Fig 1.
M2-mediated acidification of influenza virus interior.
(A) An endocytosed virion bound to the inner leaflet of an early endosome membrane. (B) As the endosome matures and acidifies, protons enter M2 channels and (C) enter the virus interior. (D) Low pH in the virus triggers disassembly of the M1 shell and low pH in the endosome triggers membrane fusion and release of the virus contents into the cytoplasm.
Table 1.
Values taken from the experiments of Leiding et al for M2 channels reconstituted into liposomes.
Table 2.
Titratable residues within the influenza virion.
Residues may reside at the c-terminus (c), n-terminus (n), or within the chain (s). For each residue type, the corresponding pKa value (from ref [12]) is given. The total count of each residue in a single virion was estimated by counting each residue type in the sequence of each protein, then multiplying by the copy number of that protein, then summing for all proteins.
Fig 2.
Model predictions for the in vitro experiments of Ivanovic et al.
(A) At time zero, external pH (dashed line) begins a rapid decrease to pH 4.5. Internal pH (solid line) lags behind by as as much as ~1 minute but eventually equilibrates with the external pH after ~2 minutes. (B) In the experiments, acidification was monitored by fluorescein fluorophores loaded into the virions. These photo-deactivate once protonated. Shown here is the predicted fluorescent intensity changes over time. Inset: the representative fluorescence trace from the Ivanovic et al publication (raw data in pale green, their fit of the data in gray) is superimposed on the model curve (black). We rescaled the experimental fluorescence (published in arbitrary units) such that the initial fluorescence before acidification and the final background fluorescence correspond to 100% and 0%, respectively.
Table 3.
Dependence of acidification kinetics on parameter values.
Values taken from the experiments of Leiding et al for M2 channels reconstituted into liposomes. Model parameters were individually adjusted by the amount shown in the table, and the resulting change in the time for the internal pH to reach 5.7 was determined by solving the model equations. The relative effect of each parameter adjustment is the ratio of the acidification time change to the parameter change (column 3 divided by column 2) multiplied by 100%.
Fig 3.
Acidification time increases with virion size.
(A) Model predictions indicate that acidification time increases roughly linearly with virion length. (B) Model predictions for the distribution of acidification times for the conditions and distribution of virion lengths of Ivanovic et al. The predicted distribution of acidification times is comparable to the experimental distribution.
Fig 4.
Virion acidification in the endosome.
Dashed lines show approximations of endosomal pH evolution for three acidification times: 1 min (black), 2 min (red) and 5 min (blue). Solid lines give predictions for the interior pH of the virion corresponding to each endosomal acidification time. Note that the horizontal axis was trimmed for clarity. For longer times the blue curves approach pH 5 together in pseudoequilibrium.