question: The difference in membrane (M) protein compositions between the transmissible gastroenteritis coronavirus (TGEV) virion and the core has been studied. The TGEV M protein adopts two topologies in the virus envelope, a Nexo-Cendo topology (with the amino terminus exposed to the virus surface and the carboxy terminus inside the virus particle) and a Nexo-Cexo topology (with both the amino and carboxy termini exposed to the virion surface). The existence of a population of M molecules adopting a Nexo-Cexo topology in the virion envelope was demonstrated by (i) immunopurification of (35)S-labeled TGEV virions using monoclonal antibodies (MAbs) specific for the M protein carboxy terminus (this immunopurification was inhibited only by deletion mutant M proteins that maintained an intact carboxy terminus), (ii) direct binding of M-specific MAbs to the virus surface, and (iii) mass spectrometry analysis of peptides released from trypsin-treated virions. Two-thirds of the total number of M protein molecules found in the virion were associated with the cores, and one-third was lost during core purification. MAbs specific for the M protein carboxy terminus were bound to native virions through the M protein in a Nexo-Cexo conformation, and these molecules were removed when the virus envelope was disrupted with NP-40 during virus core purification. All of the M protein was susceptible to N-glycosidase F treatment of the native virions, which indicates that all the M protein molecules are exposed to the virus surface. Cores purified from glycosidase-treated virions included M protein molecules that completely or partially lost the carbohydrate moiety, which strongly suggests that the M protein found in the cores was also exposed in the virus envelope and was not present exclusively in the virus interior. A TGEV virion structure integrating all the data is proposed. According to this working model, the TGEV virion consists of an internal core, made of the nucleocapsid and the carboxy terminus of the M protein, and the envelope, containing the spike (S) protein, the envelope (E) protein, and the M protein in two conformations. The two-thirds of the molecules that are in a Nexo-Cendo conformation (with their carboxy termini embedded within the virus core) interact with the internal core, and the remaining third of the molecules, whose carboxy termini are in a Nexo-Cexo conformation, are lost during virus core purification.