Glycan Array Technology

Glycan (or carbohydrate) arrays have become an essential tool in glycomics, providing fast and high-throughput data on protein-carbohydrate interactions with small amounts of carbohydrate ligands. The general concepts of glycan arrays have been adopted from other microarray technologies such as those used for nucleic acid and proteins. However, carbohydrates have presented their own challenges, in particular in terms of access to glycan probes, linker attachment chemistries and analysis, which will be reviewed in this chapter. As more and more glycan probes have become available through chemical and enzymatic synthesis and robust linker chemistries have been developed, the applications of glycan arrays have dramatically increased over the past 10 years, which will be illustrated with recent examples.

IRMPD Spectroscopy Sheds New (Infrared) Light on the Sulfate Pattern of Carbohydrates

IR spectroscopy of gas-phase ions is proposed to resolve positional isomers of sulfated carbohydrates. Mass spectrometric fingerprints and gas-phase vibrational spectra in the near and mid-IR regions were obtained for sulfated monosaccharides, yielding unambiguous signatures of sulfated isomers. We report the first systematic exploration of the biologically relevant but notoriously challenging deprotonated state in the near IR region. Remarkably, anions displayed very atypical vibrational profiles, which challenge the well-established DFT (Density Functionnal Theory) modeling. The proposed approach was used to elucidate the sulfate patterns in glycosaminoglycans, a ubiquitous class of mammalian carbohydrates, which is regarded as a major challenge in carbohydrate structural analysis. Isomeric glycosaminoglycan disaccharides from heparin and chondroitin sources were resolved, highlighting the potential of infrared multiple photon dissociation spectroscopy as a novel structural tool for carbohydrates.

Discrimination of epimeric glycans and glycopeptides using IM-MS and its potential for carbohydrate sequencing

Mass spectrometry is the primary analytical technique used to characterize the complex oligosaccharides that decorate cell surfaces. Monosaccharide building blocks are often simple epimers, which when combined produce diastereomeric glycoconjugates indistinguishable by mass spectrometry. Structure elucidation frequently relies on assumptions that biosynthetic pathways are highly conserved. Here, we show that biosynthetic enzymes can display unexpected promiscuity, with human glycosyltransferase pp-α-GanT2 able to utilize both uridine diphosphate N-acetylglucosamine and uridine diphosphate N-acetylgalactosamine, leading to the synthesis of epimeric glycopeptides in vitro. Ion-mobility mass spectrometry (IM-MS) was used to separate these structures and, significantly, enabled characterization of the attached glycan based on the drift times of the monosaccharide product ions generated following collision-induced dissociation. Finally, ion-mobility mass spectrometry following fragmentation was used to determine the nature of both the reducing and non-reducing glycans of a series of epimeric disaccharides and the branched pentasaccharide Man3 glycan, demonstrating that this technique may prove useful for the sequencing of complex oligosaccharides.

In vivo anti-malarial effect of the beta-amino alcohol 1t on Plasmodium berghei

Glycerol derivatives are a class of compounds, which are easy and inexpensive to produce with potent anti-malarial activities against blood stages of Plasmodium falciparum in vitro. In the present study, one of these compounds, termed 1t, which had the lowest IC(50) values, was assessed in a murine malarial model. Nuclear magnetic resonance imaging and Balb/c mice infected with Plasmodium berghei ANKA strain were treated in a 4-day suppressive test. Mice received a once-daily intraperitoneal administration of 50 mg/Kg of the drug for 4 days. Although no parasitaemia clearance was reached, a slower parasite proliferation and a slightly longer survival time compared with the placebo group were observed.

P450(camr), a cytochrome P450 catalysing the stereospecific 6- endo-hydroxylation of (1 R)-(+)-camphor

Rhodococcus sp. NCIMB 9784 accumulated 6- endo-hydroxycamphor 3 when grown on (1 R)-(+)-camphor 1 as sole carbon source. The structure of 3 has been unambiguously assigned for the first time using X-ray crystallography. A soluble cytochrome P450 hydroxylase, induced by growth on (1 R)-(+)-camphor and designated P450(camr), has been isolated from the bacterium Rhodococcus sp. NCIMB 9784. Using authentic 6- endo hydroxycamphor as standard, a cell-free system consisting of pure P450(camr) and putidaredoxin and putidaredoxin reductase from Pseudomonas putida confirmed that the enzyme hydroxylates (1 R)-(+)-camphor specifically in the 6- endoposition, in contrast to the 5- exo hydroxylation catalysed by the well-studied P450(cam) from P. putida. P450(camr) has a molecular mass of approximately 44 kDa, and a pI of 4.8.

