Growth restriction of influenza A virus by M2 protein antibody is genetically linked to the M1 protein

The M2 protein of influenza A virus is a 97-amino acid integral membrane protein expressed at the surface of infected cells. Recent studies have shown that a monoclonal antibody (14C2) recognizes the N terminus of M2 and restricts the replication of certain influenza A viruses. To investigate the mechanism of M2 antibody growth restriction, 14C2 antibody-resistant variants of strain A/Udorn/72 have been isolated. Most of the variant viruses are not conventional antigenic variants as their M2 protein is still recognized by the 14C2 antibody. A genetic analysis of reassortant influenza viruses prepared from the 14C2 antibody-resistant variants and an antibody-sensitive parent virus indicates that M2 antibody growth restriction is linked to RNA segment 7, which encodes both the membrane protein (M1) and the M2 integral membrane protein. Nucleotide sequence analysis of RNA segment 7 from the variant viruses predicts single amino acid substitutions in the cytoplasmic domain of M2 at positions 71 and 78 or at the N terminus of the M1 protein at residues 31 and 41. To further examine the genetic basis for sensitivity and resistance to the 14C2 antibody, the nucleotide sequences of RNA segment 7 of several natural isolates of influenza virus have been obtained. Differences in the M1 and M2 amino acid sequences for some of the naturally resistant strains correlate with those found for the M2 antibody variant viruses. The possible interaction of M1 and M2 in virion assembly is discussed.