Identification of a Contiguous 6-Residue Determinant in the MHV Receptor That Controls the Level of Virion Binding to Cells()

Murine carcinoembryonic antigens serve as receptors for the binding and entry of the enveloped coronavirus mouse hepatitis virus (MHV) into cells. Numerous receptor isoforms are now known, and each has extensive differences in its amino terminal immunoglobulin-like domain (NTD) to which MHV binds via its protruding spike proteins. Some of these receptor alterations may affect the ability to bind viral spikes. To identify individual residues controlling virus binding differences, we have used plasmid and vaccinia virus vectors to express two forms of MHV receptor differing only in their NTD. The two receptors, designated biliary glycoproteins (Bgp) 1(a)and 1(b)(NTD), varied by 29 residues in the 107 amino acid NTD. When expressed from cDNAs in receptor-negative HeLa cells, these two Bgp molecules were displayed on cell surfaces to equivalent levels, as both were equally modified by a membrane-impermeant biotinylation reagent. Infectious center assays revealed that the 1(a)isoform was 10 to 100 times more effective than 1(b)(NTD)in its ability to confer sensitivity to MHV (strain A59) infection. Bgp1(a)was also more effective than Bgp1(b)(NTD)in comparative virus adsorption assays, binding 6 times more MHV (strain A59) and 2.5 times more MHV (strain JHMX). Bgp1(a)was similarly more effective in promoting the capacity of viral spikes to mediate intercellular membrane fusion as judged by quantitation of syncytia following cocultivation of spike and receptor-bearing cells. To identify residues influencing these differences, we inserted varying numbers of 1(b)residues into the Bgp1(a)background via restriction fragment exchange and site-directed mutagenesis. Analysis of the resulting chimeric receptors showed that residues 38 to 43 of the NTD were key determinants of the binding and fusion differences between the two receptors. These residues map to an exposed loop (C-C′ loop) in a structural model of the closely related human carcinoembryonic antigen.