NL63 coronavirus (NL63-CoV), a prevalent human respiratory virus, is the only group I coronavirus known to use angiotensin-converting enzyme 2 (ACE2) as its receptor. Incidentally, ACE2 is also used by group II SARS coronavirus (SARS-CoV). We investigated how different groups of coronaviruses recognize the same receptor, whereas homologous group I coronaviruses recognize different receptors. We determined the crystal structure of NL63-CoV spike protein receptorbinding domain (RBD) complexed with human ACE2. NL63-CoV RBD has a novel ␤-sandwich core structure consisting of 2 layers of ␤-sheets, presenting 3 discontinuous receptor-binding motifs (RBMs) to bind ACE2. NL63-CoV and SARS-CoV have no structural homology in RBD cores or RBMs; yet the 2 viruses recognize common ACE2 regions, largely because of a ''virus-binding hotspot'' on ACE2. Among group I coronaviruses, RBD cores are conserved but RBMs are variable, explaining how these viruses recognize different receptors. These results provide a structural basis for understanding viral evolution and virus-receptor interactions.receptor protein ͉ SARS coronavirus ͉ spike protein receptor-binding domain ͉ virus-binding hotspots A fundamental yet unresolved puzzle in virology is how viruses evolve to recognize their receptor proteins (1). Specifically, how do different viruses recognize the same receptor protein, and how do similar viruses recognize different receptor proteins? Do viruses select their receptor proteins by chance, or do they target specific virus-binding hotspots on these receptor proteins? Structural information of virus-receptor interfaces can potentially answer these questions. To date, although a few studies have obtained structural information for a single virus-receptor interface (2-6), no study has provided structural information for the interfaces between different viruses and their common receptor protein.Here we provide such structural information, by showing that nonhomologous receptor-binding proteins of 2 coronaviruses bind to the same ''virus-binding hotspot'' on their common protein receptor.A recently identified human coronavirus, NL63 (NL63-CoV), is associated with common colds, croup, and other respiratory diseases (7,8). Potent neutralizing antibodies against NL63-CoV are detected in sera from nearly all humans older than 8 years, suggesting that NL63-CoV infection is common in childhood (7, 9). NL63-CoV belongs to the coronavirus family, a group of enveloped, positive-stranded RNA viruses that infect many mammalian and avian species. Coronaviruses are classified into 3 serologic and genetic groups: mammalian group I, mammalian group II, and avian group III (10). NL63-CoV is the only group I coronavirus known to use angiotensin-converting enzyme 2 (ACE2) as its receptor (9), whereas the others use aminopeptidase-N (APN) (10-12). Curiously, ACE2 is also the receptor for the severe acute respiratory syndrome (SARS) coronavirus (SARS-CoV) (13), a group II coronavirus responsible for SARS (14,15).Coronaviruses enter cells through a larg...

Vitamin K epoxide reductase (VKOR) generates vitamin K hydroquinone to sustain γ-carboxylation of many blood coagulation factors. Here, we report the 3.6Å crystal structure of a bacterial homolog of VKOR from Synechococcus sp. The structure shows VKOR in complex with its naturally fused redox partner, a thioredoxin-like domain, and corresponds to an arrested state of electron transfer. The catalytic core of VKOR is a four transmembrane helix bundle that surrounds a quinone, connected through an additional transmembrane segment with the periplasmic thioredoxin-like domain. We propose a pathway for how VKOR uses electrons from newly synthesized proteins to reduce a quinone, a mechanism confirmed by in vitro reconstitution of vitamin K-dependent disulfide bridge formation. Our results have implications for the mechanism of the mammalian VKOR and explain how mutations can cause resistance to the VKOR inhibitor warfarin, the most commonly used oral anticoagulant.

The structure of a synaptic intermediate of the site-specific recombinase gammadelta resolvase covalently linked through Ser10 to two cleaved duplex DNAs has been determined at 3.4 angstrom resolution. This resolvase, activated for recombination by mutations, forms a tetramer whose structure is substantially changed from that of a presynaptic complex between dimeric resolvase and the cleavage site DNA. Because the two cleaved DNA duplexes that are to be recombined lie on opposite sides of the core tetramer, large movements of both protein and DNA are required to achieve strand exchange. The two dimers linked to the DNAs that are to be recombined are held together by a flat interface. This may allow a 180 degrees rotation of one dimer relative to the other in order to reposition the DNA duplexes for strand exchange.