Comparison of the amino acid compositions and antigenic properties of spiralins purified from the plasma membranes of different spiroplasmas

Genome sequence of the Drosophila melanogaster male-killing Spiroplasma strain MSRO endosymbiont

Spiroplasmas are helical and motile members of a cell wall-less eubacterial group called Mollicutes. Although all spiroplasmas are associated with arthropods, they exhibit great diversity with respect to both their modes of transmission and their effects on their hosts; ranging from horizontally transmitted pathogens and commensals to endosymbionts that are transmitted transovarially (i.e., from mother to offspring). Here we provide the first genome sequence, along with proteomic validation, of an endosymbiotic inherited Spiroplasma bacterium, the Spiroplasma poulsonii MSRO strain harbored by Drosophila melanogaster. Comparison of the genome content of S. poulsonii with that of horizontally transmitted spiroplasmas indicates that S. poulsonii has lost many metabolic pathways and transporters, demonstrating a high level of interdependence with its insect host. Consistent with genome analysis, experimental studies showed that S. poulsonii metabolizes glucose but not trehalose. Notably, trehalose is more abundant than glucose in Drosophila hemolymph, and the inability to metabolize trehalose may prevent S. poulsonii from overproliferating. Our study identifies putative virulence genes, notably, those for a chitinase, the H₂O₂-producing glycerol-3-phosphate oxidase, and enzymes involved in the synthesis of the eukaryote-toxic lipid cardiolipin. S. poulsonii also expresses on the cell membrane one functional adhesion-related protein and two divergent spiralin proteins that have been implicated in insect cell invasion in other spiroplasmas. These lipoproteins may be involved in the colonization of the Drosophila germ line, ensuring S. poulsonii vertical transmission. The S. poulsonii genome is a valuable resource to explore the mechanisms of male killing and symbiont-mediated protection, two cardinal features of many facultative endosymbionts.

Most insect species, including important disease vectors and crop pests, harbor vertically transmitted endosymbiotic bacteria. These endosymbionts play key roles in their hosts’ fitness, including protecting them against natural enemies and manipulating their reproduction in ways that increase the frequency of symbiont infection. Little is known about the molecular mechanisms that underlie these processes. Here, we provide the first genome draft of a vertically transmitted male-killing Spiroplasma bacterium, the S. poulsonii MSRO strain harbored by D. melanogaster. Analysis of the S. poulsonii genome was complemented by proteomics and ex vivo metabolic experiments. Our results indicate that S. poulsonii has reduced metabolic capabilities and expresses divergent membrane lipoproteins and potential virulence factors that likely participate in Spiroplasma-host interactions. This work fills a gap in our knowledge of insect endosymbionts and provides tools with which to decipher the interaction between Spiroplasma bacteria and their well-characterized host D. melanogaster, which is emerging as a model of endosymbiosis.

Secondary structure of spiralin in solution, at the air/water interface, and in interaction with lipid monolayers

The surface of spiroplasmas, helically shaped pathogenic bacteria related to the mycoplasmas, is crowded with the membrane-anchored lipoprotein spiralin whose structure and function are unknown. In this work, the secondary structure of spiralin under the form of detergent-free micelles (average Stokes radius, 87.5 A) in water and at the air/water interface, alone or in interaction with lipid monolayers was analyzed. FT-IR and circular dichroism (CD) spectroscopic data indicate that spiralin in solution contains about 25+/-3% of helices and 38+/-2% of beta sheets. These measurements are consistent with a consensus predictive analysis of the protein sequence suggesting about 28% of helices, 32% of beta sheets and 40% of irregular structure. Brewster angle microscopy (BAM) revealed that, in water, the micelles slowly disaggregate to form a stable and homogeneous layer at the air/water interface, exhibiting a surface pressure up to 10 mN/m. Polarization modulation infrared reflection absorption spectroscopy (PMIRRAS) spectra of interfacial spiralin display a complex amide I band characteristic of a mixture of beta sheets and alpha helices, and an intense amide II band. Spectral simulations indicate a flat orientation for the beta sheets and a vertical orientation for the alpha helices with respect to the interface. The combination of tensiometric and PMIRRAS measurements show that, when spiroplasma lipids are used to form a monolayer at the air/water interface, spiralin is adsorbed under this monolayer and its antiparallel beta sheets are mainly parallel to the polar-head layer of the lipids without deep perturbation of the fatty acid chains organization. Based upon these results, we propose a 'carpet model' for spiralin organization at the spiroplasma cell surface. In this model, spiralin molecules anchored into the outer leaflet of the lipid bilayer by their N-terminal lipid moiety are composed of two colinear domains (instead of a single globular domain) situated at the lipid/water interface. Owing to the very high amount of spiralin in the membrane, such carpets would cover most if not all the lipids present in the outer leaflet of the bilayer.

