The matrix 1 protein of influenza A virus inhibits the transcriptase activity of a model influenza reporter genome in vivo.

The M1 protein of influenza virus inhibits the in vitro transcriptase activity of ribonucleoprotein cores from virions. This inhibitory activity is thought to be relevant in vivo because accumulation of M1 at the late stages of viral replication may be the cue to halt viral mRNA production. A model influenza reporter genome was used to explore the effect of M1 on the activity of the influenza virus transcriptase complex within cultured cells. Expression of M1 in cells bearing the model influenza virus reporter genome was accompanied by a reduction of CAT gene expression to 12% of control levels. Quantification of RNA by ribonuclease protection assay revealed that the influenza reporter genome mRNA levels in M1-expressing cells were reduced by approximately 74% compared with those of cells expressing a control protein. These findings are consistent with the proposed model in which M1 is responsible for limiting viral transcription during late stages of infection. By expressing truncated forms of M1, the inhibitory activity was found to reside within the amino-terminal half of the M1 protein. Two independent inhibitory domains were identified in this region: one between amino acid residues 1-90 and the other spanning residues 91-127.