The morphology and composition of influenza A virus particles are not affected by low levels of M1 and M2 proteins in infected cells.

We generated a recombinant influenza A virus (Mmut) that produced low levels of matrix (M1) and M2 proteins in infected cells. Mmut virus propagated to significantly lower titers than did wild-type virus in cells infected at low multiplicity. By contrast, virion morphology and incorporation of viral proteins and vRNAs into virus particles were similar to those of wild-type virus. We propose that a threshold amount of M1 protein is needed for the assembly of viral components into an infectious particle and that budding is delayed in Mmut virus-infected cells until sufficient levels of M1 protein accumulate at the plasma membrane.