Conformational changes in the spike glycoprotein of murine coronavirus are induced at 37°C either by soluble murine CEACAM1 receptors or by pH 8

The spike glycoprotein (S) of the murine coronavirus mouse hepatitis virus (MHV) binds to viral murine CEACAM receptor glycoproteins and causes membrane fusion. On virions, the 180-kDa S glycoprotein of the MHV-A59 strain can be cleaved by trypsin to form the 90-kDa N-terminal receptor-binding subunit (S1) and the 90-kDa membrane-anchored fusion subunit (S2). Incubation of virions with purified, soluble CEACAM1a receptor proteins at 37degreesC and pH 6.5 neutralizes virus infectivity (B. D. Zelus, D. R. Wessner, R. K. Williams, M. N. Pensiero, F. T. Phibbs, M. deSouza, G. S. Dveksler, and K. V. Holmes, J. Virol. 72:7237-7244,1998). We used liposome flotation and protease sensitivity assays to investigate the mechanism of receptor-induced, temperature-dependent virus neutralization. After incubation with soluble receptor at 37degreesC and pH 6.5, virions became hydrophobic and bound to liposomes. Receptor binding induced a profound, apparently irreversible conformational change in S on the viral envelope that allowed S2, but not SI, to be degraded by trypsin at 4degreesC. Various murine CEACAM proteins triggered conformational changes in S on recombinant MHV strains expressing S glycoproteins of MHV-A59 or MHV-4 (MHV-JHM) with the same specificities as seen for virus neutralization and virus-receptor activities. Increased hydrophobicity of virions and conformational change in S2 of MHV-A59 could also be induced by incubating virions at pH 8 and 37degreesC, without soluble receptor. Surprisingly, the S protein of recombinant MHV-A59 virions with a mutation, H716D, that precluded cleavage between SI and S2 could also be triggered to undergo a conformational change at 37degreesC by soluble receptor at neutral pH or by pH 8 alone. A novel 120-kDa subunit was formed following incubation of the receptor-triggered S(A59)H716D virions with trypsin at 4degreesC. The data show that unlike class 1 fusion glycoproteins of other enveloped viruses, the murine coronavirus S protein can be triggered to a membrane-binding conformation at 37degreesC either by soluble receptor at neutral pH or by alkaline pH alone, without requiring previous activation by cleavage between S1 and S2.