A host small GTP-binding protein ARL8 plays crucial roles in tobamovirus RNA replication.

Tomato mosaic virus (ToMV), like other eukaryotic positive-strand RNA viruses, replicates its genomic RNA in replication complexes formed on intracellular membranes. Previous studies showed that a host seven-pass transmembrane protein TOM1 is necessary for efficient ToMV multiplication. Here, we show that a small GTP-binding protein ARL8, along with TOM1, is co-purified with a FLAG epitope-tagged ToMV 180K replication protein from solubilized membranes of ToMV-infected tobacco (Nicotiana tabacum) cells. When solubilized membranes of ToMV-infected tobacco cells that expressed FLAG-tagged ARL8 were subjected to immunopurification with anti-FLAG antibody, ToMV 130K and 180K replication proteins and TOM1 were co-purified and the purified fraction showed RNA-dependent RNA polymerase activity that transcribed ToMV RNA. From uninfected cells, TOM1 co-purified with FLAG-tagged ARL8 less efficiently, suggesting that a complex containing ToMV replication proteins, TOM1, and ARL8 are formed on membranes in infected cells. In Arabidopsis thaliana, ARL8 consists of four family members. Simultaneous mutations in two specific ARL8 genes completely inhibited tobamovirus multiplication. In an in vitro ToMV RNA translation-replication system, the lack of either TOM1 or ARL8 proteins inhibited the production of replicative-form RNA, indicating that TOM1 and ARL8 are required for efficient negative-strand RNA synthesis. When ToMV 130K protein was co-expressed with TOM1 and ARL8 in yeast, RNA 5'-capping activity was detected in the membrane fraction. This activity was undetectable or very weak when the 130K protein was expressed alone or with either TOM1 or ARL8. Taken together, these results suggest that TOM1 and ARL8 are components of ToMV RNA replication complexes and play crucial roles in a process toward activation of the replication proteins' RNA synthesizing and capping functions.

High accumulation of the Man5GlcNAc2 structure by combining N-acetylglucosaminyltransferase I gene suppression and mannosidase I gene overexpression in Nicotiana tabacum SR1.

High accumulation of a single high-mannose glycan structure is important to ensure the quality of therapeutic proteins. We developed a glyco-engineering strategy for ensuring high accumulation of the Man5GlcNAc2 structure by combining N-acetylglucosaminyltransferase I (GnT I) gene suppression and mannosidase I (Man I) gene overexpression. Nicotiana tabacum SR1 was used as the glyco-engineered host owing to the lower risk of pathogenic contamination than that in mammalian cells. We generated three glyco-engineered plant strains (gnt, gnt-MANA1, and gnt-MANA2) with suppression of GnT I or the combined suppression of GnT I and overexpression of Man I A1 or A2. The quantitative reverse transcriptase-PCR analysis showed a higher level of upregulation of Man I expression in gnt-MANA1/A2 plants than in the wild-type plants. Man I activity assay showed that the gnt-MANA1 plants had a higher Man I activity than did the wild-type and gnt-MANA2 plants. N-glycan analysis independently performed on two plants of each plant strain showed that gnt-MANA1 plants had a low abundance of the Man6-9GlcNAc2 structure (2.8%, 7.1%) and high abundance of the Man5GlcNAc2 structure (80.0%, 82.8%) compared with those in the wild-type and gnt plants. These results indicated that GnT I knockdown suppressed further modification of the Man5GlcNAc2 structure, and Man I overexpression enhanced the conversion of Man6-9GlcNAc2 structures to the Man5GlcNAc2 structure. The developed glyco-engineered plants have potential for serving as novel expression hosts for therapeutic proteins.

Inducible viral inoculation system with cultured plant cells facilitates a biochemical approach for virus-induced RNA silencing.

