In the still growing family of sialic acids with more than 40 different derivatives of neuraminic acid, the interest of the scientific community is increasingly directed towards the O-acetylated species. In the past two decades it evolved that O-acetylated sialic acids play fundamental roles in the development of organisms, in the regulation of the immune system, in cancer processes and many other biological and pathophysiological events.

ABSTRACT 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 not 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 -acetyl-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 serum glycoproteins to Neu5Ac. By expression of the MHV esterase with recombinant vaccinia virus and incubation with guinea pig serum, we demonstrated that 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 was abolished by chemical de-O-acetylation. Since Neu4,5Ac 2 has not been identified in mice, the nature of potential substrates and/or secondary receptors 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 affinity toward 4-O-acetylated sialic acids.