The influenza virus M2 protein has an ion channel activity that permits ions to enter the virion during its uncoating and also modulates pH of intracellular compartments. M2 protein is a homotetramer consisting of either a pair of disulfide-linked dimers or a disulfide-linked tetramer. The M2 trans-membrane domain peptide adopts an alfa helical secondary structure. In polarized cells, M2 protein is expressed at the apical cell surface. The amantadine-induced, M2-mediated conversion of influenza A virus haemagglutinin (HA) to the low pH conformation occurs in an acidic trans-Golgi compartment. The M2 protein ion channel activity can affect the conformation of cleaved HA during intracellular transport. The equine influenza virus 1 HA expressed from cDNA does not require coexpression of a functional M2 protein to maintain HA in its native conformation.