Monoclonal Antibody #3-9-16 Recognizes One of the Two Isoforms of Rabies Virus Matrix Protein That Exposes Its N-Terminus on the Virion Surface

Generation of Monoclonal Antibodies against Variable Epitopes of the M Protein of Rabies Virus

Rabies virus (RABV), the causative agent of rabies, is highly neurovirulent for warm-blooded animals with a mortality rate of up to 100%. The RABV matrix protein (M) is required for virus particle assembly and budding. However, little is known about antigenic differences in the M protein. In this study, five monoclonal antibodies (mAbs), designated 3B9, 4A1, 2B11, 2C1, and 4B11, against the RABV M protein were generated using a recombinant M protein. All five mAbs reacted with the CVS-11 strain but showed no reactivity against the HEP-Flury strain in indirect immunofluorescence and western blotting. The epitope targeted by these mAbs was further identified by peptide scanning using GST-fused peptides. The ²⁵PPYDDD³⁰ peptide was defined as the minimal linear epitope. Alignment of amino acid sequences and phylogenetic analysis of different RABV strains indicated that the variable epitope ²⁵PPDGDD³⁰ is only present in the HEP-Flury and variant Flury strains of clade III, while the other strains resembling ERA and SRVA9 within the clade had another variable epitope, ²⁵PLDDDD³⁰. A Y27D mutation within the epitope was found among the rest of the RABV strains distributed in different clades. However, a single D28G mutation eliminated the reactivity of these five mAbs. In addition, the mAbs were able to recognize wildtype RABV strain in indirect immunofluorescence and western blotting and detect RABV-infected brain tissue using immunohistochemistry. The newly established mAbs and identified epitope may facilitate future investigations in the structure and function of the M protein and the development of diagnostic methods for the detection of different RABV strains worldwide. Most importantly, the epitope recognized by the mAbs against M protein might serve as a novel target for the development of a vaccine targeting RABV virulent strains.

Characterization of a Slow-Migrating Component of the Rabies Virus Matrix Protein Strongly Associated with the Viral Glycoprotein

We investigated multiple forms of rabies virus matrix (M) protein. Under non-reducing electrophoretic conditions, we detected, in addition to major bands of monomer forms (23- and 24-kDa) of M protein, an M antigen-positive slow-migrating minor band (about 54 kDa) in both the virion and infected cells. Relative contents of the 54-kDa and monomer components in the virion were about 20-30% and 70-80% of the whole M protein, respectively, while the content of the 54-kDa component was smaller (about 10-20% of the total M protein) in the cell than in the virion. The 54-kDa components could be extracted from the infected cells with sodium deoxycholate, but they were quite resistant to extraction with 1% nonionic detergents by which most monomer components were solubilized. The 54-kDa component was precipitated more efficiently than the monomer by a monoclonal antibody (mAb; #3-9-16), which recognized a linear epitope located at the N-terminal of the M protein. The mAb #3-9-16 coprecipitated the viral glycoprotein (G), which was demonstrated to be due to strong association between the G and 54-kDa component of the M protein. Monomers and the 54-kDa polypeptide migrated to the same isoelectric point (pI) in twodimensional (2-D) gel electrophoresis, implicating that the 54-kDa component was composed of component(s) of the same pI as that of the M protein monomers. From these results, we conclude that the M antigen-positive 54-kDa polypeptide is a homodimer of M protein, taking an N-terminal-exposed conformation, and is strongly associated with the viral glycoprotein. Possible association with a membrane microdomain of the cell will be discussed.

Membrane and inclusion body targeting of lyssavirus matrix proteins

Lyssavirus matrix proteins (M) support virus budding and have accessory functions that may contribute to host cell manipulation and adaptation to specific hosts. Here, we show that rabies virus (RABV) and European Bat Lyssavirus Type 1 (EBLV-1) M proteins differ in targeting and accumulation at cellular membranes. In contrast to RABV M, EBLV-1 M expressed from authentic EBLV-1 or chimeric RABV accumulated at the Golgi apparatus. Chimeric M proteins revealed that Golgi association depends on the integrity of the entire EBLV-1 M protein. Since RABV and EBLV-1 M differ in the use of cellular membranes for particle formation, differential membrane targeting and transport of M might determine the site of virus production. Moreover, both RABV and EBLV-1 M were for the first time detected within the nucleus and in Negri body-like inclusions bodies. Whereas nuclear M may imply hitherto unknown functions of lyssavirus M in host cell manipulation, the presence of M in inclusion bodies may correlate with regulatory functions of M in virus RNA synthesis. The data strongly support a model in which targeting of lyssavirus M proteins to distinctintracellular sites is a key determinant of diverse features in lyssavirus replication, host adaptation and pathogenesis.