Browsing by Author "Ni, Fengyun"

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    Functional and structural studies of influenza B virus hemagglutinin
    (2013-09-16) Ni, Fengyun; Ma, Jianpeng; Tao, Yizhi Jane; Matthews, Kathleen S.
    Influenza A and B viruses are major causes of seasonal flu epidemics each year. Hemagglutinin (HA) mediates the binding of virus to host cell and the fusion with host membrane. The crystal of HA in complex with antibody that reveals the mechanism by which antibody recognizes HA may not diffract to high resolution, thereby preventing the accurate interpretation of the structural model. The application of normal mode refinement that aims for improving the structure quality at the low resolution is tested. These studies provide some guidelines for future refinement of HA-antibody complex structures. By comparing the residues constituting the base of the receptor binding site of influenza A and B virus HAs, it is found that they share some similarities, except for a Phe at position 95 of influenza B virus hemagglutinin (BHA) versus Tyr in of influenza A virus hemagglutinin (AHA). The recombinant protein BHA containing the F95Y mutation exhibits the increased receptor binding affinity and specificity. However, recombinant viruses with the Phe95Tyr mutation show lower erythrocyte agglutination titer and decreased binding abilities with different cell lines. The replication of the Phe95Tyr mutant virus in mice is also attenuated. These data suggest that the increased receptor binding ability of HA alone is not advantageous to the pathogenesis of the viruses. The structure of BHA2 (a portion of BHA near the C-terminus) at the post-fusion state has been determined to 2.45 Å resolution. This protein forms a hairpin-like conformation rich in -helices. About 70 residues from the N-terminus is a three-stranded coiled coil, and the remaining of the protein packs in anti-parallel against the groove formed by the central helices. In the post-fusion state of BHA2, the helix converted from the B-loop in pre-fusion state contacts the C-terminal fragment of this protein with more hydrophobic interactions as compared to AHA2. This structure illustrates the distinct stabilization strategy employed by BHA2 to form a post-fusion state that resembles that for AHA2. These studies will further the understanding of BHA with respect to its role in receptor binding ability and fusion.
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    Role of Phe95 In the Receptor Binding of Influenza B Virus Hemagglutinin
    (2011) Ni, Fengyun; Ma, Jianpeng
    Influenza A and B viruses are significant human pathogens responsible for the annual seasonal "flu". Diverged some 2000 years ago, influenza B virus has several important differences from influenza A virus, including lower receptor-binding affinity and very limited host range. Based on sequence comparison and our prior structural studies, we hypothesized that a key difference in the receptor-binding site of influenza virus hemagglutinin (HA), phenylalaline (Phe) 95 in influenza B virus HA (BHA), versus tyrosine (Tyr) in influenza A virus HA (AHA), is possibly the molecular basis for the different receptor-binding affinity. We further hypothesized that this could be at least partially responsible for the very limited host range of influenza B virus. By using glycan and red blood cell binding assays, we demonstrated that the mutation Phe95[arrow right]Tyr in BHA substantially enhanced receptor-binding affinity. Furthermore, this mutation efficiently competed against the infection of influenza A virus and greatly improved the binding of BHA to three mammalian cell lines. Taken together, residue 95 of BHA appears to be a key determinant for the receptor binding affinity and host range of influenza B virus.
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    Structural Basis of Actin Filament Nucleation by Tandem W Domains
    (Cell Press, 2013) Chen, Xiaorui; Ni, Fengyun; Tian, Xia; Kondrashkina, Elena; Wang, Qinghua; Ma, Jianpeng; Bioengineering
    Spontaneous nucleation of actin is very inefficient in cells. To overcome this barrier, cells have evolved a set of actin filament nucleators to promote rapid nucleation and polymerization in response to specific stimuli. However, the molecular mechanism of actin nucleation remains poorly understood. This is hindered largely by the fact that actin nucleus, once formed, rapidly polymerizes into filament, thus making it impossible to capture stable multisubunit actin nucleus. Here, we report an effective doublemutant strategy to stabilize actin nucleus by preventing further polymerization. Employing this strategy, we solved the crystal structure of AMPPNP-actin in complex with the first two tandem W domains of Cordon-bleu (Cobl), a potent actin filament nucleator. Further sequence comparison and functional studies suggest that the nucleation mechanism of Cobl is probably shared by the p53 cofactor JMY, but not Spire. Moreover, the double-mutant strategy opens the way for atomic mechanistic study of actin nucleation and polymerization.