The desymmetrization of bicyclic beta -diketones by an enzymatic retro-Claisen reaction. A new reaction of the crotonase superfamily

The enzyme 6-oxocamphor hydrolase, which catalyzes the desymmetrization of 6-oxocamphor to yield (2R,4S)-alpha-campholinic acid, has been purified with a factor of 35.7 from a wild type strain of Rhodococcus sp. NCIMB 9784 grown on (1R)-(+)-camphor as the sole carbon source. The enzyme has a subunit molecular mass of 28,488 Da by electrospray mass spectrometry and a native molecular mass of approximately 83,000 Da indicating that the active protein is trimeric. The specific activity was determined to be 357.5 units mg(-)1, and the K(m) was determined to be 0.05 mm for the natural substrate. The N-terminal amino acid sequence was obtained from the purified protein, and using this information, the gene encoding the enzyme was cloned. The translation of the gene was found to bear significant homology to the crotonase superfamily of enzymes. The gene is closely associated with an open reading frame encoding a ferredoxin reductase that may be involved in the initial step in the biodegradation of camphor. A mechanism for 6-oxocamphor hydrolase based on sequence homology and the known mechanism of the crotonase enzymes is proposed.

Selective in vitro glycosylation of recombinant proteins: semi-synthesis of novel homogeneous glycoforms of human erythropoietin

BACKGROUND

A natural glycoprotein usually exists as a spectrum of glycosylated forms, where each protein molecule may be associated with an array of oligosaccharide structures. The overall range of glycoforms can have a variety of different biophysical and biochemical properties, although details of structure-function relationships are poorly understood, because of the microheterogeneity of biological samples. Hence, there is clearly a need for synthetic methods that give access to natural and unnatural homogeneously glycosylated proteins. The synthesis of novel glycoproteins through the selective reaction of glycosyl iodoacetamides with the thiol groups of cysteine residues, placed by site-directed mutagenesis at desired glycosylation sites has been developed. This provides a general method for the synthesis of homogeneously glycosylated proteins that carry saccharide side chains at natural or unnatural glycosylation sites. Here, we have shown that the approach can be applied to the glycoprotein hormone erythropoietin, an important therapeutic glycoprotein with three sites of N-glycosylation that are essential for in vivo biological activity.

RESULTS

Wild-type recombinant erythropoietin and three mutants in which glycosylation site asparagine residues had been changed to cysteines (His(10)-WThEPO, His(10)-Asn24Cys, His(10)-Asn38Cys, His(10)-Asn83CyshEPO) were overexpressed and purified in yields of 13 mg l(-1) from Escherichia coli. Chemical glycosylation with glycosyl-beta-N-iodoacetamides could be monitored by electrospray MS. Both in the wild-type and in the mutant proteins, the potential side reaction of the other four cysteine residues (all involved in disulfide bonds) were not observed. Yield of glycosylation was generally about 50% and purification of glycosylated protein from non-glycosylated protein was readily carried out using lectin affinity chromatography. Dynamic light scattering analysis of the purified glycoproteins suggested that the glycoforms produced were monomeric and folded identically to the wild-type protein.

CONCLUSIONS

Erythropoietin expressed in E. coli bearing specific Asn-->Cys mutations at natural glycosylation sites can be glycosylated using beta-N-glycosyl iodoacetamides even in the presence of two disulfide bonds. The findings provide the basis for further elaboration of the glycan structures and development of this general methodology for the synthesis of semi-synthetic glycoproteins.

