Cloning and characterization of the antigenic membrane protein (Amp) gene and in situ detection of Amp from malformed flowers infected with Japanese hydrangea phyllody phytoplasma.

A Japanese hydrangea phyllody (JHP) disease found throughout Japan causes economic damage to the horticultural industry. JHP phytoplasma-infected Japanese hydrangea plants show several disease symptoms involved in floral malformations, such as virescence, phyllody and proliferation. Here, we cloned and characterized the antigenic membrane protein (Amp) gene homolog from the JHP phytoplasma (JHP-amp), expressed the JHP-Amp protein in Escherichia coli cells, and then obtained an antibody against JHP-Amp. The antibody against JHP-Amp had no cross-reactions with the antibody against the Amp protein from a closely related onion yellows phytoplasma. This serologic specificity is probably due to the high diversity of the hydrophilic domains in the Amp proteins. The in situ detection of the JHP-Amp protein revealed that the JHP phytoplasma was localized to the phloem tissues in the malformed flower. This study shows that the JHP-Amp protein is indeed a membrane protein, which is expressed at detectable level in the JHP phytoplasma-infected hydrangea.

Nucleotide sequence analysis of seven Japanese isolates of Plantago asiatica mosaic virus (PlAMV): a unique potexvirus with significantly high genomic and biological variability within the species.

The genomic sequences of Plantago asiatica mosaic virus (PlAMV), six lily isolates and one primrose isolate from Japan, were determined. The genomic size of all isolates was 6102 nucleotides, containing the five open reading frames typical of members of the genus Potexvirus. Pairwise comparison analyses confirmed the close relationship between PlAMV and tulip virus X. However, quite low identities were observed between different PlAMV isolates, including foreign isolates; nucleotide sequence identities of the RNA-dependent RNA polymerase (RdRp) gene between a Russian isolate (PlAMV-Ru), a Nandina isolate (PlAMV-Na) and Japanese isolates were 75-77%. These values were the lowest amongst different isolates of the same species of any potexviruses.

Genome sequence of a Japanese isolate of Radish mosaic virus: the first complete nucleotide sequence of a crucifer-infecting comovirus.

The complete nucleotide sequences of RNA1 and RNA2 of a Japanese isolate of Radish mosaic virus (RaMV-J), a crucifer-infecting comovirus, were determined. RNA1 is 6064 nucleotides long and encodes a 210-kDa polyprotein containing conserved motifs that are required for replication. RNA2 is 4020 nucleotides long and encodes a 123-kDa polyprotein containing the putative movement protein and two coat proteins. Comparisons of the encoded proteins confirmed that RaMV-J and a Czech RaMV isolate are isolates of the same species in the genus Comovirus. A phylogenetic analysis of RaMV-J and other comoviruses revealed that legume-infecting comoviruses constitute a single branch to which RaMV is distantly related.

A single amino acid in the RNA-dependent RNA polymerase of Plantago asiatica mosaic virus contributes to systemic necrosis.

From a lily isolate of Plantago asiatica mosaic virus (PlAMV-Li), two sub-isolates (Li1 and Li6) were obtained. Although the nucleotide sequences of Li1 and Li6 were highly conserved, they showed different pathogenicity in Nicotiana benthamiana. Li1 caused necrosis, whereas Li6 infected the plant asymptomatically. Inoculation tests with chimeric and point-mutated viruses revealed that amino acid 1154 of the RNA-dependent RNA polymerase (RdRp) contributes to the necrotic symptoms. The accumulation of the mutant viruses, in which amino acid 1154 of the RdRp was exchanged to the wild-type codon in Li1 and Li6, was almost equal.

Effect of brefeldin A on melatonin secretion of chick pineal cells.

Melatonin is secreted from the pineal gland in a circadian manner. It is well established that the synthesis of melatonin shows a diurnal rhythm reflecting a daily change in serotonin N-acetyltransferase (NAT) activity, and the overall secretion of melatonin requires a cellular release process, which is poorly understood. To investigate the possible involvement of Golgi-derived vesicles in the release, we examined the effect of brefeldin A (BFA), a reversible inhibitor of Golgi-mediated secretion, on melatonin secretion of cultured chick pineal cells. We show here that treatment with BFA completely disassembles the Golgi apparatus and reduces melatonin secretion. In more detailed time course experiments, however, the inhibition of melatonin secretion is only observed after the removal of BFA in parallel with the reassembly of the Golgi apparatus. This inhibition of melatonin secretion is not accompanied by accumulation of melatonin in the cells. These observations indicate that chick pineal melatonin is released independently of the Golgi-derived vesicles, and suggest inhibition of melatonin synthesis after the removal of BFA. By measuring the activities and mRNA levels of melatonin-synthesizing enzymes, we found that the removal of BFA specifically inhibits NAT activity at the protein level. On the other hand, BFA causes no detectable phase-shift of the chick pineal oscillator regulating the circadian rhythm of melatonin secretion. The results presented here suggest that the Golgi-mediated vesicular transport is involved in neither the melatonin release nor the time-keeping mechanism of the circadian oscillator, but rather contributes to the regulation of NAT activity.

