Core structure of S2 from the human coronavirus NL63 spike glycoprotein.
ABSTRACT Human coronavirus NL63 (HCoV-NL63) has recently been identified as a causative agent of acute respiratory tract illnesses in infants and young children. The HCoV-NL63 spike (S) protein mediates virion attachment to cells and subsequent fusion of the viral and cellular membranes. This viral entry process is a primary target for vaccine and drug development. HCoV-NL63 S is expressed as a single-chain glycoprotein and consists of an N-terminal receptor-binding domain (S1) and a C-terminal transmembrane fusion domain (S2). The latter contains two highly conserved heptad-repeat (HR) sequences that are each extended by 14 amino acids relative to those of the SARS coronavirus or the prototypic murine coronavirus, mouse hepatitis virus. Limited proteolysis studies of the HCoV-NL63 S2 fusion core identify an alpha-helical domain composed of a trimer of the HR segments N57 and C42. The crystal structure of this complex reveals three C42 helices entwined in an oblique and antiparallel manner around a central triple-stranded coiled coil formed by three N57 helices. The overall geometry comprises distinctive high-affinity conformations of interacting cross-sectional layers of the six helices. As a result, this structure is unusually stable, with an apparent melting temperature of 78 degrees C in the presence of the denaturant guanidine hydrochloride at 5 M concentration. The extended HR regions may therefore be required to prime the group 1 S glycoproteins for their fusion-activating conformational changes during viral entry. Our results provide an initial basis for understanding an intriguing interplay between the presence or absence of proteolytic maturation among the coronavirus groups and the membrane fusion activity of their S glycoproteins. This study also suggests a potential strategy for the development of improved HCoV-NL63 fusion inhibitors.
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ABSTRACT: Coronavirus-cell entry programs involve virus-cell membrane fusions mediated by viral spike (S) proteins. Coronavirus S proteins acquire membrane fusion competence by receptor interactions, proteolysis, and acidification in endosomes. This review describes our current understanding of the S proteins, their interactions with and their responses to these entry triggers. We focus on receptors and proteases in prompting entry and highlight the type II transmembrane serine proteases (TTSPs) known to activate several virus fusion proteins. These and other proteases are essential cofactors permitting coronavirus infection, conceivably being in proximity to cell-surface receptors and thus poised to split entering spike proteins into the fragments that refold to mediate membrane fusion. The review concludes by noting how understanding of coronavirus entry informs antiviral therapies.Viruses 04/2012; 4(4):557-80. · 1.50 Impact Factor
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ABSTRACT: Even though coronavirus infection of humans is not normally associated with severe diseases, the identification of the coronavirus responsible for the outbreak of severe acute respiratory syndrome showed that highly pathogenic coronaviruses can enter the human population. Shortly thereafter, in Holland in 2004, another novel human coronavirus (HCoV-NL63) was isolated from a seven-month old infant suffering from respiratory symptoms. This virus has subsequently been identified in various countries, indicating a worldwide distribution. HCoV-NL63 has been shown to infect mainly children and the immunocommpromised, who presented with either mild upper respiratory symptoms (cough, fever and rhinorrhoea) or more serious lower respiratory tract involvement such as bronchiolitis and croup, which was observed mainly in younger children. In fact, HCoV-NL63 is the aetiological agent for up to 10% of all respiratory diseases. This review summarizes recent findings of human coronavirus HCoV-NL63 infections, including isolation and identification, phylogeny and taxonomy, genome structure and transcriptional regulation, transmission and pathogenesis, and detection and diagnosis.The Open Virology Journal 01/2010; 4:76-84.