MEMBRANE-PROTEIN MOLECULES OF TRANSMISSIBLE GASTROENTERITIS CORONAVIRUS ALSO EXPOSE THE CARBOXY-TERMINAL REGION ON THE EXTERNAL SURFACE OF THE VIRION

The binding domains of four monoclonal antibodies (MAbs) specific for the M protein of the PUR46-MAD strain of transmissible gastroenteritis coronavirus (TGEV) have been located in the 46 carboxy-terminal amino acids of the protein by studying the binding of MAbs to recombinant M protein fragments. Immunoelectron microscopy using these MAbs demonst rated that in a significant proportion of the M protein molecules, the carboxy terminus is exposed on the external surface both in purified viruses and in nascent TGEV virions that recently exited infected swin e testis cells. The same MAbs specifically neutralized the infectivity df the PUR46-MAD strain, indicating that the C-terminal domain of M p rotein is exposed on infectious viruses. This topology of TGEV M prote in probably coexists with the structure currently described for the M protein of coronaviruses, which consists of an exposed amino terminus and an intravirion carboxy-terminal domain. The presence of a detectab le number of M protein molecules with their carboxy termini exposed on the surface of the virion has relevance for viral function, since it has been shown that the carboxy terminus of M protein is immunodominan t and that antibodies specific for this domain both neutralize TGEV an d mediate the complement-dependent lysis of TGEV-infected cells.