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Published online 4 April 2005. doi:10.1083/jcb.200412059
The Rockefeller University Press, 0021-9525 $8.00
JCB, Volume 169, Number 1, 167-177
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Article

 Class II fusion protein of alphaviruses drives membrane fusion through the same pathway as class I proteins

Elena Zaitseva1, Aditya Mittal1,2, Diane E. Griffin3, and Leonid V. Chernomordik1

1 Section on Membrane Biology, Laboratory of Cellular and Molecular Biophysics, National Institute of Child Health and Human Development, National Institutes of Health, Bethesda, MD 20892
2 Department of Biochemical Engineering and Biotechnology, Indian Institute of Technology, New Delhi, India 110016
3 W. Harry Feinstone Department of Molecular Microbiology and Immunology, Johns Hopkins Bloomberg School of Public Health, Baltimore, MD 21205

Correspondence to Leonid V. Chernomordik: chernoml{at}mail.nih.gov

Viral fusion proteins of classes I and II differ radically in their initial structures but refold toward similar conformations upon activation. Do fusion pathways mediated by alphavirus E1 and influenza virus hemagglutinin (HA) that exemplify classes II and I differ to reflect the difference in their initial conformations, or concur to reflect the similarity in the final conformations? Here, we dissected the pathway of low pH–triggered E1-mediated cell–cell fusion by reducing the numbers of activated E1 proteins and by blocking different fusion stages with specific inhibitors. The discovered progression from transient hemifusion to small, and then expanding, fusion pores upon an increase in the number of activated fusion proteins parallels that established for HA-mediated fusion. We conclude that proteins as different as E1 and HA drive fusion through strikingly similar membrane intermediates, with the most energy-intensive stages following rather than preceding hemifusion. We propose that fusion reactions catalyzed by all proteins of both classes follow a similar pathway.

Abbreviations used in this paper: CF, 6-carboxyfluorescein; CPZ, chlorpromazine; FD, 70-kD fluorescent dextran; FFD, FITC-tagged FD; FL, fusion loop, LPC, lysophosphatidylcholine; LAS, LPC-arrested fusion stage; NEP, nonexpanding fusion pores; RH, restricted hemifusion; RFD, rhodamine-tagged FD; SFV, Semliki Forrest virus; SIN, Sindbis virus; UH, unrestricted hemifusion; ZnAS, Zn-arrested fusion stage.


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