The hemagglutinin-esterase of mouse hepatitis virus strain S is a sialate-4-O-acetylesterase

By comparative analysis of the hemagglutinin-esterase (HE) protein of mouse hepatitis virus strain S (MHV-S) and the HE protein of influenza C virus, we found major differences in substrate specificities. In striking contrast to the influenza C virus enzyme, the MHV-S esterase was unable to release acetate from bovine submandibulary gland mucin. Furthermore, MHV-S could no t remove influenza C virus receptors from erythrocytes. Analysis with free sialic acid derivatives revealed that the MHV-S HE protein specifically de- O-acetylates 5-N-acetyl-4-O-acetyl sialic acid (Neu4,5Ac(2)) but not 5-N-ac etyl-9-O-acetyl sialic acid (Neu5,9Ac(2)), which is the major substrate for esterases of influenza C virus and bovine coronaviruses. In addition, the MHV-S esterase converted glycosidically bound Neu4,5Ac(2) of guinea pig ser um glycoproteins to Neu5Ac. By expression of the MHV esterase with recombin ant vaccinia virus and incubation with guinea pig serum, we demonstrated th at the viral HE possesses sialate-4-O-acetylesterase activity, In addition to observed enzymatic activity, MHV-S exhibited affinity to guinea pig and horse serum glycoproteins. Binding required sialate-4-O-acetyl groups and w as abolished by chemical de-O-acetylation. Since Neu4,Ac-2 has not been ide ntified in mice, the nature of potential substrates and/or secondary recept ors for MHV-S in the natural host remains to be determined. The esterase of MHV-S is the first example of a viral enzyme with high specificity and aff inity toward 4-O-acetylated sialic acids.