The epitope structure of Citrus tristeza virus coat protein mapped by recombinant proteins and monoclonal antibodies

A new single-chain variable fragment (scFv) antibody provides sensitive and specific detection of citrus tristeza virus

Citrus tristeza virus (CTV) is the most economically important virus disease of citrus worldwide. To develop a specific serological assay for CTV, a Tomlinson phage display antibody library of single chain variable fragments (scFv) was screened with a recombinant CTV coat protein (CTV-CP) heterologously expressed in Escherichia coli. The phage clones were checked by ELISA to identify clones with high specificity for CTV-CP. Eight clones were strongly reactive with CTV-CP. Nucleotide sequencing of these clones revealed that all of them contained the same sequence. Thus, the phage-displayed scFv antibody was termed scFvF10. Evaluation of scFvF10 binding to CTV-CP by plate-trapped antigen ELISA (PTA-ELISA) and immunoblotting, showed that it was specific and allowed sensitive detection of CTV-CP. Homology-based molecular modeling and docking analysis confirmed that the interaction between CTV-CP and scFvF10, with a binding energy of -738 kj mol-1, occurred mainly by 12 intermolecular hydrogen bonds. Moreover, triple-antibody sandwich (TAS)-ELISA using scFvF10 as second antibody showed high sensitivity in the detection of CTV infected samples. The CTV detection limit of scFvF10 by PTA-ELISA and TAS-ELISA were 0.05 and 0.01 μg CP/mL, respectively. Our results with different diagnostic assays demonstrated that scFvF10 has the potential to be used as an efficient tool for CTV-infected plant diagnosis.

Discovery and Characterization of a Novel Bipartite Botrexvirus From the Phytopathogenic Fungus Botryosphaeria dothidea

In this study, we describe a novel positive, single-stranded (+ss) RNA mycovirus, named Botryosphaeria dothidea botrexvirus 1 (BdBV1), from a phytopathogenic fungus Botryosphaeria dothidea showing abnormal morphology and attenuated virulence. BdBV1 is phylogenetically related to Botrytis virus X (BotVX) and is the second potential member of the proposed genus Botrexvirus in the family Alphaflexiviridae. However, it differs from the monopartite BotVX in that BdBV1 possesses a bipartite genome comprised of two ssRNA segments (RNA1 and RNA2 with lengths of 5,035 and 1,063 nt, respectively). BdBV1 RNA1 and RNA2 encode putative RNA-dependent RNA polymerase (RdRp) and coat protein (CP) genes, which share significant identity with corresponding genes in both fungal and plant viruses. Moreover, open reading frames (ORFs) 2–4 of BdBV1 RNA1 shared no detectable identity with any known viral proteins. Immunosorbent electron microscopy (ISEM) analysis using an antibody against the virus CP generated in vitro revealed that BdBV1 is encapsidated in filamentous particles. A comparison of the biological effects of BdBV1 infection on symptoms and growth in isogenic lines of virus-free and virus-infected B. dothidea revealed that BdBV1 is probably involved in reduced growth and virulence of the host fungus. This study describes and characterizes a novel bipartite botrexvirus, which is closely related to uni- and multi-partite fungal and plant viruses and contributes useful information to a better understanding of virus evolution.

Development of a reverse transcription recombinase polymerase based isothermal amplification coupled with lateral flow immunochromatographic assay (CTV-RT-RPA-LFICA) for rapid detection of Citrus tristeza virus

