Oligomerization and assembly of the matrix protein of Borna disease virus

Abstract

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 StokeÕs radius of 35 Å 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 matrix protein (BDV-M) reveals ssRNA binding properties

“…Because of the disk-like shape of the BDV-M tetramer, distances between the tetramers range from 3.95 Å (Pro 142 O-Arg 98 NE), through 6-8 Å at the edges, up to 20-25 Å between their centers, resulting in large solvent channels with dimensions 35 ϫ 35 ϫ 35 Å. Interactions between the 2 tetramers are mediated by solvent molecules, including SO 4 2Ϫ ions. This finding is consistent with experimental results showing that higher oligomeric states of the M-protein appear after long incubation at high salt concentration and ultracentrifugation studies confirming that an equilibrium exists between tetramers and octamers of BDV-M (16). Because the membrane binding face is not accessible within the crystallographic octamer, it is unlikely that it is able to bind to lipid bilayers.…”

“…4B), whereas the tetramer edges exhibit alternating patches of acidic/basic nature ( Fig. 2B) that could mediate lateral association to form the planar arrays observed previously (16).…”

“…In previous work, UV spectra of BDV-M oligomeric fractions showed 2 maxima with a ratio OD 280 to OD 260 of Ϸ1.1 (16), suggesting the presence of nucleic acid. Although the density described above is clear, showing that a pyrimidine base is bound to the heterologously produced BDV-M near the tetramer axis, the limited resolution and high overall B value of the data does not allow differentiation between RNA and DNA.…”

“…BDV-M forms oligomers both in vivo and in vitro, with tetramers as the most stable structural unit (15,16). The tetramers are noncovalently associated and can form 2D lattice-like structures (16); analytical ultracentrifugation and electron microscopic analysis also indicate higher-order association states of the tetrameric BDV-M.…”

“…It is located beneath the viral envelope and associates with the inner layer of the viral membrane and as such is responsible for the structural integrity and individual form of virus particles by bridging the nucleocapsid and the envelope (14). BDV-M forms oligomers both in vivo and in vitro, with tetramers as the most stable structural unit (15,16). The tetramers are noncovalently associated and can form 2D lattice-like structures (16); analytical ultracentrifugation and electron microscopic analysis also indicate higher-order association states of the tetrameric BDV-M.…”