E. coli C41 (DE3) was routinely grown in LB medium (liquid or solid) for cloning and in 26 LB medium for enzyme overexpression, both supplemented with 50 mg mL 21 kanamycin.

Fractionation of Synechocystis Membranes
The pellets obtained as described above were washed once again with chilled lysis buffer (20 mM potassium phosphate buffer, pH 7.5), pelleted and resuspended in 1 mL lysis buffer containing protease inhibitors (Complete, EDTA-free, Roche). This suspension was then lysed by vortexing vigorously in the presence of 0.17-0.18 mm glass beads (Sartorius). Sucrose gradients were performed and aqueous two-phase partitioning carried out according to previously established protocols [24], with these modifications. Two-phase systems were incubated on ice for 15 minutes between centrifugations and thoroughly mixed by inversion in closed ice box. This procedure yields ''light'' (lowdensity) plasma membrane (PM1, less than 200 mg protein/L culture with an OD 730 = 1) and ''heavy'' (high-density) plasma membrane (PM2, roughly 2-3 mg protein/L culture), as well as purified TM (approximately 20 mg protein/L culture). PM1 separates in an orange band between 10 and 30% (w/w) borders in the sucrose gradient, while PM2 and TM are isolated from the 38-42% (w/w) region of this gradient using aqueous two-phase partitioning [24]. Protein was quantified by the Peterson procedure [25] and samples stored at 280uC for further use.

Protein Electrophoresis and Western Blotting
To assess purity, 5 mg protein from each fraction was solubilized in SDS-PAGE sample buffer supplemented with 7M urea, incubated for 30 minutes at room temperature and eletrophoresed in 12% TGX precast SDS-PAGE gels (BioRad). The gels were then transferred to PVDF membranes, which were subsequently probed with antibodies for characteristic TM (PsaA and CP43, Agrisera) and PM proteins (KtrE, obtained from Nobuyuki Uozumi and PixJ1, peptide antibody [4]), as well as for MgdA (a kind gift from Mie Shimojima). The blots were developed using an anti-rabbit antibody conjugated with horseradish peroxidase (GE Healthcare), enhanced chemiluminescence (ECL Prime, GE Healthcare) and a CCD camera (LAS-4000 Fujifilm), in the ''Precision'' mode and with automatic exposure time. For comparison of the migration of the bands detected to that of the prestained markers (PageRuler Plus, Thermo Scientific), blot images were merged with the MultiGauge software (v. 3.2, Fujifilm). Each individual blot was performed in triplicate.

Cloning MgdA Variants
Truncated MgdA constructs were generated in order to examine the function of each individual transmembrane region. All primers consisted of a gene-specific annealing sequence of approximately 20 bp and common overhangs containing the recombination sites (see Table S1). The major vector employed was pNIC28-Bsa4 (GenBank accession EF198106), developed by Posher Gileadi at SGC Oxford. All targets were fused with an Nor C-terminal hexahistidine tag to simplify purification, as well as a cleavage site for TEV protease to facilitate removal of this tag. Ligation independent cloning (LIC) was applied to all variants, in accordance with published procedures [26,27]. In brief, a RecAstrain of E. coli was utilized for the initial cloning and plasmid production, followed by re-transformation of the plasmid containing the target gene into the expression strain (a T1-phage resistant BL21-DE3 Rosetta strain developed in-house at the Protein Production and Purification Platform, SBS/NTU).

