Human coronavirus (hCoV) HKU1 is one of six hCoVs identified to date and the only one with an unidentified cellular receptor. hCoV-HKU1 encodes a
hemagglutinin-esterase (HE)
protein that is unique to the group a betacoronaviruses (group 2a). The function of HKU1-HE remains largely undetermined. In this study, we examined binding of the S1 domain of hCoV-HKU1 spike to a panel of cells and found that the S1 could specifically bind on the cell surface of a human
rhabdomyosarcoma cell line, RD. Pretreatment of RD cells with
neuraminidase (NA) and
trypsin greatly reduced the binding, suggesting that the binding was mediated by
sialic acids on
glycoproteins. However, unlike other group 2a CoVs, e.g., hCoV-OC43, for which 9-O-acetylated
sialic acid (9-O-Ac-Sia) serves as a receptor determinant, HKU1-S1 bound with neither 9-O-Ac-Sia-containing
glycoprotein(s) nor rat and mouse erythrocytes. Nonetheless, the HKU1-HE was similar to OC43-HE, also possessed
sialate-O-acetylesterase activity, and acted as a receptor-destroying
enzyme (RDE) capable of eliminating the binding of HKU1-S1 to RD cells, whereas the O-
acetylesterase-inactive HKU1-HE mutant lost this capacity. Using primary human ciliated airway epithelial (HAE) cell cultures, the only in vitro replication model for hCoV-HKU1
infection, we confirmed that pretreatment of HAE cells with HE but not the enzymatically inactive mutant blocked hCoV-HKU1
infection. These results demonstrate that hCoV-HKU1 exploits O-Ac-Sia as a cellular attachment receptor determinant to initiate the
infection of host cells and that its HE
protein possesses the corresponding
sialate-O-acetylesterase RDE activity.
IMPORTANCE: Human coronaviruses (hCoV) are important human respiratory pathogens. Among the six hCoVs identified to date, only hCoV-HKU1 has no defined cellular receptor. It is also unclear whether
hemagglutinin-esterase (HE)
protein plays a role in viral entry. In this study, we found that, similarly to other members of the group 2a CoVs,
sialic acid moieties on
glycoproteins are critical receptor determinants for the hCoV-HKU1
infection. Interestingly, the virus seems to employ a type of
sialic acid different from those employed by other group 2a CoVs. In addition, we determined that the HKU1-HE
protein is an O-
acetylesterase and acts as a receptor-destroying
enzyme (RDE) for hCoV-HKU1. This is the first study to demonstrate that hCoV-HKU1 uses certain types of O-acetylated
sialic acid residues on
glycoproteins to initiate the
infection of host cells and that the HKU1-HE
protein possesses
sialate-O-acetylesterase RDE activity.