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Journal of Virology, March 2000, p. 2714-2720, Vol. 74, No. 6
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Role of Hemagglutinin Surface Density in the Initial Stages of Influenza Virus Fusion: Lack of Evidence for Cooperativity

Susanne Günther-Ausborn,1,dagger Pieter Schoen,2,Dagger Ingrid Bartoldus,1 Jan Wilschut,2,* and Toon Stegmann3,*

Department of Biophysical Chemistry, Biozentrum of the University of Basel, CH 4056 Basel, Switzerland,1 Institut de Pharmacologie et de Biologie Structurale, CNRS UPR 9062, 31077 Toulouse Cedex, France,3 and Laboratory of Molecular Virology, Department of Medical Microbiology, University of Groningen, 9713 AV Groningen, The Netherlands2

Received 16 August 1999/Accepted 14 December 1999

Membrane fusion mediated by influenza virus hemagglutinin (HA) is believed to proceed via the cooperative action of multiple HA trimers. To determine the minimal number of HA trimers required to trigger fusion, and to assess the importance of cooperativity between these HA trimers, we have generated virosomes containing coreconstituted HAs derived from two strains of virus with different pH dependencies for fusion, X-47 (optimal fusion at pH 5.1; threshold at pH 5.6) and A/Shangdong (optimal fusion at pH 5.6; threshold at pH 6.0), and measured fusion of these virosomes with erythrocyte ghosts by a fluorescence lipid mixing assay. Virosomes with different X-47-to-A/Shangdong HA ratios, at a constant HA-to-lipid ratio, showed comparable ghost-binding activities, and the low-pH-induced conformational change of A/Shangdong HA did not affect the fusion activity of X-47 HA. The initial rate of fusion of these virosomes at pH 5.7 increased directly proportional to the surface density of A/Shangdong HA, and a single A/Shangdong trimer per virosome appeared to suffice to induce fusion. The reciprocal of the lag time before the onset of fusion was directly proportional to the surface density of fusion-competent HA. These results support the notion that there is no cooperativity between HA trimers during influenza virus fusion.

* Corresponding author. Mailing address for Jan Wilschut: Department of Medical Microbiology, University of Groningen, Ant. Deusinglaan 1, 9713 AV Groningen, The Netherlands. Phone: 31 50 3632733. Fax: 31 50 3632728. E-mail: J.C.Wilschut{at}med.rug.nl. Mailing address for Toon Stegmann: Institut de Pharmacologie et de Biologie Structurale, CNRS UPR 9062, 205 Route de Narbonne, 31077 Toulouse Cedex, France. Phone: 33 5 61 17 54 63. Fax: 33 5 61 17 59 94. E-mail: stegmann{at}ipbs.fr.

dagger Present address: s_ausborn{at}hotmail.com.

Dagger Present address: Haemoprobe BV, 9713 GX Groningen, The Netherlands.

Journal of Virology, March 2000, p. 2714-2720, Vol. 74, No. 6
0022-538X/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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