OC43 clinical isolate spike proteins have distinct carbohydrate-binding properties

The human coronavirus HCoV-OC43 (OC43) is the most widespread of the four common cold-causing seasonal coronaviruses, and tissue culture-adapted strains of it have been used for ~50 years. Nevertheless, clinical isolates of OC43 differ from tissue culture-adapted OC43 in ways that call into question the value of the latter as a model. Among these are differences in their entry mechanisms and the activities of their hemagglutinin-esterases (HE). We now show that the spike proteins of OC43 clinical isolates differ from that of the tissue culture-adapted reference strain (OC43-Lab) in their carbohydrate-binding properties and ability to bind mucins, decoy receptors cleaved by the HE. We also show that, unlike HCoV-HKU1 (HKU1), they do not bind with high affinity and specificity the 9-O-acetylated α2-8-linked disialic acid moiety implicated in viral entry for OC43-Lab and HKU1. The spike proteins of the OC43 clinical isolates possess two inserts, not found in OC43-Lab, that flank the carbohydrate-binding site. Our structural analysis of a representative clinical isolate shows that insert-2 is a determinant of these specificity differences and that the carbohydrate-binding site undergoes conformational changes on carbohydrate binding. These structural features are shared by HKU1 and suggest common mechanisms for adaptation to the human sialoglycome.