Chemical and enzymatic synthesis of glycopolymers

The development of efficient, fast, flexible and general synthetic routes to glycopolymers is an ongoing challenge and much progress has been made in recent years. Chemical coupling methods have become increasingly sophisticated to fine-tune reactivity of reagents by fortuitous choices of anomeric activating group and protecting groups. As a result, oligosaccharide synthesis has become more predictable and reliable even to the extent that first examples of saccharide library syntheses in solution and on the solid phase have been published. In biology, the repertoire of biocatalysts that can be used for glycoside synthesis is ever-increasing, and enzyme-catalysed glycosylation steps have been successfully incorporated into synthetic strategies.

Phytanyl-pyrophosphate-linked substrate for a bacterial alpha-mannosyltransferase

The biochemical characterization of bacterial glycosyltransferases involved in the assembly of cell-wall-associated polysaccharides is often hindered by the lack of the appropriate undecaprenyl-pyrophosphate-linked acceptor substrate. In order to find a suitable synthetic substrate for the alpha1,3-mannosyltransferase AceA from Acetobacter xylinum, phytanyl-pyrophosphate-linked cellobiose was prepared. In the presence of GDP-[14C]mannose and recombinant AceA, the phytanyl-pyrophosphate-linked cellobiose afforded a 14C-labeled trisaccharide that was sensitive to alpha-mannosidase degradation in a fashion analogous to the natural undecaprenyl-pyrophosphate-linked cellobiose substrate. These results suggest that phytanyl-pyrophosphate-linked oligosaccharides may be useful substrates for other important bacterial glycosyltransferases.

Novel mechanism of inhibition of elastase by beta-lactams is defined by two inhibitor crystal complexes

Two structurally related beta-lactams form different covalent complexes upon reaction with porcine elastase. The high resolution x-ray structures of these two complexes provide a clear insight into the mechanism of the reaction and suggest the design of a new class of serine protease inhibitors that resist enzyme reactivation by hydrolysis of the acyl intermediate. The presence of a hydroxyethyl substituent on the beta-lactam ring provides a new reaction pathway resulting in the elimination of the hydroxyethyl group and the formation of a stabilizing conjugated double bond system. In contrast, the presence of a diethyl substituent on the beta-lactam ring leads to addition of water. The two enzyme complexes show very different binding modes in the enzyme active site.

Development of recombinant, immobilised beta-1,4-mannosyltransferase for use as an efficient tool in the chemoenzymatic synthesis of N-linked oligosaccharides

The preparation of the conserved core structure of asparagine-linked oligosaccharides found in eukaryotic glycoproteins is an important step towards the synthesis of homogeneous neoglycoproteins. So far, however, the convenient generation of the Manbeta4GlcNAcbeta4GlcNAc (Gn2M) core trisaccharide has proved to be a major obstacle because of the inherent difficulties associated with the synthesis of beta-mannosides. Here we report the overproduction in Escherichia coli of full-length and transmembrane-deleted yeast beta-1, 4-mannosyltransferases as novel N-terminal fusions bearing a decahistidinyl sequence and the minimal human Myc epitope. The recombinant enzymes were highly active and were amenable to immobilisation by nickel(II) chelation and to immunodetection with an anti-Myc monoclonal antibody. The immobilised, transmembrane-deleted enzyme exhibited an apparent Km of 14 microM for the synthetic acceptor substrate analogue, phytanyl-pyrophosphoryl-alpha-N,N'-diacetylchitobioside (PPGn2), under saturating donor conditions. This figure is comparable to those previously reported for native and recombinant yeast beta-1, 4-mannosyltransferases with, respectively, the natural dolichyl-linked acceptor and PPGn2. The validity of the reaction product was confirmed by chromatographic and spectroscopic analysis.

The chemoenzymatic synthesis of the core trisaccharide of N-linked oligosaccharides using a recombinant beta-mannosyltransferase

The chemical synthesis of the beta-mannosyl linkage of N-glycans has presented a great challenge to synthetic carbohydrate chemists. We have therefore investigated the application of beta-mannosyltransferases to the preparative synthesis N-linked core oligosaccharides. In this paper we report the chemoenzymatic synthesis of beta-D-mannopyranosyl-(1-->4)-2-acetamido-2-deoxy-beta-D-glucopyranosyl- (1-->4)-2-acetamido-2-deoxy-alpha-D-glucopyranose on a preparative scale using a phytanyl-linked acceptor in the presence of a recombinant beta-(1-->4)-mannosyltransferase.

