A novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2) recently emerged and is rapidly spreading in humans, causing COVID-19 1,2 . A key to tackling this pandemic is to understand the receptor recognition mechanism of the virus, which regulates its infectivity, pathogenesis and host range. SARS-CoV-2 and SARS-CoV recognize the same receptor-angiotensin-converting enzyme 2 (ACE2)-in humans 3,4 . Here we determined the crystal structure of the receptor-binding domain (RBD) of the spike protein of SARS-CoV-2 (engineered to facilitate crystallization) in complex with ACE2. In comparison with the SARS-CoV RBD, an ACE2-binding ridge in SARS-CoV-2 RBD has a more compact conformation; moreover, several residue changes in the SARS-CoV-2 RBD stabilize two virus-binding hotspots at the RBD-ACE2 interface. These structural features of SARS-CoV-2 RBD increase its ACE2-binding affinity. Additionally, we show that RaTG13, a bat coronavirus that is closely related to SARS-CoV-2, also uses human ACE2 as its receptor. The differences among SARS-CoV-2, SARS-CoV and RaTG13 in ACE2 recognition shed light on the potential animal-to-human transmission of SARS-CoV-2. This study provides guidance for intervention strategies that target receptor recognition by SARS-CoV-2.The sudden emergence and rapid spread of SARS-CoV-2 is endangering global health and economy 1,2 . SARS-CoV-2 has caused many more infections, deaths and economic disruptions than SARS-CoV in 2002 . The origin of SARS-CoV-2 remains unclear. Bats are considered the original source of SARS-CoV-2 because a closely related coronavirus, RaTG13, has been isolated from bats 7 . However, the molecular events that led to the possible bat-to-human transmission of SARS-CoV-2 are unknown. Clinically approved vaccines or drugs that specifically target SARS-CoV-2 are also lacking. Receptor recognition by coronaviruses is an important determinant of viral infectivity, pathogenesis and host range 8,9 . It presents a major target for vaccination and antiviral strategies 10 . Here we elucidate the structural and biochemical mechanisms of receptor recognition by SARS-CoV-2.Receptor recognition by SARS-CoV has been extensively studied. A virus-surface spike protein mediates the entry of coronavirus into host cells. The spike protein of SARS-CoV contains a RBD that specifically recognizes ACE2 as its receptor 3,4 . A series of crystal structures of the SARS-CoV RBD from different strains in complex with ACE2 from different hosts has previously been determined 3,11,12 . These structures showed that SARS-CoV RBD contains a core and a receptor-binding motif (RBM); the RBM mediates contacts with ACE2. The surface of ACE2 contains two virus-binding hotspots that are essential for SARS-CoV binding. Several naturally selected mutations in the SARS-CoV RBM surround these hotspots and regulate the infectivity, pathogenesis, and cross-species and human-to-human transmissions of SARS-CoV 3,11,12 .Because of the sequence similarity between the spike proteins of SARS-CoV and SARS-C...

A novel severe acute respiratory syndrome (SARS)-like coronavirus (SARS-CoV-2) is causing the global coronavirus disease 2019 (COVID-19) pandemic. Understanding how SARS-CoV-2 enters human cells is a high priority for deciphering its mystery and curbing its spread. A virus surface spike protein mediates SARS-CoV-2 entry into cells. To fulfill its function, SARS-CoV-2 spike binds to its receptor human ACE2 (hACE2) through its receptor-binding domain (RBD) and is proteolytically activated by human proteases. Here we investigated receptor binding and protease activation of SARS-CoV-2 spike using biochemical and pseudovirus entry assays. Our findings have identified key cell entry mechanisms of SARS-CoV-2. First, SARS-CoV-2 RBD has higher hACE2 binding affinity than SARS-CoV RBD, supporting efficient cell entry. Second, paradoxically, the hACE2 binding affinity of the entire SARS-CoV-2 spike is comparable to or lower than that of SARS-CoV spike, suggesting that SARS-CoV-2 RBD, albeit more potent, is less exposed than SARS-CoV RBD. Third, unlike SARS-CoV, cell entry of SARS-CoV-2 is preactivated by proprotein convertase furin, reducing its dependence on target cell proteases for entry. The high hACE2 binding affinity of the RBD, furin preactivation of the spike, and hidden RBD in the spike potentially allow SARS-CoV-2 to maintain efficient cell entry while evading immune surveillance. These features may contribute to the wide spread of the virus. Successful intervention strategies must target both the potency of SARS-CoV-2 and its evasiveness.

The omicron variant of SARS-CoV-2 has been spreading rapidly across the globe. The virus-surface spike protein plays a critical role in the cell entry and immune evasion of SARS-CoV-2. Here we determined the 3.0 Å cryo-EM structure of the omicron spike protein ectodomain. In contrast to the original strain of SARS-CoV-2 where the receptor-binding domain (RBD) of the spike protein takes a mixture of open (“standing up”) and closed (“lying down”) conformations, the omicron spike molecules are predominantly in the open conformation, with one upright RBD ready for receptor binding. The open conformation of the omicron spike is stabilized by enhanced inter-domain and inter-subunit packing, which involves new mutations in the omicron strain. Moreover, the omicron spike has undergone extensive mutations in RBD regions where known neutralizing antibodies target, allowing the omicron variant to escape immune surveillance aimed at the original viral strain. The stable open conformation of the omicron spike sheds light on the cell entry and immune evasion mechanisms of the omicron variant.

Porcine epidemic diarrhea virus (PEDV), a member of Alphacoronavirus, has caused huge economic losses for the global pork industry recently. The spike (S) protein mediates PEDV entry into host cells. Herein, we investigated the interactions between the S protein and its receptor porcine aminopeptidase N (pAPN) or co-receptor sugars. The C-terminal domain (CTD) of the S1 domain is bound to pAPN. The prototype strain demonstrated similar receptor-binding activity compared with the variant field isolate. Three loops at the tips of the β-barrel domains did not play crucial roles in the PEDV S-pAPN association, indicating that PEDV conforms to a different receptor recognition model compared with transmissible gastroenteritis virus (TGEV), porcine respiratory CoV (PRCV), and human coronavirus NL63 (HCoV-NL63). The N-terminal domain (NTD) of the PEDV S1 domain could bind sugar, a possible co-receptor for PEDV. The prototype strain exhibited weaker sugar-binding activity compared with the variant field isolate. Strategies targeting the receptor binding domain (RBD) may be helpful for developing vaccines or antiviral drugs for PEDV. Understanding the differences in receptor binding between the prototype and the variant strains may provide insight into PEDV pathogenesis.

Porcine epidemic diarrhea virus (PEDV), a member of the genus Alphacoronavirus, has caused significant damage to the Asian and American pork industries. Coronavirus 3C-like protease (3CL(pro)), which is involved in the processing of viral polyproteins for viral replication, is an appealing antiviral drug target. Here, we present the crystal structures of PEDV 3CL(pro) and a molecular complex between an inactive PEDV 3CL(pro) variant C144A bound to a peptide substrate. Structural characterization, mutagenesis and biochemical analysis reveal the substrate-binding pockets and the residues that comprise the active site of PEDV 3CL(pro). The dimerization of PEDV 3CL(pro) is similar to that of other Alphacoronavirus 3CL(pro)s but has several differences from that of SARS-CoV 3CL(pro) from the genus Betacoronavirus. Furthermore, the non-conserved motifs in the pockets cause different cleavage of substrate between PEDV and SARS-CoV 3CL(pro)s, which may provide new insights into the recognition of substrates by 3CL(pro)s in various coronavirus genera.