Expression of MgdA Variants in E. coli
For expression of MgdA variants, single colonies from LB-agar plates were grown overnight in 5 mL 26 LB medium supplemented with 50 mg.mL 21 kanamycin at 37uC, shaking at 200 RPM. The following morning, 2 mL of the starter culture was inoculated into 100 mL fresh 26LB medium containing antibiotic and grown for another 2 hours, after which the temperature was reduced to 22uC and the cultures incubated for an additional 60 minutes. Expression was induced by addition of 1 mM isopropyl b-D-1-thiogalactopyranoside (IPTG) and the cells allowed to grow for 22 hours at 22uC with shaking. Cultures were harvested by centrifugation (3,000 g, 4uC, 15 minutes), washed in wash buffer (100 mM HEPES with 20 mM MgCl 2 , pH 8.0), resuspended to an OD 600 = 40 in the assay buffer (100 mM HEPES, 20 mM MgCl 2 and 20 mM CHAPS, pH 8.0) and sonicated three times for 5 minutes each in a water-bath sonicator (Fisherbrand) to obtain whole cell lysates, which were stored in aliquots at 280uC until further use.
For large-scale expression of full-length MgdA, 500-mL cultures were induced as described above and total membrane isolated as follows: after washing in wash buffer supplemented with Complete EDTA-Free protease inhibitors (Roche Applied Science), the cells were homogenized (four passes at 12,000 psi), debris removed by slow-speed centrifugation (3,000 g, 4uC, 15 minutes) and the total membrane fraction then obtained by high-speed centrifugation (200,000 g, 4uC, 45 minutes). These fractions were resuspended in storage buffer (wash buffer supplemented with 10% glycerol) and kept in aliquots at 280uC. Protein content were quantified by the Peterson method [25].

Assay of MGS Activity and Thin-layer Chromatography (TLC)
MGS activity was assayed as described previously [18], with certain modifications. In brief, either 600 mg total membrane protein or 200 mg protein of the different Synechocystis membrane fractions was washed once with wash buffer, centrifuged (250,000 g, 4uC, 20 minutes) and resuspended in 100 mL of assay buffer. These suspensions were subsequently incubated on ice for 30 min, sonicated in a water-bath sonicator (1 minute, 3 times) and combined with 80 mL of mixed lipid micelles (Avanti Polar Lipids) [18], followed by additional 30 minutes of incubation on ice. 10 mL of UDP-[ 14 C]glucose solution (Perkin-Elmer, 0.02 mCi.mL 21 , 302 mCi.mmol 21 , diluted 1:5) was then added and incubation continued for another 30 minutes at 30uC, after which the reaction was stopped by adding 390 mL chloroform:methanol (1:2) and lipid phases extracted according to Bligh and Dyer [28,29]. Thereafter, these lipid phases were vacuum-dried, redissolved in 40 mL chlorophorm:methanol (2:1) and loaded onto an aluminum-backed TLC plate (Merck), which was developed with a chloroform:methanol:28% NH 4 OH system (78:42:6). After TLC run, the plate was dried and exposed at least 22 hours on a Kodak Imaging Screen K (Kodak) and analyzed with a Typhoon scanner (Typhoon Trio, GE Healthcare) using standard settings for autoradiography. Activity was calculated based on the intensity of the radioactive GlcDAG band in comparison to serial dilutions standard of UDP-[ 14 C] glucose, employing the ImageQuant TL software (v. 7.0, GE Healthcare).
Variants expressed in E. coli were assayed with this same protocol using 100 mL of culture at OD 600 = 40. Full-length MgdA recombinant was assayed using 200 mg total E. coli membrane suspension in 100 mL MGS assay buffer.

Pulse-labeling of Newly Synthesized Lipids in Synechocystis
Synechocystis cultures (at OD 730 = 1.5) were centrifuged, cell pellets were washed three times with fresh BG11 and then divided into 4 individual cultures in 100 mL E-flasks (final OD 730 of 15 in 30 mL of BG11). Following 2 hours of incubation at 30uC with air bubbling, acyl chains were labeled by addition of 12 mCi [ 14 C]acetate (Perkin-Elmer) to each culture (corresponding to a final concentration of 7.6 mM), which can be taken up rapidly by the cells. Then, these mixtures were incubated at 30uC with air bubbling and at the time intervals indicated, labeling was stopped by addition of 1 mL of 1 M sodium acetate (pH = 7.5) to each culture. Cells were transferred to a 50-mL Falcon tube, cooled rapidly in liquid nitrogen to slow down metabolism and centrifuged (15 minutes at 8,000 g, 4uC). The resulting pellets were flash-frozen in liquid nitrogen and stored at 280uC, until preparation of total membrane, PM1, PM2 and TM fractions as described above. 5,000 CPM from the total membrane fraction at each time point and 2,000 CPM from each isolated fraction were subjected to TLC and analyzed as described above.