An inducible virus infection system was demonstrated to be an efficient protein expression system for inducing synchronous virus vector multiplication in suspension-cultured plant cells. A GFP-tagged tomato mosaic virus (ToMV-GFP) derivative that has a defect in its 130 K protein, a silencing suppressor of ToMV, was synchronously infected to tobacco BY2 cultured cells using this system. In the infection-induced cells, viral RNA was degraded rapidly, and a cytosol extract prepared from the infected cells showed RNA degradation activity specific for ToMV- or GFP-related sequences. In lysate prepared from cells infected by ToMV-GFP carrying the wild-type 130 K protein, sequence-specific RNA degradation activity was suppressed, although siRNA derived from the virus was generated. Furthermore, the 130 K protein interfered with 3'-end methylation of siRNA. The inducible virus infection system may provide a method for biochemical analysis of antiviral RNA silencing and silencing suppression by ToMV.

Long-distance, graft-transmissible action of Arabidopsis FLOWERING LOCUS T protein to promote flowering.

Day length perceived by a leaf is a major environmental factor that controls the timing of flowering. It has been believed that a mobile, long-distance signal called florigen is produced in the leaf under inductive day length conditions, and is transported to the shoot apex where it triggers floral morphogenesis. Grafting experiments have shown that florigen is transmissible from a donor plant that has been subjected to inductive day length to an uninduced recipient plant. However, the nature of florigen has long remained elusive. Arabidopsis FLOWERING LOCUS T (FT) is expressed in cotyledons and leaves in response to inductive long days (LDs). FT protein, with a basic region/leucine zipper (bZIP) transcription factor FD, acts in the shoot apex to induce target meristem identity genes such as APETALA1 (AP1) and initiates floral morphogenesis. Recent studies have provided evidence that the FT protein in Arabidopsis and corresponding proteins in other species are an important part of florigen. Our work shows that the FT activity, either from overexpressing or inducible transgenes or from the endogenous gene, to promote flowering is transmissible through a graft junction, and that an FT protein with a T7 tag is transported from a donor scion to the apical region of recipient stock plants and becomes detectable within a day or two. The sequence and structure of mRNA are not of critical importance for the long-distance action of the FT gene. These observations led to the conclusion that the FT protein, but not mRNA, is the essential component of florigen.

The "resurrection method" for modification of specific proteins in higher plants.

We describe a new method designated "the resurrection method" by which a modified protein is expressed in higher plants in place of the original protein. The modified gene constructed by introducing synonymous codon substitutions throughout the original gene to prevent the sequence-specific degradation of its mRNA during RNA silencing is expressed while the expression of the original gene is suppressed. Here, we report the successful alteration of the biochemical properties of green fluorescent protein expressed in transgenic Nicotiana benthamiana, suggesting that this method could be useful for gene control in living plants.

Improved expression and characterization of recombinant human Golgi α1,2-mannosidase I isoforms (IA2 and IC) by Escherichia coli.

Golgi α1,2-mannosidase I is involved in the N-linked oligosaccharide processing pathway. In this study, two truncated genes encoding for human Golgi α1,2-mannosidase I (hManIA2: amino acids 127-626 and hManIC: amino acids 118-617) were expressed in Escherichia coli to characterize the enzymes. These genes were fused to a T7 protein tag and a histidine tag at the N- and C-terminal ends, respectively, and purified using Co(2+) affinity chromatography. The properties including optimal temperature, optimal pH, and substrate specificity of the purified enzymes were investigated by HPLC using pyridylamino (PA)-labeled oligosaccharides as substrates. The stability of hManIA2 was dependent on the presence of Ca(2+), which was also required for its activity. On the other hand, hManIC was stable in the absence of Ca(2+), even though Ca(2+) was also effective for the activity of hManIC. While the similarity of the amino acid sequences is over 60%, hManIA2 and hManIC showed different substrate specificities particularly toward M9A and M8C.

Jack bean α-mannosidase digestion profile of hybrid-type N-glycans: effect of reaction pH on substrate preference.