Enzyme-catalyzed formation of glycosidic linkages

Significant progress has recently been achieved in the use of glycosidases and glycosyltransferases as synthetic tools. Glycosidases have been used to synthesize trisaccharides with a reasonable overall yield, as well as high-mannose neoglycoconjugates. Studies on glycosyltransferases have defined reaction mechanisms and demonstrated reasonable substrate tolerance of these enzymes. Effective methodology for the synthesis of defined glycoproteins has also been demonstrated.

COS-1 cell expression and one-step affinity protein purification and activity of epitope-tagged human erythropoietin and of site-directed mutants

Recombinant human erythropoietin (rhEPO) is an important glycoprotein hormone which has been successfully used in the treatment of anaemia. To facilitate the rapid evaluation of wild-type and mutant forms of rhEPO in structure-function studies, we have developed an expression system in which the recombinant hormone is tagged at the C-terminus with a c-myc peptide. One-step affinity purification of culture supernatants on an anti-myc antibody column yielded proteins which were greater than 50% pure with a specific activity of 300,000 U/mg, in agreement with the value of wild-type protein. We conclude that the additional myc-peptide does not affect receptor binding. The expression system was used to study three mutants in which the N-glycosylation sites were changed to cysteines (Asn24Cys, Asn38Cys and Asn83Cys). Specific activities of these cysteine mutants were significant, but reduced (60%, 22% and 70%, respectively), compared to wild-type. The reduction in specific activity may be due to reduced stability of the mutant proteins.

Evidence that bilayer bending rigidity affects membrane protein folding

The regeneration kinetics of the integral membrane protein bacteriorhodopsin have been investigated in a lipid-based refolding system. Previous studies on bacteriorhodopsin regeneration have involved detergent-based systems, and in particular mixed dimyristoylphosphatidylcholine (DMPC)/CHAPS micelles. Here, we show that the short chain lipid dihexanoylphosphatidylcholine (DHPC) can be substituted for the detergent CHAPS and that bacteriorhodopsin can be regenerated to high yield in mixed DMPC/DHPC micelles. Bacteriorhodopsin refolding kinetics are measured in the mixed DMPC/DHPC micelles. Rapid, stopped flow mixing is employed to initiate refolding of denatured bacterioopsin in SDS micelles with mixed DMPC/DHPC micelles and time-resolved fluorescence spectroscopy to follow changes in protein fluorescence during folding. Essentially identical refolding kinetics are observed for mixed DMPC/CHAPS and mixed DMPC/DHPC micelles. Only one second-order retinal/apoprotein reaction is identified, in which retinal binds to a partially folded apoprotein intermediate, and the free energy of this retinal binding reaction is found to be the same in both types of mixed micelles. Formation of the partially folded apoprotein intermediate is a rate-limiting step in protein folding and appears to be biexponential. Both apparent rate constants are found to be dependent on the relative proportion of DMPC present in the mixed DMPC/DHPC micelles as well as on the pH of the aqueous phase. Increasing the DMPC concentration should increase the bending rigidity of the amphiphilic bilayer, and this is found to slow the rate of formation of the partially folded apoprotein intermediate. Increasing the mole fraction of DMPC from 0.3 to 0.6 slows the two apparent rate constants associated with formation of this intermediate from 0.29 and 0.031 to 0.11 and 0.013 s-1, respectively. Formation of the intermediate also slows with increasing pH, from 0.11 and 0.013 s-1 at pH 6 to 0.033 and 0.0053 s-1 at pH 8. Since this pH change has no known effect on the phase behavior of lecithins, this is more likely to represent a direct effect on the protein itself. Thus, it appears to be possible to control the rate-limiting process in bacterioopsin folding through both bilayer bending rigidity and pH.