Correlation between anti-bacterial activity and pore sizes of two classes of voltage-dependent channel-forming peptides

Anti-bacterial activities were compared for two series of voltage-dependent pore-formers: (i) alamethicin (Alm) and its synthetic analogs (Alm-dUL) where alpha-amino-isobutyric acid residues (Aibs) were replaced by leucines and selected key residues substituted and (ii) homologous voltage sensors of the electric eel sodium channel (repeats S4L45 (III) and S4L45 (IV)). Spiroplasma melliferum, a bacterium related to the mycoplasmas, was used as a target cell. The data show that with respect to growth inhibition, cell deformation and plasma membrane depolarization, the highest efficient peptide remained natural Alm although the minimal inhibitory concentrations of its Leu analogs were within the same range as the parent molecule, except for Alm-dUL P14A. Thus, as for the pore-forming activity observed in artificial membranes and for the toxicity towards mammalian cells, proline-14 proved to be a critical residue for the anti-bacterial activity of alamethicin. Regarding the sodium voltage sensors, their anti-bacterial efficiency was at least 10 times lower although they promoted spiroplasma cell agglutination. The anti-bacterial activities of the peptides were correlated with their pore-forming properties, especially with the apparent and mean number of monomers per conducting aggregate (<N>) when both peptide families were considered and, secondly, with mean open times (tau(o)) within each family. This suggests that although they may form 'raft-like' structures, the mechanism underlying anti-bacterial activity of Alm and its active analogs, as well as the S4L45 voltage sensors with the S. melliferum plasma membrane, is predominantly through pore-formation according to the 'barrel-stave' mechanism.

Membrane permeabilisation and antimycoplasmic activity of the 18-residue peptaibols, trichorzins PA

The membrane permeabilisation properties of six linear natural 18-residue peptaibols, termed trichorzins PA, have been assessed on liposomes and on mollicutes (trivial name, mycoplasmas), a class of parasitic bacteria characterized by a small genome, the lack of a cell wall, a minute cell size, and the incorporation in their plasma membrane of exogenously supplied cholesterol. The trichorzins PA used in this study (PA II, PA IV-VI, PA VIII, and PA IX) differ between them by amino acid or amino alcohol substitutions at positions 4, 7, and 18, and form slightly amphipathic alpha-helices. They proved bactericidal for mollicutes belonging to the genera Acholeplasma, Mycoplasma, and Spiroplasma, with minimal inhibitory concentrations (3.12</=MICs</=50 microM) generally 2 to 4 fold higher than those of alamethicin F50, a related 20-residue peptide (1.56</=MICs</=12.5 microM). Spiroplasma cells were apparently not protected by the presence of spiralin on their surface. The activities of the six trichorzins PA were not influenced by their sequence variations and no synergistic effect was observed. Consistent with the marginal effect of cholesterol on the incorporation of the trichorzins PA into liposome bilayers, the antibiotic activity was independent of the amount of cholesterol in the membranes of the different mollicutes. The trichorzins PA and alamethicin inhibited the motility of Spiroplasma melliferum, the helical cells being deformed and split into coccoid forms. Membrane potential measurements in Acholeplasma laidlawii and S. melliferum showed that trichorzin PA V and alamethicin F50 very efficiently depolarized the plasma membrane of mollicutes. This was consistent with fluorescence and 23Na NMR measurements on liposomes that revealed the permeabilisation of the lipid bilayer and the nonselective ionophoric activity of the trichorzins PA. These data suggest that the bactericidal activity exhibited by the trichorzins PA on mollicutes is due to the permeabilisation of the plasma membrane.