Complete nucleotide sequence of Tulip virus X (TVX-J): the border between species and strains within the genus Potexvirus.

The complete nucleotide sequence of the genomic RNA of Tulip virus X Japanese isolate (TVX-J) has been determined. The sequence is 6056 nucleotides in length, excluding the poly(A) tail at the 3' terminus, and contains five open reading frames (ORFs) coding for proteins of Mr 153, 25, 12, 10, and 22 kDa (ORFs 1 through 5, respectively). The genome organization of TVX-J is similar to that of potexviruses, and the encoded proteins share a high degree of homology to the corresponding proteins of other potexviruses. Phylogenetic analyses based on the RNA-dependent RNA polymerase (RdRp) protein (the methyltransferase, helicase, and polymerase domains) encoded by ORF1 and the capsid protein (CP) encoded by ORF5, revealed a close relationship of TVX-J to Plantago asiatica mosaic virus (PlAMV). Pairwise comparison analyses revealed that the relationship between TVX and PlAMV is intermediate between that of strains and species, though previously they have not been considered related. Due to the relatively distant relationships of their replication apparatus and triple gene blocks, we conclude that TVX and PlAMV should be classified as distinct viruses. In addition, the borderline between species and strains of potexviruses is discussed.

Pleiotropic alterations in lipid metabolism in yeast sac1 mutants: relationship to "bypass Sec14p" and inositol auxotrophy.

SacIp dysfunction results in bypass of the requirement for phosphatidylinositol transfer protein (Sec14p) function in yeast Golgi processes. This effect is accompanied by alterations in inositol phospholipid metabolism and inositol auxotrophy. Elucidation of how sac1 mutants effect "bypass Sec14p" will provide insights into Sec14p function in vivo. We now report that, in addition to a dramatic accumulation of phosphatidylinositol-4-phosphate, sac1 mutants also exhibit a specific acceleration of phosphatidylcholine biosynthesis via the CDP-choline pathway. This phosphatidylcholine metabolic phenotype is sensitive to the two physiological challenges that abolish bypass Sec14p in sac1 strains; i.e. phospholipase D inactivation and expression of bacterial diacylglycerol (DAG) kinase. Moreover, we demonstrate that accumulation of phosphatidylinositol-4-phosphate in sac1 mutants is insufficient to effect bypass Sec14p. These data support a model in which phospholipase D activity contributes to generation of DAG that, in turn, effects bypass Sec14p. A significant fate for this DAG is consumption by the CDP-choline pathway. Finally, we determine that CDP-choline pathway activity contributes to the inositol auxotrophy of sac1 strains in a novel manner that does not involve obvious defects in transcriptional expression of the INO1 gene.

The Saccharomyces cerevisiae SCS2 gene product, a homolog of a synaptobrevin-associated protein, is an integral membrane protein of the endoplasmic reticulum and is required for inositol metabolism.

The Saccharomyces cerevisiae SCS2 gene has been cloned as a suppressor of inositol auxotrophy of CSE1 and hac1/ire15 mutants (J. Nikawa, A. Murakami, E. Esumi, and K. Hosaka, J. Biochem. 118:39-45, 1995) and has homology with a synaptobrevin/VAMP-associated protein, VAP-33, cloned from Aplysia californica (P. A. Skehel, K. C. Martin, E. R. Kandel, and D. Bartsch, Science 269:1580-1583, 1995). In this study we have characterized an SCS2 gene product (Scs2p). The product has a molecular mass of 35 kDa and is C-terminally anchored to the endoplasmic reticulum, with the bulk of the protein located in the cytosol. The disruption of the SCS2 gene causes yeast cells to exhibit inositol auxotrophy at temperatures of above 34 degrees C. Genetic studies reveal that the overexpression of the INO1 gene rescues the inositol auxotrophy of the SCS2 disruption strain. The significant primary structural feature of Scs2p is that the protein contains the 16-amino-acid sequence conserved in yeast and mammalian cells. The sequence is required for normal Scs2p function, because a mutant Scs2p that lacks the sequence does not complement the inositol auxotrophy of the SCS2 disruption strain. Therefore, the Scs2p function might be conserved among eukaryotic cells.