Slow alpha helix formation during folding of a membrane protein

Very little is known about the folding of proteins within biological membranes. A "two-stage" model has been proposed on thermodynamic grounds for the folding of alpha helical, integral membrane proteins, the first stage of which involves formation of transmembrane alpha helices that are proposed to behave as autonomous folding domains. Here, we investigate alpha helix formation in bacteriorhodopsin and present a time-resolved circular dichroism study of the slow in vitro folding of this protein. We show that, although some of the protein's alpha helices form early, a significant part of the protein's secondary structure appears to form late in the folding process. Over 30 amino acids, equivalent to at least one of bacteriorhodopsin's seven transmembrane segments, slowly fold from disordered structures to alpha helices with an apparent rate constant of about 0.012 s-1 at pH 6 or 0.0077 s-1 at pH 8. This is a rate-limiting step in protein folding, which is dependent on the pH and the composition of the lipid bilayer.

Retinal binding during folding and assembly of the membrane protein bacteriorhodopsin

The factors driving folding and assembly of integral membrane proteins are largely unknown. In order to determine the role that the retinal chromophore plays in assembly of bacteriorhodopsin, we have determined the kinetics and thermodynamics of retinal binding during regeneration of bacteriorhodopsin, from denatured apoprotein, in vitro. Regeneration is initiated by rapid, stopped-flow, mixing of the denatured apoprotein bacterioopsin in sodium dodecyl sulfate micelles with mixed detergent/lipid micelles containing retinal. Regeneration kinetics are measured by time-resolving changes in protein fluorescence. The dependence of each kinetic component on retinal concentration is determined. Only one experimentally observed rate constant is dependent on retinal concentration, leading to identification of only one second-order reaction involving retinal and bacterioopsin. This reaction occurs after a rate-limiting step in bacterioopsin folding, and results in formation of a noncovalent retinal/protein complex. The free energy change of this retinal binding step is determined, showing that thermodynamic information can be obtained on transient intermediates involved in membrane protein regeneration.

Chemical and biological approaches to glycoprotein synthesis

Protein glycosylation is a common posttranslational modification that produces glycoproteins that are highly complex in terms of both their structure and in their function. Systematic structure-function studies of such glycoproteins require synthetic methods that can produce homogeneous glycoproteins with defined oligosaccharide sidechains.

Dolichol is not a necessary moiety for lipid-linked oligosaccharide substrates of the mannosyltransferases involved in in vitro N-linked-oligosaccharide assembly

Dolichol is utilized in vivo as an unusually large anchor on which the precursor for N-linked oligosaccharides is assembled by a series of glycosyltransferases. The role of dolichol in enzyme substrate recognition is investigated. Thus the biosynthetic intermediate NN'-diacetylchitobiose was chemically linked to either dolichol or the much shorter fully saturated tetraisoprenoid phytanol. Both lipids were used as substrates by a recombinant, soluble beta-1,4-mannosyltransferase. beta-[3H]Mannosylated lipids from this reaction were then used as substrates for the subsequent mannosyltransferases from yeast or rat liver microsomes. It was found that both the dolichyl- and phytanyl-linked substrates were easily mannosylated to form Man5GlcNAc2, with some further mannosylation to Man7GlcNAc2 and Man9GlcNAc2 at low concentrations of lipid-linked substrate. It is concluded that dolichol is not necessary in vitro as part of the substrate for the mannosyltransferases in the biosynthetic pathway for N-glycosylation.

Expression and mutagenesis of recombinant human and murine erythropoietins in Escherichia coli

Expression of the polypeptide hormone erythropoietin (EPO) in Escherichia coli by four bacterial expression vectors was examined. Complementary DNAs encoding human and murine EPO were amplified by polymerase chain reaction (PCR) and cloned into the glutathione-S-transferase (GST) fusion vector, pGEX-2T. Human EPO DNA was also cloned into the vectors, pET14b, pIN III-Omp A2 and pT7/7. Expression of human and murine EPO was obtained using constructs based on pGEX-2T. For constructs based on the other vectors, expression of EPO was absent or occurred at low levels, despite attempts to optimise conditions. Human and murine EPO, expressed as fusion proteins with GST, were partially soluble and displayed EPO bioactivity. Soluble GST-EPO fusion proteins were affinity purified on immobilised glutathione. Insoluble protein could also be purified by elution from gel slices following SDS-PAGE to yield either fusion protein or, after treatment with thrombin, unmodified EPO which was both soluble and bioactive. The pGEX expression system was evaluated as a means of analysing the structure-function relationships of EPO by in vitro mutagenesis. Three human and three murine EPO mutants were constructed and expressed as GST fusion proteins. Following purification, biological activity was evaluated using assays for bioactivity, immunoactivity and GST activity. The pGEX expression system complements eukaryotic systems described previously for expression of EPO and should provide much useful information about the structure-function relationships of the hormone.