Spiralin polymorphism in strains of Spiroplasma citri is not due to differences in posttranslational palmitoylation

Spiralin is defined as the major membrane protein of the helical mollicute Spiroplasma citri. According to the S. citri strain used, spiralin shows polymorphism in its electrophoretic mobility. The spiralin gene sequences of eight S. citri strains were determined by direct sequencing of the PCR-amplified genes. All spiralins were found to be 241 amino acids long, except for the spiralin of strain Palmyre, which is 242 amino acids long. The molecular masses calculated from these sequences did not explain the differences observed in the electrophoretic mobilities. In all of the spiralins examined, the first 24 N-terminal amino acids were conserved, including a cysteine at position 24, and had the features of typical signal peptides of procaryotic lipoproteins. When S. citri strains were grown in the presence of [3H]palmitic acid, at least 10 proteins, including spiralin, became labeled. In the presence of globomycin, a lipoprotein signal peptidase inhibitor in eubacteria, apparently unprocessed spiralin could be detected. Formic acid hydrolysis of the [3H]palmitic acid-labeled spiralins of four representative S. citri strains yielded two peptide fragments for each spiralin, as expected from the gene sequence. On fragment was [3H]palmitic acid labeled, and it had almost the same electrophoretic mobility irrespective of the spiralins used. Samples of the unlabeled peptide fragments from the four representative strains had slightly different electrophoretic mobilities (delta Da approximately equal to 800 Da); however, these were much smaller than those of the whole spiralins before formic acid hydrolysis (delta Da approximately equal to 8,000 Da). These results suggest that spiralin polymorphism in S. citri is not due to differences in posttranslational modification by palmitic acid and is certainly a structural property of the whole protein or could result from an unidentified posttranslational modification of spiralin.

Conformation, pore-forming activity, and antigenicity of synthetic peptide analogues of a spiralin putative amphipathic alpha helix

Current models depict spiralin as a bitopic transmembrane protein with the transbilayer domain being an amphipathic alpha helix. However, though secondary structure prediction methods suggest a helical conformation for the hypothetical transmembrane segment of spiralin, no potential transmembrane helices could be detected in this protein using the method of Von Heijne (Von Heijne, G. (1992) J. Mol. Biol. 225, 487-494). Therefore, we have reconsidered the spiralin topological model by investigating the properties of the chemically synthesized peptides SM-BC3 (LNAVNTYATLAKAVLDAIQN-NH2) and SC-R8A2 (LNAVNTYATLASAVLEAIKN-NH2), corresponding to the hypothetical transmembrane segments of spiralins of two distinct spiroplasma species. The hydrophobic moment plot method suggests that these spiralin amino acid stretches are class G amphipathic alpha helices (i.e., helices localized on the surface of a globular protein domain). Circular dichroism spectra showed that both peptides have little ordered structure in aqueous solutions but adopt a mainly helical conformation in the presence of 25% trifluoroethanol or in detergent micelles (up to 74% alpha helix). Both peptides formed concentration- and voltage-dependent pores in planar lipid bilayers with a unitary conductance of 130 pS in 1 M KCl and with mean numbers of monomers per conducting aggregates of 6 for SC-R8A2 and 9 for SM-BC3. However, the two peptides displayed a haemolytic activity only at high concentrations (> 250 microM) and reacted with antibodies raised against membrane-bound spiralin. Together with previously published results, these data suggest that spiralin is a monotopic membrane protein anchored at the surface of the spiroplasma cell and that the 20-residue amphipathic segment is most probably a class G helix containing a B-cell epitope.

Solution structure determination by NMR spectroscopy of a synthetic peptide corresponding to a putative amphipathic alpha-helix of spiralin: resonance assignment, distance geometry and simulated annealing

Spiralin is the major protein of the plasma membrane of several spiroplasmas. Neither the function of this protein nor the crystallographic structure is known. Analysis of the primary structure of spiralin from Spiroplasma melliferum BC3 suggests the presence of an amphipathic peptide in the 143-162 region (Chevalier, C., Saillard, C. and Bové, J.M. (1990) J. Bacteriol. 172, 6090-6097). The structure of a synthetic peptide, H2N-L-N-A-V-N-T-Y-A-T-L-A-K-A-V-L-D-A-I-Q-N-amide, corresponding to this fragment has been examined by 1H-NMR spectroscopy. This 20 amino acid peptide adopts a random coil structure in solution, but the addition of trifluoroethanol stabilizes a structure exhibiting alpha-helical character. The 1H-NMR spectrum has been fully assigned in CF3CD2OD/H2O (30:70, v/v) and the three-dimensional structure has been elucidated using NMR-derived distance information. The calculated structures have been obtained by dynamical simulated annealing or distance geometry followed by simulated annealing. Both sets of structures have been energy-minimized using CHARMm potential. The resulting structures are very similar in terms of constraint violations and energies. It is demonstrated that whereas the first three residues exhibit a large flexibility, the remaining sequence is helical.

