The Spike but Not the Hemagglutinin/Esterase Protein of Bovine Coronavirus Is Necessary and Sufficient for Viral Infection

Abstract

The spike (S) and hemagglutinin/esterase (HE) of bovine coronavirus (BCV) are the two envelope proteins that recognize the same receptor-determinant of 9-O-acetylneuraminic acid on host cells. However, the precise and relative roles of the two proteins in BCV infectivity remain elusive. To unequivocally determine their roles in viral cytopathogenicity, we developed a system in which phenotypically chimeric viruses were generated by infecting a closely related mouse hepatitis virus (MHV) in cells that stably express an individual BCV protein (S or HE). The chimeric viruses were then used to infect human rectal tumor (HRT)-18 cells that are permissive to BCV but are nonsusceptible to MHV. Using this approach, we found that the chimeric virus containing the BCV S protein on the virion surface entered and replicated in HRT-18 cells; this was specifically blocked by prior treatment of the virus with a neutralizing antibody specific to the BCV S protein, indicating that the BCV S protein is responsible for initiating chimeric virus infection. In contrast, chimeric viruses that contain biologically active and functional BCV HE protein on the surface failed to enter HRT-18 cells, indicating that the BCV HE protein alone is not sufficient for BCV infection. Taken together, these results demonstrate that the S protein but not the HE protein of BCV is necessary and sufficient for infection of the chimeric viruses in HRT-18 cells, suggesting that BCV likely uses the S protein as a primary vehicle to infect permissive cells.

The Molecular Biology of Coronaviruses

“…The hemagglutinating property of HE raised the possibility that, in the viruses in which it appears, this protein may duplicate or replace the role that is assigned to the coronavirus S protein. However, it has been shown, through the construction of MHV-BCoV chimeric viruses, that the BCoV HE protein, in the absence of BCoV S protein, is not sufficient for initiation of infection in tissue culture (Popova and Zhang, 2002).…”

Bovine Coronavirus Associated Syndromes

“…[11][12][13] These studies have targeted the spike gene of BCoV for phylogenetic analysis because it is the most critical surface protein that binds to the receptor N-acetyl-9-O-acetylneuraminic acid of the virus. 14 The Japanese BCoV isolates (1999)(2000)(2001)(2002)(2003)(2004)(2005)(2006)(2007)(2008) cluster in 4 phylogenic groups. Japanese isolates collected after 2005 were included in antigenic Group 4.…”

Molecular Interactions in the Assembly of Coronaviruses

“…While the HE proteins of BCoV, HEV, and HCoV-OC43 hydrolyze the 9-O-acetyl group of sialic acid and therefore appear to function as receptor-destroying enzymes (Schultze et al, 1991b;Vlasak et al, 1988a), the HE proteins of MHVlike coronaviruses function as sialate-4-O-acetylesterases (Klausegger et al, 1999;Regl et al, 1999;Wurzer et al, 2002). Although inhibition of the esterase activity of BCoV resulted in a 100-to 400-fold reduction in viral infectivity (Vlasak et al, 1988a), it was shown both for BCoV and for an MHV strain expressing an HE gene that the S protein is required and sufficient for infection (Gagneten et al, 1995;Popova and Zhang, 2002). In view of these results it has been proposed that the HE protein might play a role at an even earlier step and may mediate viral adherence to the intestinal wall through the specific yet reversible binding to mucopolysaccharides.…”

“…In another study the BCoV S and HE proteins were shown to be incorporated into MHV particles when coexpressed in MHV-infected cells. Apparently, homology between the proteins of these related group 2 coronaviruses is sufficiently high for heterologous M-S and M-HE interactions to occur (Popova and Zhang, 2002).…”