Generation and Assembly of Virus-Specific Nucleocapsids of the Respiratory Syncytial Virus
Summary
For in-depth mechanistic analysis of the respiratory syncytial virus (RSV) RNA synthesis, we report a protocol of utilizing the chaperone phosphoprotein (P) for coexpression of the RNA-free nucleoprotein (N0) for subsequent in vitro assembly of the virus-specific nucleocapsids (NCs).
Abstract
The use of an authentic RNA template is critical to advance the fundamental knowledge of viral RNA synthesis that can guide both mechanistic discovery and assay development in virology. The RNA template of nonsegmented negative-sense (NNS) RNA viruses, such as the respiratory syncytial virus (RSV), is not an RNA molecule alone but rather a nucleoprotein (N) encapsidated ribonucleoprotein complex. Despite the importance of the authentic RNA template, the generation and assembly of such a ribonucleoprotein complex remain sophisticated and require in-depth elucidation. The main challenge is that the overexpressed RSV N binds non-specifically to cellular RNAs to form random nucleocapsid-like particles (NCLPs). Here, we established a protocol to obtain RNA-free N (N0) first by co-expressing N with a chaperone phosphoprotein (P), then assembling N0 with RNA oligos with the RSV-specific RNA sequence to obtain virus-specific nucleocapsids (NCs). This protocol shows how to overcome the difficulty in the preparation of this traditionally challenging viral ribonucleoprotein complex.
Introduction
Nonsegmented negative-sense (NNS) RNA viruses include many significant human pathogens, such as rabies, Ebola, and respiratory syncytial virus (RSV)1,2. RSV is the leading cause of respiratory illness such as bronchiolitis and pneumonia in young children and older adults worldwide3. Currently, no effective vaccines or antiviral therapies are available to prevent or treat RSV4. As part of the life cycle, the RSV genome serves as the template for replication by the RSV RNA dependent RNA polymerase to produce an antigenome, which in turn acts as the template to generate a progeny genome. Both genome and antigenome RNAs are entirely encapsidated by the nucleoprotein (N) to form the nucleocapsids (NCs)3. Because the NCs serve as the templates for both replication and transcription by the RSV polymerase, proper NC assembly is crucial for the polymerase to gain access to the templates for RNA synthesis5. Interestingly, based on the structural analyses of the NNS viral polymerases, it is hypothesized that several N proteins transiently dissociate from the NCs to allow the access of the polymerase and rebind to RNA after the RNA synthesis6,7,8,9,10,11,12.
Currently, the RSV RNA polymerization assay has been established using purified RSV polymerase on short naked RNA templates13,14. However, the activities of the RSV polymerase do not reach optimal, as observed in the non-processive and abortive products generated by the RSV polymerase when using naked RNA templates. The lack of NC with virus-specific RNA is a primary barrier for the further mechanistic understanding of the RSV RNA synthesis. Therefore, using an authentic RNA template becomes a critical need to advance the fundamental knowledge of RSV RNA synthesis. The known structures of the nucleocapsid-like particles (NCLPs) from RSV and other NNS RNA viruses reveal that the RNAs in the NCLPs are either random cellular RNAs or average viral genomic RNAs15,16,17,18,19. Together, the main hurdle is that N binds non-specifically to cellular RNAs to form NCLPs when N is overexpressed in the host cells.
To overcome this hurdle, we established a protocol to obtain RNA-free (N0) first and assemble N0 with authentic viral genomic RNA into NCLPs20. The principle of this protocol is to obtain a large quantity of recombinant RNA-free N (N0) by co-expressing N with a chaperone, the N-terminal domain of RSV phosphoprotein (PNTD). The purified N0P could be stimulated and assembled into NCLPs by adding RSV-specific RNA oligos, and during the assembly process, the chaperone PNTD is displaced upon the addition of RNA oligos.
Here, we detail a protocol for the generation and assembly of RSV RNA-specific NCs. In this protocol, we describe the molecular cloning, protein preparation, in vitro assembly, and validation of the complex assembly. We highlight the cloning strategy to generate bi-cistronic constructs for protein coexpression for molecular cloning. For protein preparation, we describe the procedures of cell culture, protein extraction, and the purification of the protein complex. Then we discuss the method for in vitro assembly of the RSV RNA-specific NCs. Finally, we use size exclusion chromatography (SEC) and negative stain electron microscopy (EM) to characterize and visualize the assembled NCLPs.
Protocol
Representative Results
Discussion
The known nucleocapsid-like particle (NCLP) structures of the nonsegmented negative-sense (NNS) RNA viruses show that the assembled NCLPs are the complex N with host cellular RNAs when overexpressed in bacterial or eukaryotic expression systems15,16,17,18,19. Previous studies have attempted to get the RNA free N with a variety of methods, such as the RNase A d…
Offenlegungen
The authors have nothing to disclose.
Acknowledgements
The research programs in the Liang laboratory at Emory are supported by the US National Institute of General Medical Sciences (NIGMS), National Institutes of Health (NIH) under award number R01GM130950, and the Research Start-Up Fund at Emory University School of Medicine. The author acknowledges the members of the Liang laboratory for helpful support and critical discussion.
Materials
| Agarose | SIgma | A9539-500G | making construct using LIC method |
| Amicon Ultra-15 Centrifugal Filter Unit | Millipore | UFC901024 | concentrate the protein sample |
| Ampicillin sodium | GOLD BIOTECHNOLOGY | 5118.111317A | antibiotic for cell culture |
| AseI | NEB | R0526S | making construct using LIC method |
| Cobalt (High Density) Agarose Beads | Gold Bio | H-310-500 | For purification of His-tag protein |
| Corning LSE Digital Dry Bath Heater | CORNING | 6885-DB | Heate the sample |
| dCTP | Invitrogen | 10217016 | making construct using LIC method |
| dGTP | Invitrogen | 10218014 | making construct using LIC method |
| Glycerol | Sigma | G5516-4L | making solution |
| HEPES | Sigma | H3375-100G | making solution |
| HiTrap Q HP | Sigma | GE29-0513-25 | Protein purification |
| Imidazole | Sigma | I5513-100G | making solution |
| IPTG (Isopropyl-beta-D-thiogalactopyranoside) | GOLD BIOTECHNOLOGY | 1116.071717A | induce the expression of protein |
| Microcentrifuge Tubes | VWR | 47730-598 | for PCR |
| Misonix Sonicator XL2020 Ultrasonic Liquid Processor | SpectraLab | MSX-XL-2020 | sonicator for lysing cell |
| Negative stain grids | Electron Microscopy Sciences | CF400-Cu-TH | For making negative stain grids |
| New Brunswick Innova 44/44R | eppendorf | M1282-0000 | Shaker for culturing the cell |
| Nonidet P 40 Substitute | Sigma | 74385-1L | making solution |
| OneTaq DNA Polymerase | NEB | M0480L | PCR |
| QIAquick Gel Extraction Kit | QIAGEN | 28706 | Purify DNA |
| SSPI-HF | NEB | R3132S | making construct using LIC method |
| Superose 6 Increase 10/300 GL | Sigma | GE29-0915-96 | Protein purification |
| T4 DNA polymerase | Sigma | 70099-3 | making construct using LIC method |
| Thermo Scientific Sorvall RC 6 Plus Centrifuge | Fisher Scientific | 36-101-0816 | Centrifuge, highest speed 20,000 rpm |
| Trizma hydrochloride | Sigma | T3253-250G | making solution |
| Uranyl Formate | Electron Microscopy Sciences | 22451 | making negative stain solution |
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