The envelope glycoprotein, termed the spike protein, of severe acute respiratory syndrome coronavirus (SARS-CoV) is known to mediate viral entry. Similar to other class 1 viral fusion proteins, the heptad repeat regions of SARS-CoV spike are thought to undergo conformational changes from a prefusion form to a subsequent post-fusion form that enables fusion of the viral and host membranes. Recently, the structure of a post-fusion form of SARS-CoV spike, which consists of isolated domains of heptad repeats 1 and 2 (HR1 and HR2), has been determined by x-ray crystallography. To date there is no structural information for the prefusion conformations of SARS-CoV HR1 and HR2. In this work we present the NMR structure of the HR2 domain (residues 1141-1193) from SARS-CoV (termed S2-HR2) in the presence of the co-solvent trifluoroethanol. We find that in the absence of HR1, S2-HR2 forms a coiled coil symmetric trimer with a complex molecular mass of 18 kDa. The S2-HR2 structure, which is the first example of the prefusion form of coronavirus envelope, supports the current model of viral membrane fusion and gives insight into the design of structure-based antagonists of SARS.

Background: Avian influenza virus H5N1 is a major concern as a potential global pandemic. It is thought that multiple key events must take place before efficient human-to-human transmission of the virus occurs. The first step in overcoming host restriction is viral entry which is mediated by HA, responsible for both viral attachment and viral/host membrane fusion. HA binds to glycans-containing receptors with terminal sialic acid (SA). It has been shown that avian influenza viruses preferentially bind to α2,3-linked SAs, while human influenza A viruses exhibit a preference for α2,6-linked SAs. Thus it is believed the precise linkage of SAs on the target cells dictate host tropism of the viruses. Results: We demonstrate that H5N1 HA/HIV pseudovirus can efficiently transduce several human cell lines including human lung cells. Interestingly, using a lectin binding assay we show that the presence of both α2,6-linked and α2,3-linked SAs on the target cells does not always correlate with efficient transduction. Further, HA substitutions of the residues implicated in switching SA-binding between avian and human species did not drastically affect HA-mediated transduction of the target cells or target cell binding. Conclusion: Our results suggest that a host factor(s), which is yet to be identified, is required for H5N1 entry in the host cells.

Hemagglutinin (HA) plays a critical role during influenza virus receptor binding and subsequent membrane fusion process, thus HA has become a promising drug target. For the past several decades, we and other researchers have discovered a series of HA inhibitors mainly targeting its fusion machinery. In this review, we summarize the advances in HA-targeted development of small molecule inhibitors. Moreover, we discuss the structural basis and mode of action of these inhibitors, and speculate upon future directions toward more potent inhibitors of membrane fusion and potential anti-influenza drugs.