Effect of cytoplasmic tail truncations on the activity of the M-2 ion channel of influenza A virus

The M-2 protein of influenza A virus forms a proton channel that is require d 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 methi onine 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 truncx, 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 sensit ive to the M-2-specific inhibitor amantadine) of the cytoplasmic tail trunc ation mutants expressed in oocytes of Xenopus laevis. When their conductanc e was measured over time, mutants trunc72, trunc77, and trunc92 behaved com parably to wild-type M-2 protein (a decrease of only 4% over 30 min). In co ntrast, 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 stabiliz e the pore against premature closure while the ectodomain is exposed to low pH.