A pan-coronavirus fusion inhibitor targeting the HR1 domain of human coronavirus spike

Xia, Shuai; Yan, Lei; Xu, Wei; Agrawal, Anurodh Shankar; Algaissi, Abdullah; Tseng, Chien-Te K.; Wang, Qian; Du, Lanying; Tan, Wenjie; Wilson, Ian A.; Jiang, Shibo; Yang, Bei; Lu, Lu

The S glycoprotein is a type I transmembrane glycoprotein that plays an important role in mediating viral infection and is common to all HCoVs. The S proteins consist of two subunits, S1 and S2 (Fig. 1B) . The S1 subunit binds the cellular receptor through its receptor-binding domain (RBD), followed by conformational changes in the S2 subunit, which allows the fusion peptide to insert into the host target cell membrane. The heptad repeat 1 (HR1) region in the S2 subunit forms a homotrimeric assembly, which exposes three highly conserved hydrophobic grooves on the surface that bind heptad repeat 2 (HR2). This six-helix bundle (6-HB) core structure is formed during the fusion process and helps bring the viral and cellular membranes into close proximity for viral fusion and entry ( Fig. 1B) (15) . Thus, the S protein is an important target protein for the development of specific drugs. In particular, the S1 RBD is a very good target site, and both RBD-specific antibodies and RBD-based vaccines have previously exhibited effective antiviral activity or protective effect in blocking binding of virus to host receptors (16) . However, from an evolutionary perspective, the RBD of CoV is part of a highly mutable region and, thus, is not an ideal target site for broadspectrum antiviral inhibitor development (14) . The SARS-CoV RBDspecific antibody fm6 failed to block infection mediated by the S protein of SL-CoV-SHC014 (9) (10) (11) 17) . In contrast, the HR region in the S2 subunit is conserved among various HCoVs and plays a pivotal role in HCoV infections by forming the 6-HB that mediates viral fusion (fig. S1A). Furthermore, the mode of interaction between HR1 and HR2 is conserved among CoVs such that residues located at the "e" and "g" positions in the HR1 helices interact with residues at the "a" and "d" positions in the HR2 helices (fig. S1B) (18) . Previous studies have reported that peptides derived from the HR2 (or C-terminal HR) region of class I viral fusion proteins from some enveloped viruses, including HIV-1 (19) (20) (21) , respiratory syncytial virus (RSV) (22) , Ebola virus (23) , paramyxoviruses SV5 (24) , Nipah virus (25) , and murine hepatitis virus (MHV) (26) , could competitively bind the viral HR1 (or N-terminal HR) and effectively inhibit viral infection. Therefore, it is reasonable to speculate that HR1 could also be a good target for the development of fusion inhibitors against highly pathogenic HCoVs.