RNA induced polymerization of the Borna disease virus nucleoprotein.

The Borna disease virus (BDV) nucleoprotein (N) monomer resembles the nucleoprotein structures from rabies virus (RABV) and vesicular stomatitis virus (VSV). We show that BDV N assembles into ring- and string-like structures in the presence of 5' genomic BDV RNA. RNA induced polymerization is partly RNA-specific since polymerization is inefficient in the presence of 3' genomic BDV RNA or E. coli RNA. Mutagenesis of basic residues located in the cleft made up by the N- and C-terminal domains of N abrogate RNA-induced polymerization indicating that BDV N binds RNA similarly as observed in case of RABV and VSV N-RNA complexes. Bound RNA is not protected and sensitive to degradation. N-RNA polymers form complexes with the phosphoprotein P as required for functional transcription or replication units. Our data indicate that BDV N utilizes similar structural principles for N-RNA and N-P-RNA complex formation as observed for related negative strand RNA viruses.

Crystal structure of the Borna disease virus matrix protein (BDV-M) reveals ssRNA binding properties.

Borna disease virus (BDV) is a neurotropic enveloped RNA virus that causes a noncytolytic, persistent infection of the central nervous system in mammals. BDV belongs to the order Mononegavirales, which also includes the negative-strand RNA viruses (NSVs) Ebola, Marburg, vesicular stomatitis, rabies, mumps, and measles. BDV-M, the matrix protein (M-protein) of BDV, is the smallest M-protein (16.2 kDa) among the NSVs. M-proteins play a critical role in virus assembly and budding, mediating the interaction between the viral capsid, envelope, and glycoprotein spikes, and are as such responsible for the structural stability and individual form of virus particles. Here, we report the 3D structure of BDV-M, a full-length M-protein structure from a nonsegmented RNA NSV. The BDV-M monomer exhibits structural similarity to the N-terminal domain of the Ebola M-protein (VP40), while the surface charge of the tetramer provides clues to the membrane association of BDV-M. Additional electron density in the crystal reveals the presence of bound nucleic acid, interpreted as cytidine-5'-monophosphate. The heterologously expressed BDV-M copurifies with and protects ssRNA oligonucleotides of a median length of 16 nt taken up from the expression host. The results presented here show that BDV-M would be able to bind RNA and lipid membranes simultaneously, expanding the repertoire of M-protein functionalities.

Maturation of Borna disease virus glycoprotein.

The maturation of Borna disease virus (BDV) glycoprotein GP was studied in regard to intracellular compartmentalization, compartmentalization signal-domains, proteolytic processing, and packaging into virus particles. Our data show that BDV-GP is (i) predominantly located in the endoplasmic reticulum (ER), (ii) partially exists in the ER already as cleaved subunits GP-N and GP-C, (iii) is directed to the ER/cis-Golgi region by its transmembrane and/or cytoplasmic domains in CD8-BDV-GP hybrid constructs and (iv) is incorporated in the virus particles as authentic BDV glycoprotein exclusively in the cleaved form decorated with N-glycans of the complex type. Downregulation of BDV-glycoproteins on the cell surface, their limited proteolytic processing, and protection of antigenic epitopes on the viral glycoproteins by host-identical N-glycans are different strategies for persistent virus infections.

Oligomerization and assembly of the matrix protein of Borna disease virus.

The matrix protein M of Borna disease virus (BDV) is a constituent of the viral envelope covering the inner leaflet of the lipid bilayer. BDV-M was expressed as recombinant protein in Escherichia coli, purified to homogeneity and structurally analyzed. Recombinant M (i) forms non-covalently bound multimers with a Stoke's radius of 35 Angstroms estimated by size exclusion chromatography, (ii) consists of tetramers detected by analytical ultracentrifugation, and (iii) appears by electron microscopy studies as tetramers with the tendency to assemble into high molecular mass lattice-like complexes. The structural features suggest that BDV-M possesses a dominant driving force for virus particle formation.

Crystal structure of the borna disease virus nucleoprotein.

Borna disease virus (BDV) causes an infection of the central nervous system in a wide range of vertebrates, which can fatally progress to an immune-mediated disease, called Borna disease. BDV is a member of the Mononegavirales, which also includes the highly infectious measles and Ebola viruses. The viral nucleoproteins are central to transcription, replication, and packaging of the RNA genome. We present the X-ray structure of the BDV nucleoprotein determined at 1.76 A resolution. The structure reveals a novel fold, organized into two distinct domains, and an assembly into a planar homotetramer. Surface potential calculations strongly support an RNA binding model with the RNA wrapping around the outside of the tetramer, although a positively charged central channel in the tetramer could fit single-stranded RNA in an alternative binding mode. This first structure of an RNA virus nucleoprotein provides a paradigmatic model for RNA packaging and replication of single-stranded RNA viruses.

Identification of the amino terminal subunit of the glycoprotein of Borna disease virus.

The only surface membrane glycoprotein of Borna disease virus (BDV) is synthesized as a polypeptide with a molecular mass of 57 kDa and N-glycosylated to a precursor glycoprotein (GP) of about 94 kDa. It is processed by the cellular protease furin into the C-terminal membrane-anchored subunit GP-C, also known as gp43, and a presumptive N-terminal subunit GP-N, that is highly glycosylated and has a molecular mass of about 51 kDa. However, up to now the latter remained undetected in BDV-infected material. We describe a novel approach to identify glycan masked linear antigenic epitopes. In the present study, GP-N was identified in BDV-infected cells by a combination of lectin precipitation, enzymatic deglycosylation on blot and immunochemistry using an N-terminal specific antiserum. The GP-N has an apparent molecular mass of 45-50 kDa in its glycosylated form and 27 kDa in its deglycosylated form. N-glycan analysis revealed that the precursor GP contains only mannose-rich N-glycans, whereas GP-N and GP-C contain mannose-rich and complex-type N-glycans.

Crystallization and preliminary X-ray analysis of the matrix protein of Borna disease virus.

The matrix protein M of Borna disease virus (BDV) is associated with the inner viral membrane and is thought to be a mediator between the nucleocapsid and the lipid-containing envelope in stabilizing the virus shape. The full-length BDV-M gene encoding a 16 kDa protein was expressed in Escherichia coli. M was purified to homogeneity and crystallized by the sitting-drop vapour-diffusion method. The crystals of M belong to the space group I432, with unit-cell parameters a = b = c = 144.6 A, and diffract to 3.1 A.