Intermediates in the folding of the membrane protein bacteriorhodopsin

Assembly of proteins within lipid bilayers is essential for the biogenesis and function of biological membranes. Little is known, however, about the underlying mechanism of assembly, and it is not clear whether it is possible to observe individual folding steps for integral membrane proteins either in vivo or in vitro. Fluorescence spectroscopy is used here to follow the time course of folding events for bacteriorhodopsin in mixed detergent/lipid micelles. Transient folding-intermediates are detected and binding of the retinal chromophore occurs at a late stage, when it binds to an apoprotein intermediate.

The chemoenzymatic synthesis of neoglycolipids and lipid-linked oligosaccharides using glycosyltransferases

The application of glycosyltransferases to the chemoenzymatic synthesis of neoglycosphingolipids and lipid-linked oligosaccharides allows the regio- and stereoselective formation of glycosidic bonds. In our laboratory galactosyl-, sialyl-, and fucosyltransferases have been used to assemble oligosaccharide headgroups directly on a sphingosine derivative without the need for any protection group strategies, including the Lewisx antigen. In complementary studies on N-linked oligosaccharide biosynthesis, chemically phosphorylated dolichol analogues have been tested as substrates for Dol-P-Man synthetase. Also, the substrate recognition of the core beta-1,4-mannosyltransferase from yeast has been investigated using a range of chitobiose derivatives as potential substrates.

The potential dolichol recognition sequence of beta-1,4-mannosyltransferase is not required for enzymic activity using phytanyl-pyrophosphoryl-alpha-N,N'- diacetylchitobioside as acceptor

The ALG1 gene of Saccharomyces cerevisiae encodes beta-1,4-mannosyltransferase, an essential membrane-associated enzyme involved in the assembly of dolichyl-linked oligosaccharide precursors for N-glycosylation [Albright and Robbins (1990) J. Biol. Chem. 265, 7042-7049], which catalyses the transfer of a mannose residue from GDP-mannose to dolichyl-pyrophosphoryl-alpha-N,N'- diacetylchitobioside; it also possesses a putative transmembrane domain, bearing an 11-amino-acid consensus sequence, which has been proposed to mediate dolichol recognition. Here we report the construction and bacterial expression of a mutant beta-1,4-mannosyltransferase derived from ALG1, which carries a 34-amino-acid deletion resulting in the absence of the entire N-terminal transmembrane domain. This truncated enzyme has an apparent Km value of 17 microM for phytanyl-pyrophosphoryl-alpha-N,N'-diacetylchitobioside, a known acceptor for beta-1,4-mannosyltransferase [Flitsch, Pinches, Taylor and Turner (1992) J. Chem. Soc., Perkin Trans. 1, 2087-2093]. The intact enzyme, expressed in the same system, has an apparent Km value of 25 microM. These figures are in good agreement with previously reported values for wild-type beta-1,4-mannosyl-transferase incubated with the natural dolichyl-linked substrate. Gel-filtration chromatography (before and after beta-mannosidase digestion) of the products of both forms of the enzyme verifies the formation of Man beta 1-->4GlcNAc beta 1-->4GlcNAc. We therefore conclude that the putative dolichol recognition sequence is not necessary for recognition of the phytanyl analogue of its natural dolichol substrate and suggest it probably also is not needed for its natural substrate.