Essential role for diacylglycerol in protein transport from the yeast Golgi complex.

Yeast phosphatidylinositol transfer protein (Sec14p) is required for the production of secretory vesicles from the Golgi. This requirement can be relieved by inactivation of the cytosine 5'-diphosphate (CDP)-choline pathway for phosphatidylcholine biosynthesis, indicating that Sec14p is an essential component of a regulatory pathway linking phospholipid metabolism with vesicle trafficking (the Sec14p pathway). Sac1p (refs 7 and 8) is an integral membrane protein related to inositol-5-phosphatases such as synaptojanin, a protein found in rat brain. Here we show that defects in Sac1p also relieve the requirement for Sec14p by altering phospholipid metabolism so as to expand the pool of diacylglycerol (DAG) in the Golgi. Moreover, although short-chain DAG improves secretory function in strains with a temperature-sensitive Sec14p, expression of diacylglycerol kinase from Escherichia coli further impairs it. The essential function of Sec14p may therefore be to maintain a sufficient pool of DAG in the Golgi to support the production of secretory vesicles.

Kes1p shares homology with human oxysterol binding protein and participates in a novel regulatory pathway for yeast Golgi-derived transport vesicle biogenesis.

The yeast phosphatidylinositol transfer protein (Sec14p) is required for biogenesis of Golgi-derived transport vesicles and cell viability, and this essential Sec14p requirement is abrogated by inactivation of the CDP-choline pathway for phosphatidylcholine biosynthesis. These findings indicate that Sec14p functions to alleviate a CDP-choline pathway-mediated toxicity to yeast Golgi secretory function. We now report that this toxicity is manifested through the action of yeast Kes1p, a polypeptide that shares homology with the ligand-binding domain of human oxysterol binding protein (OSBP). Identification of Kes1p as a negative effector for Golgi function provides the first direct insight into the biological role of any member of the OSBP family, and describes a novel pathway for the regulation of Golgi-derived transport vesicle biogenesis.

The yeast BSD2-1 mutation influences both the requirement for phosphatidylinositol transfer protein function and derepression of phospholipid biosynthetic gene expression in yeast.

The BSD2-1 allele renders Saccharomyces cerevisiae independent of its normally essential requirement for phosphatidylinositol transfer protein (Sec14p) in the stimulation of Golgi secretory function and cell viability. We now report that BSD2-1 yeast mutants also exhibit yet another phenotype, an inositol auxotrophy. We demonstrate that the basis for this Ino- phenotype is the inability of BSD2-1 strains to derepress transcription of INO1, the structural gene for the enzyme that catalyzes the committed step in de novo inositol biosynthesis in yeast. This constitutive repression of INO1 expression is mediated through specific inactivation of Ino2p, a factor required for trans-activation of INO1 transcription, and we show that these transcriptional regulatory defects can be uncoupled from the "bypass Sec14p" phenotype of BSD2-1 strains. Finally, we present evidence that newly synthesized phosphatidylinositol is subject to accelerated turnover in BSD2-1 mutants and that prevention of this accelerated phosphatidyl-inositol turnover in turn negates suppression of Sec14p defects by BSD2-1. We propose that, in BSD2-1 strains, a product(s) generated by phosphatidylinositol turnover coordinately modulates the activities of both the Sec14p/Golgi pathway and the pathway through which transcription of phospholipid biosynthetic genes is derepressed.

Fusion of dioleoylphosphatidylcholine vesicles induced by an amphiphilic cationic peptide and oligophosphates at neutral pH.

