Coexpression and Association of the Spike Protein and the Membrane Protein of Mouse Hepatitis Virus

The M and S envelope glycoproteins of mouse hepatitis virus associate in the process of virus assembly. We have studied the intrinsic properties of M/S heterocomplexes by coexpressing M and S in the absence of other coronaviral proteins. The formation of M/S complexes under these conditions indicates that M and S can interact independently of other coronaviral factors. Pulse-chase analysis revealed that M and S associate in a pre-Golgi compartment. M/S complexes are efficiently transported beyond the coronavirus budding compartment to the Golgi complex. The failure to detect complexes at the surface of coexpressing cells demonstrated that they are retained intracellularly. Thus, coexpression of the envelope glycoproteins drastically affects the intracellular transport of the S protein: instead of being transported to the cell surface, S is retained intracellularly by its association with M.