Jack bean α-mannosidase (JBM) is a well-studied plant vacuolar α-mannosidase, and is widely used as a tool for the enzymatic analysis of sugar chains of glycoproteins. In this study, the JBM digestion profile of hybrid-type N-glycans was examined using pyridylamino (PA-) sugar chains. The digestion efficiencies of the PA-labeled hybrid-type N-glycans Manα1,6(Manα1,3)Manα1,6(GlcNAcß1,2Manα1,3)Manß1,4GlcNAcß1,4GlcNAc-PA (GNM5-PA) and Manα1,6(Manα1,3)Manα1,6(Galß1,4GlcNAcß1,2Manα1,3)Manß1,4GlcNAcß1,4GlcNAc-PA (GalGNM5-PA) were significantly lower than that of the oligomannose-type N-glycan Manα1,6(Manα1,3)Manα1,6Manß1,4GlcNAcß1,4GlcNAc-PA (M4-PA), and the trimming pathways of GNM5-PA and GalGNM5-PA were different from that of M4-PA, suggesting a steric hindrance to the JBM activity caused by GlcNAcß1-2Man(α) residues of the hybrid-type N-glycans. We also found that the substrate preference of JBM for the terminal Manα1-6Man(α) and Manα1-3Man(α) linkages in the hybrid-type N-glycans was altered by the change in reaction pH, suggesting a pH-dependent change in the enzyme-substrate interaction.

Recombinant expression and characterization of N-acetylglucosaminyltransferase I derived from Nicotiana tabacum.

The C-terminal catalytic domain of tobacco N-acetylglucosaminyltransferase I fused to maltose-binding protein was produced in Escherichia coli as a soluble form with significant activity. The protein was affinity-purified using amylose resin, and its enzymatic properties were investigated, including its divalent cation requirements, optimal temperature, optimal pH, and substrate specificity.

Insertion in the coding region of the movement protein improves stability of the plasmid encoding a tomato mosaic virus-based expression vector.

A major obstacle in the genetic manipulation of tomato mosaic virus (ToMV) is the instability of the plasmid containing the infectious full-length cDNA of the ToMV vector, which often prevents the subcloning of a foreign gene of interest into the vector. We found that an insertion of a 0.3-1.6-kbp DNA fragment in the movement protein (MP) coding region effectively attenuated bacterial toxicity of the plasmid and greatly increased plasmid yield. Accumulation of a modified ToMV containing a 0.3-kb insertion in the MP coding region was comparable to that of a modified ToMV without an insertion in tobacco BY-2 protoplasts, while an insertion more than 0.6 kb significantly reduced accumulation of the viral RNA. The modified ToMV vector containing a 0.3-kb insertion was easily manipulated to introduce a coding sequence for human interferon-gamma (HuIFN-gamma) and successfully utilized to produce HuIFN-gamma in both BY-2 protoplasts and transgenic BY-2 cells.

Stable-isotope labeling using an inducible viral infection system in suspension-cultured plant cells.

We established a novel strategy for preparing uniformly stable isotope-labeled proteins by using suspension-cultured plant cells and an inducible virus vector encoding the research target. By using this new method, we demonstrated the expression of three proteins, namely, Escherichia coli dihydrofolate reductase (DHFR), chicken calmodulin (CaM), and porcine protein kinase C-dependent protein phosphatase-1 inhibitor with a molecular mass of 17-kDa (CPI-17). In addition, we successfully expressed bovine pancreatic trypsin inhibitor (BPTI), which contains three pairs of disulfide bonds, as the soluble form. In the most efficient case, as little as 50 ml culture yielded 3-4 mg (15)N-labeled protein suitable for NMR experiments. The (1)H-(15)N HSQC spectra of all of these proteins clearly indicated that their structures were identical to those of their counterparts reported previously. Thus, the present results suggest that our novel protocol is a potential method for NMR sample preparation.

Saccharomyces cerevisiae SSD1 orthologues are essential for host infection by the ascomycete plant pathogens Colletotrichum lagenarium and Magnaporthe grisea.

