Distinct Roles for Sialoside and Protein Receptors in Coronavirus Infection

Qing, Enya; Hantak, Michael; Perlman, Stanley; Gallagher, Tom

This differential utilization of sialoside and protein receptors raises questions about CoV evolution. The CoV S1A domains fold into a galectin-like structure (14, 15, 17-19, 25, 64) , and several CoV S1A domains demonstrably bind carbohydrate ligands (18, 21, 41, 54) , but prior to this report, the MHV-CoV S1A domains were considered to bind only protein (mCEACAM) receptors (25) . However, Peng et al. (19) proposed that ancestral MHV-CoVs bound carbohydrates, with adaptive evolution generating the present-day CEACAM-binding sites. This proposal infers the existence, past or present, of evolutionary intermediates capable of binding both mCEACAM and sialic acid. The MHV JHM-CoV strain meets the criteria for such an intermediate-a virus in which a single domain retains two "receptor" binding activities, one for carbohydrate and the other for protein. Conceivably, these two receptor sites interact, such that increased affinity for one ligand reduces affinity for the other. Mutants with relatively high affinity for sialic acid may bind poorly to mCEACAM, while strains lacking sialic acid binding may demonstrate strong mCEACAM affinity. An inverse relationship would be consistent with the study of Peng et al., who proposed that acquisition of a CEACAM-binding site concomitantly destroyed a sialate site (19) . Yet perhaps more intriguing relationships are between CoV S and hemagglutinin-esterase (HE) proteins, both of which bind (65, 66) . Esterase activities within HE proteins deacetylate sialosides, and it has been documented, in HKU1-CoV infections, that HE activity destroys an S-specific sialoside receptor, keeping viruses detached from cells and allowing for virus dissemination (67) . Several MHV strains, including JHM, express a viral HE, which binds and deacetylates 4-O-acetyl sialiate (68) . Similar to HKU-1-CoV HE, MHV HE could destroy MHV sialoside receptors and facilitate dissemination. MHV strain A59 did not bind sialic acids, and it does not retain an intact HE open reading frame (36, 37) , while strain JHM did bind sialic acids and does express HE, suggesting that HE is necessary for those CoVs with relatively high S-protein affinity for sialosides. MERS-CoV binds sialic acids, albeit weakly (21) , and increases in its affinity for sialates may be limited by the absence of a MERS-CoV HE gene.