Reconstitution of the influenza virus M₂ ion channel in lipid bilayers

M₂, an integral membrane protein of influenza A virus, was purified from either influenza A virus-infected CV-1 cells or from Spodoptera frugiperda (Sf9) cells infected with a recombinant-M₂ baculovirus. The purified protein, when incorporated into phospholipid bilayer membranes, produced ion-permeable channels with the following characteristics: (1) The channels appeared in bursts during which unit conductances of diverse magnitudes (25-500 pS) were observed. (2) The most probable open state was usually the lowest unit conductance (25-90 pS). (3) The channels were selective for cations; t_Na = 0.75 when 150 m m NaCl bathed both sides of the membrane. (4) Amantadine reduced the probability of opening of the high conductance state and also the conductance of the most probable state. (5) Reducing pH increased the mean current through the open channel as well as the conductance of the most probable state. (6) The sequence of selectivity for group IA monovalent cations was Rb > K > Cs ∼ Na > Li. The pH activation, amantadine block and ion selectivity of the M₂ protein ion channel in bilayers are consistent with those observed on expression of the M₂ protein in oocytes of Xenopus laevis as well as for those predicted for the proposed role of an ion channel in the uncoating process of influenza virus. The finding that the M₂ protein has intrinsic ion channel activity supports the hypothesis that it has ion channel activity in the influenza virus particle.