Fungal plant pathogens have evolved diverse strategies to overcome the multilayered plant defence responses that confront them upon host invasion. Here we show that pathogenicity of the cucumber anthracnose fungus, Colletotrichum lagenarium, and the rice blast fungus, Magnaporthe grisea, requires a gene orthologous to Saccharomyces cerevisiae SSD1, a regulator of cell wall assembly. Screening for C. lagenarium insertional mutants deficient in pathogenicity led to the identification of ClaSSD1. Following targeted gene replacement, appressoria of classd1 mutants retained the potential for penetration but were unable to penetrate into host epidermal cells. Transmission electron microscopy suggested that appressorial penetration by classd1 mutants was restricted by plant cell wall-associated defence responses, which were observed less frequently with the wild-type strain. Interestingly, on non-host onion epidermis classd1 mutants induced papilla formation faster and more abundantly than the wild type. Similarly, colonization of rice leaves by M. grisea was severely reduced after deletion of the orthologous MgSSD1 gene and attempted infection by the mutants was accompanied by the accumulation of reactive oxygen species within the host cell. These results suggest that appropriate assembly of the fungal cell wall as regulated by SSD1 allows these pathogens to establish infection by avoiding the induction of host defence responses.

Membrane-bound tomato mosaic virus replication proteins participate in RNA synthesis and are associated with host proteins in a pattern distinct from those that are not membrane bound.

Extracts of vacuole-depleted, tomato mosaic virus (ToMV)-infected plant protoplasts contained an RNA-dependent RNA polymerase (RdRp) that utilized an endogenous template to synthesize ToMV-related positive-strand RNAs in a pattern similar to that observed in vivo. Despite the fact that only minor fractions of the ToMV 130- and 180-kDa replication proteins were associated with membranes, the RdRp activity was exclusively associated with membranes. A genome-sized, negative-strand RNA template was associated with membranes and was resistant to micrococcal nuclease unless treated with detergents. Non-membrane-bound replication proteins did not exhibit RdRp activity, even in the presence of ToMV RNA. While the non-membrane-bound replication proteins remained soluble after treatment with Triton X-100, the same treatment made the membrane-bound replication proteins in a form that precipitated upon low-speed centrifugation. On the other hand, the detergent lysophosphatidylcholine (LPC) efficiently solubilized the membrane-bound replication proteins. Upon LPC treatment, the endogenous template-dependent RdRp activity was reduced and exogenous ToMV RNA template-dependent RdRp activity appeared instead. This activity, as well as the viral 130-kDa protein and the host proteins Hsp70, eukaryotic translation elongation factor 1A (eEF1A), TOM1, and TOM2A copurified with FLAG-tagged viral 180-kDa protein from LPC-solubilized membranes. In contrast, Hsp70 and only small amounts of the 130-kDa protein and eEF1A copurified with FLAG-tagged non-membrane-bound 180-kDa protein. These results suggest that the viral replication proteins are associated with the intracellular membranes harboring TOM1 and TOM2A and that this association is important for RdRp activity. Self-association of the viral replication proteins and their association with other host proteins may also be important for RdRp activity.

RNA-dependent RNA polymerase complex of Brome mosaic virus: analysis of the molecular structure with monoclonal antibodies.

Viral RNA-dependent RNA polymerase (RdRp) plays crucial roles in the genomic replication and subgenomic transcription of Brome mosaic virus (BMV), a positive-stranded RNA plant virus. BMV RdRp is a complex of virus-encoded 1a and 2a proteins and some cellular factors, and associates with the endoplasmic reticulum at an infection-specific structure in the cytoplasm of host cells. In this study, we investigate the gross structure of the active BMV RdRp complex using monoclonal antibodies raised against the 1a and 2a proteins. Immunoprecipitation experiments showed that the intermediate region between the N-terminal methyltransferase-like domain and the C-terminal helicase-like domain of 1a protein, and the N terminus region of 2a protein are exposed on the surface of the solubilized RdRp complex. Inhibition assays for membrane-bound RdRp suggested that the intermediate region between the methyltransferase-like and the helicase-like domains of 1a protein is located at the border of the region buried within a membrane structure or with membrane-associated material.