Furin cleavage of the SARS coronavirus spike glycoprotein enhances cell–cell fusion but does not affect virion entry

Abstract

The fusogenic potential of Class I viral envelope glycoproteins is activated by proteloytic cleavage of the precursor glycoprotein to generate the mature receptor-binding and transmembrane fusion subunits. Although the coronavirus (CoV) S glycoproteins share membership in this class of envelope glycoproteins, cleavage to generate the respective S1 and S2 subunits appears absent in a subset of CoV species, including that responsible for the severe acute respiratory syndrome (SARS). To determine whether proteolytic cleavage of the S glycoprotein might be important for the newly emerged SARS-CoV, we introduced a furin recognition site at single basic residues within the putative S1-S2 junctional region. We show that furin cleavage at the modified R667 position generates discrete S1 and S2 subunits and potentiates membrane fusion activity. This effect on the cell-cell fusion activity by the S glycoprotein is not, however, reflected in the infectivity of pseudotyped lentiviruses bearing the cleaved glycoprotein. The lack of effect of furin cleavage on virion infectivity mirrors that observed in the normally cleaved S glycoprotein of the murine coronavirus and highlights an additional level of complexity in coronavirus entry.

Genomic characterization of seven distinct bat coronaviruses in Kenya

“…In spite of lacking conserved cleavage sites, they all consist of two domains, S1 and S2, showing the conserved GxCx motif in S1 around the cleavage site and the conserved nonamer motif IPTNFSISI or similar motif in S2. These motifs have been observed in other known CoVs (Follis et al, 2006). The S1 is responsible for virus binding to the receptor on the target cells and may contain receptor binding domains (RBDs) that directly bind to host cellular receptors.…”

“…As shown in Fig. 2, spike proteins of the seven bat CoVs lack furin protease recognition site, such as RRADR-S in Murine Hepatitis Virus (MHV), RRSRG-A in human CoV OC43 (HCoV OC43), RRSRR-A in bovine CoV (BCoV) (Follis et al, 2006), and cathepsin L cleavage site (VAYT-M) as in SARS-CoV (Bosch et al, 2008). In spite of lacking conserved cleavage sites, they all consist of two domains, S1 and S2, showing the conserved GxCx motif in S1 around the cleavage site and the conserved nonamer motif IPTNFSISI or similar motif in S2.…”

Identification of two novel B cell epitopes on porcine epidemic diarrhea virus spike protein

“…In the case of PEDV, the S protein could not be cleaved into S1 and S2 subunits, due to a lack of proteolytic cleavage sites (Duarte and Laude, 1994;Park et al, 2006;Yeo et al, 2003). However, the S1 domain (residues 1-789) and the S2 domain (residues 790-1383) have been artificially defined by the conserved nonamer and the GxCx motifs at the position of S protein cleavage sites in coronavirus group II members (Follis et al, 2006). Based on bioinformatics analysis of the S protein structure of other coronaviruses, the S1 domain is thought to form the spherical head of the S protein, and is easily accessible to neutralizing antibodies.…”

Identification of N-linked carbohydrates from severe acute respiratory syndrome (SARS) spike glycoprotein

“…As with several coronaviruses, the spike glycoprotein can be proteolytically cleaved into two non-covalently associated glycoproteins: the virion-associated S1 (666 amino acid residues) containing the receptor attachment site and the transmembrane-anchored S2 (583 residues) responsible for fusion activity (Bergeron et al, 2005; Follis et al, 2006; Li et al, 2005). …”