Tristeza is a highly destructive disease of citrus caused by the phloem-limited, flexuous filamentous Citrus tristeza virus (CTV) in the genus Closterovirus and the family Closteroviridae. It has been a major constraint for higher productivity and has destroyed millions of citrus trees globally. CTV is graft transmissible and spread through use of virus infected nursery plants. Therefore, virus detection by using specific and reliable diagnostic tools is very important to mitigate disease outbreaks. Currently, the standard molecular techniques for CTV detection include RT-PCR and RT-qPCR. These diagnostic methods are highly sensitive but time consuming, labor intensive and require sophisticated expensive instruments, thus not suitable for point-of-care use. In the present study, we report the development of a rapid, sensitive, robust, reliable, and highly specific reverse transcription-RPA technique coupled with a lateral flow immunochromatographic assay (CTV-RT-RPA-LFICA). RT-RPA technique was standardized to amplify the coat protein gene of CTV (CTV-p25) and detect double labeled amplicons on a sandwich immunoassay by designing specific labeled primer pair and probe combinations. The optimally performing primer set (CTRPA-F1/CTRPA-R9-Btn) and the corresponding TwistAmp nfo probe (CTRPA-Probe) was optimized for temperature and reaction time using purified cDNA and viral RNA as template. The sensitivity of the developed assay was compared with other detection techniques using in vitro-transcribed RNA. The efficacy and specificity of the assay was evaluated using CTV positive controls, healthy samples, field grown citrus plants of unknown status, and other virus and bacterial pathogens that infect citrus plants. The RT-RPA-LFICA was able to detect ≤ 141 fg of RNA when cDNA used as a template. The assay detected ≤ 0.23 ng/µl of CTV RNA when directly used as template without cross-reactivity with other citrus pathogens. Best results were achieved at the isothermal temperature of 40 °C within 15-20 min. The study demonstrated that RT-RPA-LFICA has potential to become an improved detection technique for end users in bud-wood certification and quarantine programs and a promising platform for rapid point-of-care diagnostics for citrus farmers and small nurseries in low resource settings.

In-silico characterization and RNA-binding protein based polyclonal antibodies production for detection of citrus tristeza virus

Citrus tristeza virus (CTV) is the etiologic agent of the destructive Tristeza disease, a massive impediment for the healthy citrus industry worldwide. Routine indexing of CTV is an essential component for disease surveys and citrus budwood certification for production of disease-free planting material. Therefore, the present study was carried out to develop an efficient serological assay for CTV detection based on the RNA binding protein (CTV-p23), which is translated from a subgenomic RNA (sgRNA) that accumulates at higher levels in CTV-infected plants. CTV-p23 gene was amplified, cloned and polyclonal antibodies were raised against recombinant CTV-p23 protein. The efficacy of the produced polyclonal antibodies was tested by Western blots and ELISA to develop a quick, sensitive and economically affordable CTV detection tool and was used for indexing of large number of plant samples. The evaluation results indicated that the developed CTV-p23 antibodies had an excellent diagnostic agreement with RT-PCR and would be effective for the detection of CTV in field samples. Furthermore, CTV-p23 gene specific primers designed in the present study were found 1000 times more sensitive than the reported coat protein (CTV-p25) gene specific primers for routine CTV diagnosis. In silico characterizations of CTV-p23 protein revealed the presence of key conserved amino acid residues that involved in the regulation of protein stability, suppressor activity and protein expression levels. This would provide precious ground information towards understanding the viral pathogenecity and protein level accumulation for early diagnosis of virus.

Dynamin-Like Proteins of Endocytosis in Plants Are Coopted by Potyviruses To Enhance Virus Infection

Endocytosis and endosomal trafficking regulate the proteins targeted to the plasma membrane and play essential roles in diverse cellular processes, including responses to pathogen attack. Here, we report the identification of Glycine max (soybean) endocytosis dynamin-like protein 5A (GmSDL5A) associated with purified soybean mosaic virus (SMV) virions from soybean using a bottom-up proteomics approach. Knockdown of GmSDL5A and its homologous gene GmSDL12A inhibits SMV infection in soybean. The role of analogous dynamin-like proteins in potyvirus infection was further confirmed and investigated using the Arabidopsis/turnip mosaic virus (TuMV) pathosystem. We demonstrate that dynamin-related proteins 2A and 2B in Arabidopsis thaliana (AtDRP2A, AtDRP2B), homologs of GmSDL5A, are recruited to the virus replication complex (VRC) of TuMV. TuMV infection is inhibited in both A. thalianadrp2a (atdrp2a) and atdrp2b knockout mutants. Overexpression of AtDRP2 promotes TuMV replication and intercellular movement. AtRDP2 interacts with TuMV VPg, CP, CI, and 6K2. Of these viral proteins, VPg, CP, and CI are essential for viral intercellular movement, and 6K2, VPg, and CI are critical components of the VRC. We reveal that VPg and CI are present in the punctate structures labeled by the endocytic tracer FM4-64, suggesting that VPg and CI can be endocytosed. Treatment of plant leaves with a dynamin-specific inhibitor disrupts the delivery of VPg and CI to endocytic structures and suppresses TuMV replication and intercellular movement. Taken together, these data suggest that dynamin-like proteins are novel host factors of potyviruses and that endocytic processes are involved in potyvirus infection.IMPORTANCE It is well known that animal viruses enter host cells via endocytosis, whereas plant viruses require physical assistance, such as human and insect activities, to penetrate the host cell to establish their infection. In this study, we report that the endocytosis pathway is also involved in virus infection in plants. We show that plant potyviruses recruit endocytosis dynamin-like proteins to support their infection. Depletion of them by knockout of the corresponding genes suppresses virus replication, whereas overexpression of them enhances virus replication and intercellular movement. We also demonstrate that the dynamin-like proteins interact with several viral proteins that are essential for virus replication and cell-to-cell movement. We further show that treatment of a dynamin-specific inhibitor disrupts endocytosis and inhibits virus replication and intercellular movement. Therefore, the dynamin-like proteins are novel host factors of potyviruses. The corresponding genes may be manipulated using advanced biotechnology to control potyviral diseases.