Subcellular Localization of MgdA Protein and Activity in Synechocystis
In plant chloroplasts, lipid synthesis occurs exclusively in the two envelope membranes, rather than in the intraorganellar TM. Therefore, formation of the TM should be entirely dependent on lipid transport from these envelopes, most likely in the form of lipid vesicles [30]. To localize synthesis of the galactolipid precursor GlcDAG in Synechocystis, we investigated the distribution of both MgdA protein and activity in the different membranes.
The purity of the Synechocystis membrane fractions isolated by sucrose gradient and two-phase partition [24] was confirmed by western blots (Figure 1). The marker proteins for the TM (CP43 and PsaA) only exist in TM fraction, while PM marker proteins (PixJ1 [4] and KtrE (also known as DgdA [31])) are only present in PM1 and PM2 fractions. In comparison to the unambiguous distribution of these markers, MgdA was detected in both PM1 and PM2, as well as in TM, although the amount present in TM was significantly lower (Fig. 1).
To our surprise, the TM fraction exhibited the highest MGS activity, followed by the PM1 and the PM2 fractions (Fig. 2). Previously, Omata and Murata [32] reported that ''lighter'', ''cytoplasmic'' membranes isolated from Synechococcus on a sucrose gradient demonstrated higher MGS activity than ''heavier'' membranes, which is consistent with our present findings, since the PM1 fraction should correspond to their lighter fraction. Moreover, using our aqueous two-phase system we were able to further separate their ''heavy'', ''thylakoid'' fraction into the TM and PM2 fractions (as shown in [24]) and found that the highest specific activity was present in TM (Fig. 2). Accordingly, the lower specific activity observed by Omata and Murata for the ''thylakoid membrane'' fractions [32] may be due to the simultaneous presence of PM2 (with low specific activity) in the same fraction as TM.

Localization of de novo Lipid Synthesis in Synechocystis Membranes
When examining de novo lipid synthesis in Synechocystis membranes by labeling acyl chains with [ 14 C]-acetate, it was necessary to label for at least 10 minutes ( Figure S1). Within 10 minutes, selectively labeled GlcDAG had already accumulated in the total membrane fraction; during the following 3 hours, over 90% of this labeled precursor was converted into GalDAG, while, surprisingly, labeling of GalGalDAG required more than 24 hours (Fig. 3A). The relatively rapid biosynthesis of GlcDAG and its conversion to GalDAG highlight the indispensability of these lipids in cyanobacteria. It was shown previously that deletion of the enzyme responsible for synthesis of GalGalDAG (DgdA) exerts only minor effects on cell growth and photosynthesis reactions [17] and its slow rate of synthesis here also supports a less critical role for this lipid in Synechocystis. The non-sugar lipids PG and SQDAG were also labeled, even at the earliest time-points, and the level of this labeling appeared to remain rather constant. In experiments designed to further clarify the localization of GlcDAG synthesis, purified PM (PM1 and PM2) and TM fractions from the pulse-labeled cells were all found to contain radio-labeled GlcDAG even at the first time-point (10 minutes) tested after addition of [ 14 C]-acetate (Fig. 3B). This observation is consistent with the presence of MGS activity in all these membranes (Fig. 2). Furthermore, the PM1, PM2 and TM fractions all epimerized GlcDAG to GalDAG. Therefore, it can be concluded that all of these membranes harbor the machinery required to synthesize GlcDAG and convert it to GalDAG.

