The Furin Cleavage Site of Feline Coronavirus Type 1 (FCoV-1) and Its Structural Localization Within the S1 Domain D

Abstract

Feline coronavirus type 1 (FCoV-1 is an alphacoronavirus (species _alphacoronavirus-1_) present in a distinct genetic clade. The FCoV-1 spike protein contains an identifiable furin cleavage site (FCS), which is highly unusual for an alphacoronavirus. FCoV-1 is a widespread and highly transmissible virus of both domestic and non-domestic felids. Notably, following infection with a low-pathogenicity virus, highly pathogenic variants of FCoV-1 are selected in individual cats by a process of ‘internal mutation’; these variants appear to have robust tropism for macrophages and are strongly linked to the disease outcome known as feline infectious peritonitis (FIP). Also strongly linked to FIP disease outcome are a range of point mutations that are proposed to modify and disrupt the furin cleavage site. There is only a single structure of an FCoV-1 spike protein—for the UU4 variant sequenced from an FIP cat. Here, we structurally localized the FCS of FCoV-1 UU4 in comparison with that of a reference non-pathogenic variant (UU2). We show that the FCS is located within S1 domain D. Topology domain mapping revealed the FCoV-1 FCS to be within a solvent-exposed structural loop (“loop 2”) located between two of the beta strands that comprise domain D of the spike protein—upstream of the interface of the S1 and S2 sub-domains. An equivalent “loop 2” of SARS-CoV-2 has also been identified as a proteolytic cleavage site for cathepsin L, suggesting conserved fusion-activation regulation of the coronavirus spike protein embedded within domain D.