Multimerization- and glycosylation-dependent receptor binding of SARS-CoV-2 spike proteins

Author summary The ongoing COVID19 pandemic necessitates additional tools to study SARS-CoV receptor binding properties. This increases our understanding of how these viruses attach to cells, essential to monitor possible adaptations and determine the zoonotic capabilities of these viruses. Here we created fluorescent multimeric RBD proteins with two distinct glycosylation profiles. The results demonstrate that trimeric fully glycosylated RBD trimers are excellent tools to study receptor binding properties. Receptor binding studies on sarbecoviruses would benefit from an available toolkit of recombinant spike proteins, or domains thereof, that recapitulate receptor binding properties of native viruses. We hypothesized that trimeric Receptor Binding Domain (RBD) proteins would be suitable candidates to study receptor binding properties of SARS-CoV-1 and -2. Here we created monomeric and trimeric fluorescent RBD proteins, derived from adherent HEK293T, as well as in GnTI-/- mutant cells, to analyze the effect of complex vs high mannose glycosylation on receptor binding. The results demonstrate that trimeric, complex glycosylated proteins are superior in receptor binding compared to monomeric and immaturely glycosylated variants. Although differences in binding to commonly used cell lines were minimal between the different RBD preparations, substantial differences were observed when respiratory tissues of experimental animals were stained. The RBD trimers demonstrated distinct ACE2 expression profiles in bronchiolar ducts and confirmed the higher binding affinity of SARS-CoV-2 over SARS-CoV-1. Our results show that complex glycosylated trimeric RBD proteins are attractive to analyze sarbecovirus receptor binding and explore ACE2 expression profiles in tissues.