Influence of the Lipid Environment on MGS Activity
TM is the most abundant membrane fraction in most cyanobacterial cells and large amounts of sugar lipids are required as building blocks for its biogenesis. This explains the necessity for higher glucosyltransferase activity in the TM rather than PM2. However, the lower level of MgdA protein in the TM (Fig. 1) raises the question as to how sugar lipid synthesis is regulated in Synechocystis. Since the lipid composition of the membrane influences lipid glycosyltransferase (GT) activities in other organisms [18][19][20][21]33], we tested this possibility here as well.
Under normal conditions GalDAG and GalGalDAG can account for approximately 50 and 26 mol%, respectively, of the total lipid in Synechocystis membranes [17]. However, the presence of either of these lipids in our mixed-micelle assay system at any of the concentrations tested lowered MgdA activity (Fig. 4A), with GalDAG exerting a more pronounced effect. We propose that this represents feedback inhibition. In Synechocystis, the pathways for sulfolipid and galactolipid synthesis share the same initial substrate (DAG) and must therefore be delicately balanced. Koichiro Awai and co-workers [17] have shown that when GalGalDAG synthesis is prevented (by a dgdA knock-out mutation), the amount of SQDAG in the membranes increases by 60% (from 14.3 to 22.5 mol%), while the level of GalDAG increases by only 20% (from 49.7 to 60.4 mol%). Together with our data, these observations indicate that regulation of these two synthetic pathways is closely coordinated.
PG and other anionic phospholipids stimulate the activity of several different lipid GTs in various organisms, such as A. thaliana [18,19,33] and Homo sapiens (unpublished observation) substantially. However, here PG exerted only a marginal stimulatory effect (2568%) on MgdA activity at the maximal concentration tested (40 mol%) and was actually an entirely unnecessary component of the mixed micelle (Fig. 4B), in complete contrast to similar enzymes in other organisms (e.g., A. laidlawii). In E. coli, PG appears to be the major regulator of lipid synthesis, also controlling a key step in synthesis of PE, the most abundant lipid in this organism, by a mechanism involving surface charge and density [34]. Phosphatidylethanolamine (PE), which stimulates certain lipid GTs but is not found in the inner membranes of Synechocystis, also exhibited a mild activity-enhancing effect on MgdA, here 6067%, at 40 mol% concentration (Fig. 4B).
The most potent activator of MgdA activity was a constituent of the photosynthetic membrane, SQDAG, which doubled this activity at a concentration of 10 mol% (Fig. 4B) and stimulated 7fold at the highest concentration tested (40 mol%). This is the first time such pronounced stimulation of a lipid GT by SQDAG has been reported. The more detailed discussion will be elucidated in the later paragraphs.

Membrane Curvature has Little Effect on MgdA Activity
The ratio between lipids that are prone to form bilayers or not influences membrane curvature and lipid phase equilibria, which in turn affect functional features of membrane proteins [18,35,36]. For example, the activity of lipid GTs is sensitive to membrane curvature and surface charge [18,19,21,33,37]. In addition, in the presence of anionic lipids MGS and DGS in A. laidlawii (alMGS and alDGS) are stimulated by the zwitterionic (neutral) PE and other nonbilayer-prone lipids [18,35,38].
Both GalDAG and GalGalDAG have neutral headgroups, but only the latter forms bilayers [36]. However, addition of either of these two major sugar lipids with different effects on membrane curvature impaired MgdA activity (Fig. 4A). Moreover, although GalDAG and PE both have neutral headgroups and are not prone to form bilayers, PE stimulated MgdA activity somewhat (Fig. 4A  and B). Similarly, GalGalDAG and SQDAG, both of which form bilayers, exert opposite effects (Fig. 4A and B). It appears likely that in Synechocystis MgdA activity is not influenced by the bilayer curvature, in contrast to alMGS and alDGS in A. laidlawii [18,35]. We suggest that, as final products of the pathway, GalDAG and GalGalDAG interact with and allosterically regulate MgdA (or a regulatory interacting partner). This would allow the cells to maintain the proper lipid composition of their membranes by reducing production of the precursor GlcDAG when sufficient galactolipids are already present (Fig. 5).

