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Journal of Virology, April 2001, p. 3647-3656, Vol. 75, No. 8
Institut für Virologie,
Universitätsklinikum Charité der
Humboldt-Universität zu Berlin, D-10098 Berlin, Germany
Received 9 October 2000/Accepted 16 January 2001
The viral ion channel protein M2 supports the transit of influenza
virus and its glycoproteins through acidic compartments of the cell. M2
conducts endosomal protons into the virion to initiate uncoating and,
by equilibrating the pH at trans-Golgi membranes,
preserves the native conformation of acid-sensitive viral
hemagglutinin. The exceptionally low conductance of the M2 channel
thwarted resolution of single channels by electrophysiological techniques. Assays of liposome-reconstituted M2 yielded the average unitary channel current of the M2 tetramer
0022-538X/01/$04.00+0 DOI: 10.1128/JVI.75.8.3647-3656.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Definitive Assignment of Proton Selectivity and Attoampere
Unitary Current to the M2 Ion Channel Protein of Influenza A
Virus
1.2 aA (1.2 × 10
18 A) at neutral pH and 2.7 to 4.1 aA at pH 5.7
which
activates the channel. Extrapolation to physiological
temperature predicts 4.8 and 40 aA, respectively, and a unitary
conductance of 0.03 versus 0.4 fS. This minute activity, below previous
estimates, appears sufficient for virus reproduction, but low enough to
avert abortive cytotoxicity. The unitary permeability of M2 was within the range reported for other proton channels. To address the ion selectivity of M2, we exploited the coupling of ionic influx and efflux
in sealed liposomes. Metal ion fluxes were monitored by proton
counterflow, employing a pH probe 1,000 times more sensitive than
available Na+ or K+ probes. Even
low-pH-activated M2 did not conduct Na+ and K+.
The proton selectivity of M2 was estimated to be at least 3 × 106 (over sodium or potassium ions), in agreement with
electrophysiological studies. The stringent proton selectivity of M2
suggests that the cytopathology of influenza virus does not
involve direct perturbation of cellular sodium or potassium gradients.
*
Corresponding author. Mailing address: Institut
für Virologie, Universitätsklinikum Charité der
Humboldt-Universität zu Berlin, D-10098 Berlin, Germany.
Phone: 4930 96209060. Fax: 4930 28023562. E-mail:
corneliaschroeder{at}hotmail.com.
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