Effect of Cytoplasmic Tail Truncations on the Activity of the M 2 Ion Channel of Influenza A Virus

Abstract

The M 2 protein of influenza A virus forms a proton channel that is required for viral replication. The M 2 ion channel is a homotetramer and has a 24-residue N-terminal extracellular domain, a 19-residue transmembrane domain, and a 54-residue cytoplasmic tail. We show here that the N-terminal methionine residue is cleaved from the mature protein. Translational stop codons were introduced into the M 2 cDNA at residues 46, 52, 62, 72, 77, 82, 87, and 92. The deletion mutants were designated trunc x , according to the amino acid position that was changed to a stop codon. We studied the role of the cytoplasmic tail by measuring the ion channel activity (the current sensitive to the M 2 -specific inhibitor amantadine) of the cytoplasmic tail truncation mutants expressed in oocytes of Xenopus laevis . When their conductance was measured over time, mutants trunc72, trunc77, and trunc92 behaved comparably to wild-type M 2 protein (a decrease of only 4% over 30 min). In contrast, conductance decreased by 28% for trunc82, 27% for trunc62, and 81% for trunc52 channels. Complete closure of the channel could be observed in some cells for trunc62 and trunc52 within 30 min. These data suggest that a role of the cytoplasmic tail region of the M 2 ion channel is to stabilize the pore against premature closure while the ectodomain is exposed to low pH.