SQDAG is a Potent and Specific Activator of Synechocystis MgdA
Synechocystis MgdA activity was specifically and very potently stimulated by SQDAG, a negatively charged lipid component of cyanobacterial membranes (Fig. 4B). In an attempt to understand the underlying mechanism, we performed an experiment similar to the one previously conducted with the A. laidlawii enzymes [40]. Since the SQDAG headgroup contains a sulfonic acid rather than phosphate moiety, MgdA activity was assayed in the presence of increasing concentrations of sulfate ions and unexpectedly, as shown in Fig. 4C, as little as 20 mM sulfate, in the presence of 30 mol% PG, decreased this activity by 52% and this suppression was even greater at higher concentrations of sulfate. Thus, the stimulation by SQDAG cannot be explained either by its negative surface or sulfate/sulfonic acid moiety.
When mgdA gene was expressed in E. coli, only a small amount of the recombinant enzyme was produced but the bacterial membranes exhibited potent MGS activity that was stimulated by SQDAG in the same manner as above (Fig. 4D). This observation provides strong support for the unique and probably specific interaction of SQDAG with MgdA. SQDAG contains a large polar head with a negative charge located some distance from the hydrophobic portion of the membrane. As sulfate alone did not activate MgdA, activation by SQDAG must be somehow dependent on the nature of the head group and/or of the entire molecule. It is noteworthy that lipids with a similar (sugar) head group, but no negative charge (such as GalDAG and GalGal-DAG), were inhibitory (Fig. 4A).
Curiously, synthesis of SQDAG is not an absolute requirement for all cyanobacteria. Synechocystis strains lacking the sqdB gene require exogenous SQDAG for survival, whereas in Synechococcus elongatus sp. PCC7942 (Synechococcus) such a knockout produced no obvious effect on cell growth [39]. This difference might be explained by the recent observation that deletion of the sqdB gene affects DNA replication and cell division in Synechocystis, but not in Synechococcus [40].
We propose that this unique regulation of MgdA activity by SQDAG might be involved in maintaining lipid homeostasis in cyanobacteria, especially under stress conditions. For example, in the absence of inorganic phosphate, both Synechocystis and Synechococcus accumulate SQDAG and GalGalDAG at the expense of PG [41]. Such an exchange of phospholipids for sulfolipids has been suggested to play an important evolutionary role for picocyanobacteria, which must compete with heterotrophic bacteria for a limited supply of phosphate [42]. The situation is similar in freshwater environments [43][44][45] and Synechocystis can also take up and incorporate exogenous lipids such as SQDAG [43].
Control of lipid-synthesizing enzymes in both Synechococcus and Synechocystis must be at the post-translational, rather than the transcriptional level, since in neither organism is MgdA among the proteins whose expression is regulated by the SphS-SphR twocomponent system in response to inorganic phosphate [46,47]. In fact, MgdA can be regulated post-translationally by temperature, both in vivo and in vitro [16], and, as our present results demonstrate (Fig. 4D), by SQDAG, GalDAG and GalGalDAG. As a freshwater cyanobacterium, Synechocystis is often subjected to restricted and/or fluctuating supplies of phosphate [7] and in such circumstances it may be more efficient to utilize a lipid species that does not contain phosphate, i.e., SQDAG, to regulate lipid biosynthesis and/or as part of an effective rescue mechanism when little phosphate is available. A higher SQDAG concentration would then respond to inadequate levels of galactolipids by activating MgdA and vice-versa (Fig. 5). The level of SQDAG is also related to the availability of inorganic phosphate and, since the amount of galactolipid (namely GalGalDAG) is elevated during phosphate starvation [17], the synthesis of phospho-, sulfoand galactolipids is probably coordinated in cyanobacteria by regulating the activity of appropriate enzymes.
However, the mechanism underlying this regulation on MgdA by SQDAG remains unknown. Several laboratories have observed direct interaction with or binding of SQDAG to different proteins in vitro, e.g., a protein that initiates replication in humans [48] and E. coli DNA polymerase I, which is inhibited by SQDAG [49]. In addition, this lipid stimulates protein import through the chloroplast outer envelope mediated by the precursor protein translocon [50]. To determine whether or not SQDAG regulates MgdA through direct interaction will require further experimental investigations. Organisms related evolutionarily often retain specific features in the sequences and structures of their proteins and common regulatory mechanisms. Transmembrane domains are not only an important structural feature of proteins, but can also serve important regulatory roles. We chose typical model cyanobacteria, from single-cell to filamentous and both nitrogen-fixing and nonnitrogen-fixing species, to investigate conservation of sequence and secondary structure. When we analyzed the amino acid sequence of MgdA in silico, the 4 prediction programs employed all identified 5 putative transmembrane helices (TMH) that appear to be a conserved feature of cyanobacterial homologs (Fig. 6). In MgdA these are localized essentially to the N-and C-terminal regions (at residues TMH1 46-66, TMH2 68-88, TMH3 361-381, TMH4 389-409, and TMH5 435-455), flanking the putative catalytic domain and active site that face the cytoplasmic surface, with pronounced sequence similarity and remarkable conservation of these five transmembrane domains. Even in the case of the MGS homologue in the ''chromatophore'' inclusions of the amoeba Paulinella chromatophora, which are thought to be of cyanobacterial origin and the product of a recent endosymbiotic event [51], there is pronounced sequence similarity, with two TMH in the Nterminal and three in the C-terminal region (Fig. 6).
To evaluate the importance of different TMH segments in Synechocystis experimentally, several deletion variants were expressed in E. coli and assayed for activity. Even though many of these were expressed at a level higher than the full-length protein, none of the truncated constructs exhibited any glucosyltransferase activity, even in the presence of SQDAG or any other lipid activator (Fig. S5). In comparison, the full-length construct produced GlcDAG under the same conditions as those used for MgdA in Synechocystis (Fig. 4D). Thus, the transmembrane helices appear to be important for correct folding and/or enzymatic activity and may be one reason why MgdA responds preferentially to SQDAG, rather than PG, as do lipid GT enzymes without transmembrane domains.

