Single amino acid changes in the S2 subunit of the MHV surface glycoprotein confer resistance to neutralization by S1 subunit-specific monoclonal antibody.

We have isolated 10 monoclonal-antibody-resistant variants (MAR variants) of the murine hepatitis virus (MHV) by selection with a neutralizing monoclonal antibody that binds to the S1 subunit of the surface glycoprotein (MAb 11F, E. Routledge et al., J. Virol. 65, 254-262, 1991). The variant viruses escape neutralization and are able to mediate membrane fusion in the presence of high concentrations of antibody. Sequence analysis of the variant S protein genes showed that they are not mutated in the codons for the amino acids that bind MAb 11F (positions 33 to 40). Instead, they have mutations that encode single amino acid changes in the S2 subunit of the protein (positions 1109, 1110, and 1116). These amino acid substitutions are conservative. Using a T7/vaccinia virus system, we have confirmed that each of the S2 subunit substitutions is able to confer the MAR phenotype on transiently expressed S proteins. The indirect immunoprecipitation of metabolically labeled S protein from cell lysates of MHV- or MAR variant-infected cells shows that the MAR-variant S proteins no longer bind MAb 11F, although they are still bound by a large panel of monoclonal antibodies that recognize many different discontinuous and linear determinants on both the S1 and S2 subunits of the protein. These data provide new insights into the interaction between defined regions of the S1 and S2 subunits of the MHV S protein.