To infect humans and cause a pandemic, avian influenza must first adapt to use human versions of the proteins the virus hijacks for replication, instead of the avian orthologues found in bird cells. One critical host protein is ANP32.

Host restriction limits the emergence of novel pandemic strains from the Influenza A Virus avian reservoir. For efficient replication in mammalian cells, the avian influenza RNA-dependent RNA polymerase must adapt to use human orthologues of the host factor ANP32, which lack a 33 amino acid insertion relative to avian ANP32A. Here we find that influenza polymerase requires ANP32 proteins to support both steps of replication: cRNA and vRNA synthesis. Nevertheless, avian strains are only restricted in vRNA synthesis in human cells. Therefore, avian polymerase can use human ANP32 orthologues to support cRNA synthesis, without acquiring mammalian adaptations. This implies a fundamental difference in the mechanism by which ANP32 proteins support cRNA vs vRNA synthesis.