Spiralins of Spiroplasma citri and Spiroplasma melliferum: amino acid sequences and putative organization in the cell membrane

Spiralin is the major membrane protein of the helical mollicute Spiroplasma citri. A similar protein occurs in the membrane of Spiroplasma melliferum, an organism related to S. citri. The gene encoding spiralin has been sequenced. A restriction fragment of the spiralin gene has been used as a probe to detect the gene encoding S. melliferum spiralin. A 4.6-kilobase-pair ClaI DNA fragment from S. melliferum strongly hybridized with the probe. This fragment was inserted in pBR322 and cloned in Escherichia coli. It was further subcloned in the replicative forms of M13mp18 and M13mp19, and its nucleotide sequence was determined (GenBank accession number M33991). An open reading frame showing 88.6% base sequence homology with the S. citri spiralin gene could be identified and was assumed to be the gene encoding S. melliferum spiralin. The deduced amino acid sequence of the protein had 75% homology with the spiralin sequence. In particular, the two proteins possess a stretch of 20 amino acids which can form an alpha-helix, in which all polar amino acids occupy approximately one-third of the axial projection down the helix. On the basis of these data and published data, we propose a topological model for the structural organization of the spiralin in the cell membrane of spiroplasmas.

Antigenic relatedness between the spiralins of Spiroplasma citri and Spiroplasma melliferum

Four spiralins were compared by rocket immunoelectrophoresis, quantitative immunoblotting techniques, and the spiroplasma deformation test with the use of antispiralin (polyclonal) monospecific antibodies. This investigation revealed that the spiralins of Spiroplasma citri and S. melliferum are antigenically related and that probably no more than two epitopes simultaneously saturable with antibodies are shared by the two proteins. One at least of these epitopes is accessible to antibodies on the spiroplasma cell surface.

Organization and nucleotide sequences of the Spiroplasma citri genes for ribosomal protein S2, elongation factor Ts, spiralin, phosphofructokinase, pyruvate kinase, and an unidentified protein

The gene for spiralin, the major membrane protein of the helical mollicute Spiroplasma citri, was cloned in Escherichia coli as a 5-kilobase-pair (kbp) DNA fragment. The complete nucleotide sequence of the 5.0-kbp spiroplasmal DNA fragment was determined (GenBank accession no. M31161). The spiralin gene was identified by the size and amino acid composition of its translational product. Besides the spiralin gene, the spiroplasmal DNA fragment was found to contain five additional open reading frames (ORFs). The translational products of four of these ORFs were identified by their amino acid sequence homologies with known proteins: ribosomal protein S2, elongation factor Ts, phosphofructokinase, and pyruvate kinase, respectively encoded by the genes rpsB, tsf, pfk, and pyk. The product of the fifth ORF remains to be identified and was named protein X (X gene). The order of the above genes was tsf--X--spiralin gene--pfk--pyk. These genes were transcribed in one direction, while the gene for ribosomal protein S2 (rpsB) was transcribed in the opposite direction.

