Twenty-nine independent hybridomas producing monoclonal antibodies to the matrix (M) protein of vesicular stomatitis virus (Indiana serotype) were prepared by fusion of SP2/0 myeloma cells with spleen lymphocytes obtained from BALB/c mice which had been immunized with the purified M protein. The specific reactivity of each monoclonal antibody was determined by an enzyme-linked immunosorbent assay and a competitive binding assay. Most of the antibodies were of the immunoglobulin G2a and G2b isotypes, although some were immunoglobulin M. By measuring the competitive binding of 125I-antibody, we identified four antigenic determinants in the M protein of the virus; two of these determinants, however, exhibited a large degree of overlap. Western blot analysis revealed little or no cross-reactivity of the antibodies with other viral proteins or with the M protein of the New Jersey serotype. Prolonged trypsin proteolysis removed the first 43 amino acids from the amino-terminal region of the M protein, but it retained its reactivity with monoclonal antibodies to each epitope, except for diminished reactivity with one. To aid in future mapping of these epitopes, we inserted a cDNA clone of the mRNA encoding the M protein of vesicular stomatitis virus into an inducible lac expression vector; the M protein produced in the JM103 strain of Escherichia coli under induced conditions was found to be approximately the same size as native M protein and was recognized by the monoclonal antibodies. These monoclonal antibodies and the cDNA clone should be useful for studying the role of M protein in virus maturation and the regulation of viral transcription
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