Monoclonal antibody-based serological methods for detecting Citrus tristeza virus in citrus groves

Citrus tristeza virus (CTV) is one of the most economically important citrus viruses and harms the citrus industry worldwide. To develop reliable and effective serological detection assays of CTV, the major capsid protein (CP) gene of CTV was expressed in Escherichia coli BL21 (DE3) using the expression vector pET-28a and purified through Ni+-NTA affinity chromatography. The recombinant protein was used to immunize BALB/c mice. Four hybridoma cell lines (14B10, 14H11, 20D5, and 20G12) secreting monoclonal antibodies (MAbs) against CTV were obtained through conventional hybridoma technology. The titers of MAb-containing ascitic fluids secreted by the four hybridoma lines ranged from 10(-6) to 10(-7) in indirect enzyme-linked immunosorbent assay (ELISA). Western blots showed that all four MAbs could specifically react with CTV CP. Using the prepared MAbs, dot-ELISA, Tissue print-ELISA, and triple antibody sandwich (TAS)-ELISA were developed to detect CTV in tree nurseries and epidemiological studies. The developed dot-ELISA and TAS-ELISA methods could detect CTV in crude extracts of infected citrus leaves with dilutions of 1:2560 and 1:10, 240 (w/v, g/mL), respectively. Tissue print-ELISA was particularly useful for large-scale field sample detection, mainly owing to its simplicity and lack of sample preparation requirements. The field survey revealed that CTV is prevalent on citrus trees in the Chongqing Municipality, Jiangxi Province, and Zhejiang Province of China. The coincidence rate of serological and RT-PCR test results reached more than 99.5%. The prepared MAbs against CTV and established sensitive and specific serological assays have a significant role in the detection and prevention and control of CTV in our country.

Towards a Unified Theory of Calmodulin Regulation (Calmodulation) of Voltage-Gated Calcium and Sodium Channels

Voltage-gated Na and Ca(2+) channels represent two major ion channel families that enable myriad biological functions including the generation of action potentials and the coupling of electrical and chemical signaling in cells. Calmodulin regulation (calmodulation) of these ion channels comprises a vital feedback mechanism with distinct physiological implications. Though long-sought, a shared understanding of the channel families remained elusive for two decades as the functional manifestations and the structural underpinnings of this modulation often appeared to diverge. Here, we review recent advancements in the understanding of calmodulation of Ca(2+) and Na channels that suggest a remarkable similarity in their regulatory scheme. This interrelation between the two channel families now paves the way towards a unified mechanistic framework to understand vital calmodulin-dependent feedback and offers shared principles to approach related channelopathic diseases. An exciting era of synergistic study now looms.

Soybean actin-depolymerizing factor 2 interacts with Soybean mosaic virus-encoded P3 protein

Soybean mosaic virus (SMV), a member of the Potyvirus genus, is one of the most prevalent and devastating viral pathogens in soybean-growing regions worldwide. It is generally accepted that symptom development of a viral plant disease results from molecular interactions between the virus and its host plant. P3 protein is the most variable polyprotein in potyviruses, which potentially plays an important role in the process of the evolution of virus type specialization. However, P3 not only plays a major role in virus replication and movement, but it is also responsible for symptom development in SMV-infected plants. This study provides evidence that actin-depolymerizing factor 2 (designated as ADF2) of soybean interacts with SMV P3 via a two-hybrid yeast system by screening a soybean cDNA library. Bimolecular fluorescence complementation assay further confirmed the interaction, which occurred in both the cytomembrane and cytoskeleton of Nicotiana benthamiana cells. The results support the hypothesis that SMV P3 might have a role in virus movement within cells.