Topology and acylation of spiralin

Of the 51 polypeptides detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the plasma membrane of the helical mollicute Spiroplasma melliferum, 21 are acylated, predominantly with myristic (14:0) and palmitic (16:0) chains. This is notably the case for spiralin, the major membrane protein of this bacterium, which contains an average of 0.7 acyl chains per polypeptide, attached very probably by ester bonds to alcohol amino acids. The amphiphilicity of spiralin was demonstrated by the behavior of the protein in charge-shift electrophoresis, its incorporation into liposomes, and its ability to form in the absence of lipids and detergents, globular protein micelles (diameter, approximately 15 nm). The presence of epitopes on the two faces of the cell membrane, as probed by antibody adsorption and crossed immunoelectrophoresis, and the strong interaction between spiralin and the intracytoplasmic fibrils show that spiralin is a transmembrane protein. The mean hydropathy of the amino acid composition of spiralin (-0.30) is on the hydrophilic side of the scale. Surprisingly, the water-insoluble core of spiralin micelles, which is the putative membrane anchor, has a still more hydrophilic amino acid composition (mean hydropathy, -0.70) and is enriched in glycine and serine residues. Taking into account all these properties, we propose a topological model for spiralin featuring a transbilayer localization with hydrophilic domains protruding on the two faces of the membrane and connected by a small domain embedded within the apolar region of the lipid bilayer. In this model, the membrane anchoring of the protein is strengthened by a covalently bound acyl chain.

Production of monoclonal antibodies against phloem-limited prokaryotes of plants: a general procedure using extracts from infected periwinkles as immunogen

Using Spiroplasma citri-infected periwinkle extracts as the source of antigens, together with monoclonal antibody technology, we determined the conditions for mouse immunization and developed a screening assay to detect those hybridomas which produce immunoglobulins specifically directed against the pathogen.

Spiroplasma virus 4: nucleotide sequence of the viral DNA, regulatory signals, and proposed genome organization

The replicative form (RF) of spiroplasma virus 4 (SpV4) has been cloned in Escherichia coli, and the cloned RF has been shown to be infectious by transfection (M. C. Pascarel-Devilder, J. Renaudin, and J.-M. Bové, Virology 151:390-393, 1986). The cloned SpV4 RF was randomly subcloned and was fully sequenced by the dideoxy chain termination technique, using the M13 cloning and sequencing system. The nucleotide sequence of the SpV4 genome contains 4,421 nucleotides with a G+C content of 32 mol%. The triplet TGA is not a termination codon but, as in Mycoplasma capricolum (F. Yamao, A. Muto, Y. Kawauchi, M. Iwami, S. Iwagani, Y. Azumi, and S. Osawa, Proc. Natl. Acad. Sci. USA 82:2306-2309, 1985), probably codes for tryptophan. With these assumptions, nine open reading frames (ORFs) were identified. All nine are characterized by an ATG or GTG initiation codon, one or several termination codons, and a Shine-Dalgarno sequence upstream of the initiation codon. The nine ORFs are distributed in all three reading frames. One of the ORFs (ORF1) corresponds to the 60,000-dalton capsid protein gene. Analysis of codon usage showed that T- and A-terminated codons are preferably used, reflecting the low G+C content (32 mol%) of the SpV4 genome. The viral DNA contains two G+C-rich inverted repeat sequences. One could be involved in transcription termination and the other in initiation of cDNA strand synthesis. The SpV4 genome was found to contain at least three promoterlike sequences quasi-identical to those of eubacteria. These results fully support the bacterial origin of spiroplasmas.

Highly selective extraction of spiralin from the Spiroplasma citri cell membrane with alkyl-N-sulfobetaines

The extraction of proteins from the membrane of the mollicute (mycoplasma) Spiroplasma citri by sodium N-dodecyl-N,N-dimethyl-3-amino-1-propane sulfonate (SB12) and sodium N-tetradecyl-N,N-dimethyl-3-amino-1-propane sulfonate (SB14) was studied with electrophoretic methods. The membranes were prepared by osmotic lysis of the cells and depleted of the bulk of extrinsic proteins. It was possible to extract up to 35 and 45% of membrane proteins with SB12 and SB14, respectively. Maximal yield was obtained in both cases with detergent concentrations greater than or equal to 5 mumoles/mg of membrane protein. Spiralin, the major protein in the S. citri membrane, was highly selectively solubilized without the loss of antigenicity, with a yield of about 90% with SB12 and close to 100% with SB14, for a detergent concentration greater than or equal to 0.2 M. The degree of selectivity in favour of spiralin was higher with SB12 (purity approximately equal to 70%) than with SB14 (purity approximately equal to 50%). Treatment of the S. citri membrane with high concentrations of SB12 is a simple and fast procedure for partial purification of spiralin. This example shows that, in some cases, it should be possible to modulate the selectivity of the extraction of membrane proteins simply by varying the relative concentration of detergent.