Influenza virus M₂ protein ion channel activity stabilizes the native form of fowl plague virus hemagglutinin during intracellular transport

The influenza A/fowl plague virus/Rostock/34 hemagglutinin (HA), which is cleaved intracellularly and has a high pH threshold (pH 5.9) for undergoing its conformational change to the low-pH form, was expressed from cDNA in CV- 1 and HeLa T4 cells in the absence of other influenza virus proteins. It was found, by biochemical assays, that the majority of the HA molecules were in a form indistinguishable from the low-pH form of HA. The acidotropic agent, ammonium chloride, stabilized the accumulation of HA in its native form. Coexpression of HA and the homotypic influenza virus M₂ protein, which has ion channel activity, stabilized the accumulation of HA in its pH neutral (native) form, and the M₂ protein ion channel blocker, amantadine, prevented the rescue of HA in its native form. These data provide direct evidence that the influenza virus M₂ protein ion channel activity can affect the status of the conformational form of cleaved HA during intracellular transport.