Co-incorporation of the PB2 and PA polymerase subunits from human H3N2 influenza virus is a critical determinant of the replication of reassortant ribonucleoprotein complexes.

The influenza virus RNA polymerase, composed of the PB1, PB2 and PA subunits, has a potential role in influencing genetic reassortment. Recent studies on the reassortment of human H3N2 strains suggest that the co-incorporation of PB2 and PA from the same H3N2 strain appears to be important for efficient virus replication; however, the underlying mechanism remains unclear. Here, we reconstituted reassortant ribonucleoprotein (RNP) complexes and demonstrated that the RNP activity was severely impaired when the PA subunit of H3N2 strain A/NT/60/1968 (NT PA) was introduced into H1N1 or H5N1 polymerase. The NT PA did not affect the correct assembly of the polymerase trimeric complex, but it significantly reduced replication-initiation activity when provided with a vRNA promoter and severely impaired the accumulation of RNP, which led to the loss of RNP activity. Mutational analysis demonstrated that PA residues 184N and 383N were the major determinants of the inhibitory effect of NT PA and 184N/383N sequences were unique to human H3N2 strains. Significantly, NT PB2 specifically relieved the inhibitory effect of NT PA, and the PB2 residue 627K played a key role. Our results suggest that PB2 from the same H3N2 strain might be required for overcoming the inhibitory effect of H3N2 PA in the genetic reassortment of influenza virus.