Peptide E5 is an analogue of the fusion peptide of influenza virus hemagglutinin and K5 is a cationic peptide which has an arrangement of electric charges complementary to that of E5. We reported that a stoichiometric mixture of E5 and K5 caused fusion of large unilamellar vesicles (LUV) of neutral phospholipids (Murata, M., Kagiwada, S., Takahashi, S. and Ohnishi, S. (1991) J. Biol. Chem. 266, 14353-14358). K5 caused fusion of LUV composed of dioleoylphosphatidylcholine (DOPC) at pH > 10, but not at neutral pH. In the presence of oligophosphates, such as 1 mM ATP, GTP, or polyphosphate, K5 caused rapid and efficient fusion of DOPC LUV at neutral pH without hydrolysis of oligophosphate groups, but another anions such as citrate, acetate, AMP, phosphate, or EDTA were ineffective. The peptide/oligophosphate-induced fusion behaviors have been investigated by a fluorescence resonance energy transfer assay for lipid mixing of LUV and negative staining electron microscopy. At higher ionic strengths ( > 0.3 M KCl) or in the presence of 5.0 mM MgCl2, the fusion was inhibited. Even at the inhibitory conditions, the association of K5 with lipid vesicles at neutral pH was directly confirmed by the Ficoll gradient assay method and by blue shifts of the tryptophan fluorescence of the peptide. A nonhydrolyzable GTP analogue, GTP gamma S, also induced fusion. These observations suggested that the electrostatic interactions between the positive and negative charges of K5 and oligophosphate, respectively, induced complex formation, triggering membrane fusion.

Specificity of amphiphilic anionic peptides for fusion of phospholipid vesicles.

We have synthesized five amphiphilic anionic peptides derived from E5 peptide [Murata, M., Takahashi, S., Kagiwada, S., Suzuki, A., Ohnishi, S. 1992. Biochemistry 31:1986-1992. E5NN and E5CC are duplications of the N-terminal and the C-terminal halves of E5, respectively, and E5CN is an inversion of the N- and the C-terminal halves. E5P contains a Pro residue in the center of E5 and E8 has 8 Glu residues and 9 Leu residues. We studied fusion of dioleoylphosphatidylcholine (DOPC) large unilamellar vesicles assayed by fluorescent probes. The peptides formed alpha-helical structure with different degrees; E5NN, E5CN, and E8 with high helical content and E5CC and E5P with low helical content. These peptides bound to DOPC vesicles at acidic pH in proportion to the helical content of peptide. The peptides caused leakage of DOPC vesicles which increased with decreasing pH. The leakage was also proportional to the helicity of peptide. Highly helical peptides E5NN, E5CN, and E8 caused hemolysis at acidic pH but not at neutral pH. The fusion activity was also dependent on the helicity of peptides. In fusion induced by an equimolar mixture of E5 analogues and K5 at neutral pH, E8, E5NN, and E5CN were most active but E5CC did not cause fusion. In fusion induced by E5-analogue peptides alone, E5CN was active at acidic pH but not at neutral pH. Other peptides did not cause fusion. Amphiphilic peptides also appear to require other factors to cause fusion.

In vitro fusion of rabbit liver Golgi membranes with liposomes.

Fusion of Golgi membranes isolated from rabbit liver with liposomes was studied by lipid mixing of fluorescent lipid analogues and internal content mixing and by electron microscopic observation of transfer of horseradish peroxidase from liposomes into Golgi membranes. A monoclonal antibody was used to confirm fusion of Golgi membranes but not other contaminating vesicles. Fusion was rapid and efficient, reaching about 20% of the maximum after a 5-min incubation using small or large unilamellar dioleoylphosphatidylcholine vesicles. The fusion was dependent on temperature, decreasing at lower temperatures, and becoming nearly zero below 10 degrees C. The addition of ATP, GTP, cytosolic factors, or N-ethylmaleimide did not affect fusion. Treatments of Golgi membranes with 0.1 M Na2CO3 or 1 M KCl did not cause any changes in fusion. However, treatment with proteases inhibited fusion. These results suggest that Golgi integral membrane protein(s) are involved in fusion. Changing the medium to an isoosmotic substance, sucrose, in place of KCl or NaCl inhibited fusion. The binding assay of fluorescent liposomes to Golgi membranes showed that lowering the temperature or replacing salts with sucrose did not affect binding. However, treatment of Golgi membranes with proteases inhibited binding. Addition of phosphatidylserine or phosphatidylethanolamine to dioleoylphosphatidylcholine liposomes caused a 2-fold increase in binding and fusion. Fusion between Golgi membranes by themselves did not occur. These results provide some information on the mechanism of intracellular vesicular transport.

pH-dependent membrane fusion and vesiculation of phospholipid large unilamellar vesicles induced by amphiphilic anionic and cationic peptides.

