Carr C M, Chaudhry C, Kim P S
Howard Hughes Medical Institute, Whitehead Institute for Biomedical Research, Department of Biology, Massachusetts Institute of Technology, Nine Cambridge Center, Cambridge, MA 02142, USA.
Proc Natl Acad Sci U S A. 1997 Dec 23;94(26):14306-13. doi: 10.1073/pnas.94.26.14306.
Enveloped viruses enter cells by protein-mediated membrane fusion. For influenza virus, membrane fusion is regulated by the conformational state of the hemagglutinin (HA) protein, which switches from a native (nonfusogenic) structure to a fusion-active (fusogenic) conformation when exposed to the acidic environment of the cellular endosome. Here we demonstrate that destabilization of HA at neutral pH, with either heat or the denaturant urea, triggers a conformational change that is biochemically indistinguishable from the change triggered by low pH. In each case, the conformational change is coincident with induction of membrane-fusion activity, providing strong evidence that the fusogenic structure is formed. These results indicate that the native structure of HA is trapped in a metastable state and that the fusogenic conformation is released by destabilization of native structure. This strategy may be shared by other enveloped viruses, including those that enter the cell at neutral pH, and could have implications for understanding the membrane-fusion step of HIV infection.
包膜病毒通过蛋白质介导的膜融合进入细胞。对于流感病毒，膜融合受血凝素（HA）蛋白构象状态的调节，当暴露于细胞内体的酸性环境时，HA蛋白会从天然（非融合）结构转变为融合活性（融合）构象。在这里，我们证明，在中性pH值下，用加热或变性剂尿素使HA不稳定，会引发一种构象变化，这种变化在生化上与低pH值引发的变化无法区分。在每种情况下，构象变化都与膜融合活性的诱导同时发生，这有力地证明了融合结构的形成。这些结果表明，HA的天然结构被困在亚稳态，而融合构象是通过天然结构的不稳定释放出来的。这种策略可能为其他包膜病毒所共有，包括那些在中性pH值下进入细胞的病毒，并且可能对理解HIV感染的膜融合步骤有启示意义。