A novel mono-branched lipid phosphate acts as a substrate for dolichyl phosphate mannose synthetase

Dolichyl phosphate mannose synthetase (GDP-mannose: dolichyl-phosphate O-beta-D-mannosyltransferase; EC 2.4.1.83) is an enzyme that is involved in glycoconjugate biosynthesis and possesses a putatively conserved dolichol binding site. In order to probe the interaction between the enzyme and the dolichol chain, lipid phosphates varying in length and extent of branching have been tested as substrates in crude microsomal preparations from Saccharomyces cerevisiae. It was found that phytanyl (3,7,11,15-tetramethylhexadecanyl) phosphate was utilized at 60-70% of the efficiency of the natural dolichyl lipid in transfer of [3,4,-3H]mannose from GDP-Man to organic soluble material, whereas addition of S-3-methyloctadecanyl phosphate, which is of similar length to the phytanyl analogue but with only one branch, resulted in approximately 25% of the incorporation of the natural substrate. Incubations with the unbranched tetradecanyl phosphate and with the short, doubly branched R- and S-dihydrocitronellyl (3,7-dimethyloctanyl) phosphates exhibited levels of activity similar to incubations with no exogenous acceptor. These results were qualitatively confirmed with experiments on Escherichia coli harbouring the S. cerevisiae DPM1 gene. The [3H]mannosylated lipid-linked material from microsomal incubations was purified by anion-exchange chromatography. The major saccharide component recovered after hydrolysis was determined to be mannose, but a mannose-containing disaccharide was also present. It is concluded that branching of lipid phosphates is essential for substrates of dolichyl phosphate mannose synthetase and that significant transfer of mannose occurs even if only branching at C-3 is present.

Structural studies on transmembrane proteins. 1. Model study using bacteriorhodopsin mutants containing single cysteine residues

In developing new approaches to structural studies of polytopic transmembrane proteins, we have prepared bacteriorhodopsin mutants containing single cysteine residues at selected sites in different topological domains. Four such mutants were prepared: Gly-72----Cys and Ser-169----Cys in the presumed looped-out regions on the opposite sides of the membrane bilayer and Thr-90----Cys and Leu-92----Cys in the membrane-embedded helix C. The four mutants folded and regenerated the characteristic chromophore in detergent/phospholipid micelles and pumped protons like the wild-type bacteriorhodopsin. After reconstitution in asolectin vesicles, the sulfhydryl groups in the mutants Gly-72----Cys and Ser-169----Cys reacted with iodo[2-3H]acetic acid, while the sulfhydryl groups in the membrane-embedded mutants, Thr-90----Cys and Leu-92----Cys, did not. The sulfhydryl groups in all four mutants could be derivatized in the denatured state by reaction with iodoacetic acid or 6-acryloyl-2-(dimethylamino)naphthalene. Of these derivatives, the two from the mutants Gly-72----Cys and Ser-169----Cys folded like the wild-type bacterioopsin, whereas of the two from the helix C mutants, Thr-90----Cys and Leu-92----Cys, only the latter folded normally. However, the folding of Leu-92----Cys was also impaired when treated with the bulky 5-(iodoacetamido)fluorescein. The reactivity and the folding behavior of the cysteine mutants can thus report on the topographic domain as well as on the orientation of the helices within the membrane.

Structural studies on transmembrane proteins. 2. Spin labeling of bacteriorhodopsin mutants at unique cysteines

Site-directed mutagenesis was used to produce mutants of bacteriorhodopsin where either glycine-72, threonine-90, leucine-92, or serine-169 was replaced by a cysteine. Two different spin labels were then covalently attached to these sites. The selection of attachment sites covered two postulated loops (72,169) and a membrane-spanning segment (90,92). It was not possible to properly refold the protein labeled at position 90, presumably due to steric problems, but the EPR spectra of the other mutants that were successfully reconstituted in phospholipid vesicles provided information on the dynamics of protein side chains in the vicinity of the label site. A power saturation approach was used to investigate the spin relaxation times, which in turn can be influenced by collisions with paramagnetic species. The differential effect of oxygen and a water-soluble chromium complex on the power-saturation behavior of the spin-labeled mutants was used to obtain topographical information on the sites in the membrane-bound protein. The results are consistent with residues 72 and 169 being located in structured loops exposed to the aqueous phase and residue 92 being localized in the membrane interior, possibly near a helix-helix contact region.