We studied fusion induced by a 20-amino acid peptide derived from the amino-terminal segment of hemagglutinin of influenza virus A/PR/8/34 [Murata, M., Sugahara, Y., Takahashi, S., & Ohnishi, S. (1987) J. Biochem. (Tokyo) 102, 957-962]. To extend the study, we have prepared several water-soluble amphiphilic peptides derived from the HA peptide; the anionic peptides D4, E5, and E5L contain four and five acidic residues and the cationic peptide K5 has five Lys residues in place of the five Glu residues in E5. Fusion of egg phosphatidylcholine large unilamellar vesicles induced by these peptides is assayed by two different fluorescence methods, lipid mixing and internal content mixing. Fusion is rapid in the initial stage (12-15% within 20 s) and remains nearly the same or slightly increasing afterward. The anionic peptides cause fusion at acidic pH lower than 6.0-6.5, and the cationic peptide causes fusion at alkaline pH higher than 9.0. Leakage and vesiculation of vesicles are also measured. These peptides are bound and associated with vesicles as shown by Ficoll discontinuous gradients and by the blue shift of tryptophan fluorescence. They take an alpha-helical structure in the presence of vesicles. They become more hydrophobic in the pH regions for fusion. When the suspension is made acidic or alkaline, the vesicles aggregate, as shown by the increase in light scattering. The fusion mechanism suggests that the amphiphilic peptides become more hydrophobic by neutralization due to protonation of the carboxyl groups or deprotonation of the lysyl amino groups, aggregate the vesicles together, and interact strongly with lipid bilayers to cause fusion. At higher peptide concentrations, E5 and E5L cause fusion transiently at acidic pH followed by vesiculation.

Interaction of the Golgi membranes isolated from rabbit liver with microtubules in vitro.

We have developed a reconstituted model system to study the interaction of the Golgi membranes isolated from rabbit liver with taxol-stabilized bovine-brain microtubules without microtubule-associated proteins (MAPs). The Golgi membranes are associated with microtubules. The sheets of vesicles and the membranous tubules are observed along microtubules by direct visualization using differential-interference-contrast, dark field, or fluorescence microscopy. The monoclonal antibody against Golgi membranes suggests that the Golgi membranes, but not the contaminating vesicles, are interacting with microtubules. The degree of association is assayed quantitatively using rhodamine-labeled microtubules after separation of the complex from unbound microtubules by centrifugation upon sucrose gradient. The association is inhibited by crude MAPs, purified MAP2, or 1.0 mM ATP. However, the association neither requires the cytosol from rat liver or bovine brain nor N-ethylmaleimide, brefeldin A, or GTP-gamma-S. The association is mediated by trypsin-sensitive peripheral protein(s) on the Golgi membranes.

Modification of the N-terminus of membrane fusion-active peptides blocks the fusion activity.

The amphiphilic anionic peptides E5 and E5L can mimic the fusogenic activity of influenza hemagglutinin(HA). These peptides induced fusion of egg yolk phosphatidylcholine small or large unilamellar vesicles only at acidic pH in a similar manner to viral HA. Acetylation or acetimidylation of the N-terminus of the peptides drastically reduced the fusion activity of the intact peptides, while C-terminal amidation left the activity unchanged. The binding assay suggested that the interaction of the modified peptides with lipid membranes was almost unchanged in comparison with those of the parent peptides, and the CD spectra showed that these peptides were alpha-helical. The results showed the importance of the N-terminus of the peptides on the membrane fusion activity, although why the N-terminal modifications affect the activity is still unclear.

Membrane fusion induced by mutual interaction of the two charge-reversed amphiphilic peptides at neutral pH.

An anionic amphiphilic peptide and the charge-reversed cationic peptide are synthesized. They contain 20 amino acids with the same sequence except for 5 Glu residues for the anionic versus 5 Lys residues for the cationic peptides. Fusion of egg phosphatidylcholine large unilamellar vesicles is assayed with the fluorescent probes by the lipid mixing and the internal content mixing at neutral pH. The peptide mixture causes a rapid and efficient membrane fusion, in spite of no fusions with each peptide by itself. Each peptide takes nearly random coils with a small amount of helix, but the peptide mixture has an ordered helical structure. The equimolar peptide mixture forms a much more hydrophobic complex than those of different molar ratios of peptides and also that of each peptide itself. The equimolar peptide mixture causes the most efficient fusion. Preincubations of two peptides before addition to vesicles cause the slower rates of fusion. The fusion is greatly reduced at higher ionic strength and nearly zero at 800 mM NaCl and 40 mM sodium phosphate. Each peptide and the peptide mixture show the same alpha-helical structure, interact with vesicles, but do not induce fusion at higher ionic strengths. These results suggest that the two peptides interact mutually through the electrostatic Coulombic interaction between the charged groups. The electrically neutralized hydrophobic complex aggregates the separate vesicles together and interacts with the hydrocarbon region of lipid bilayers to cause fusion.