Regulation of the Synthesis of Sugar Lipids in
Synechocystis -One Step Closer to the Answer?
Our findings that the MgdA protein and activity are not distributed in the same manner among Synechocystis membranes could have two explanations (Fig. 7). First, an as yet unknown MGS-type enzyme located exclusively in the TM, may be responsible for the high activity observed there. As previously mentioned, deletion of the sll1377 gene is lethal to Synechocystis [15] so such an enzyme, if it exists is unable to compensate for the loss of MgdA under these circumstances.
A second alternative could be the presence of a stimulator of MgdA activity in PM1 and TM. Such a regulator has already been suggested to enhance MgdA activity and thereby maintain membrane integrity under conditions of heat shock [52]. We attempted to isolate complexes containing MgdA, both by twodimensional clear-native/SDS-PAGE electrophoresis, as well as by co-immunoprecipitation with the antibody against MgdA (Figs. S3 and S4). Regrettably, MgdA appeared to migrate alone in the gels and the antibody failed to pull-down putative MgdA-containing complexes, which may indicate that such a protein regulator does not exist or that its interaction with MgdA is not preserved during use of these approaches.
To examine whether reversible phosphorylation is involved in the regulation of MgdA, prior to addition of the enzymatic substrate we treated both PM2 and TM preparations with alkaline phosphatase in the presence of SQDAG. Since both treated and untreated preparations showed similar levels of activity (Fig. S2) this does not seem to be the case. Another possible explanation is that MgdA is regulated by a small molecule, as c-di-GMP regulates the GT enzymes PgaC and PgaD that synthesize the exopolysaccharides in E. coli [53].
The Synechocystis enzyme Slr1508 (DgdA) has GalGalDAG synthase activity in vivo [17] and deletion of the dgdA gene results in cells containing none of this lipid [17]. Strangely, this enzyme has also been reported to be part of a sodium-dependent potassium membrane transporter complex (Ktr) in Synechocystis [31,54] and details concerning its reaction mechanism and the galactose donor are still unclear, since no assay for its in vitro activity is presently available. We detected the DgdA protein exclusively in PM (Fig. 1), implying that GalGalDAG synthesis should occur only at this location. This raises the question as to whether GalGalDAG is transported by vesicular traffic or by intermembrane diffusion through connections between the PM and TM, previously postulated to be PM1 [4]. While no consensus has been reached, accumulating evidence for a connection between the PM and TM [1,3,4,55] supports lateral membrane diffusion (as hypothesized in Fig. 7).

Concluding Remarks
Here, our initial observation of the discrepancies between the subcellular localization of protein and enzymatic activity involved in the crucial first step in galactolipid synthesis by Synechocystis motivated us to investigate the regulation of MgdA in greater detail. We have demonstrated that this MgdA has unique regulatory features, in particular the potent activation by sulfolipid SQDAG, which distinguishes it from other GlcDAG or GalDAG synthases. Pulse-labelling experiments with [ 14 C]-acetate, as well as measurements of enzymatic activity revealed that GlcDAG and GalDAG are synthesized in both PM and TM, with highest specific activity in the latter. Whether MgdA is regulated posttranslationally, by interaction with a small effector molecule or by a protein partner remains to be discovered. The possible existence of second MGS in Synechocystis and the interaction of SQDAG with MgdA are still open questions that will require further investigation.