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
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