Structures of influenza A and B replication complexes explain avian to human host adaption and reveal a role of ANP32 as an electrostatic chaperone for the apo-polymerase.

Replication of influenza viral RNA depends on at least two viral polymerases, a parental replicase and an encapsidase, and cellular factor ANP32. ANP32 comprises an LRR domain and a long C-terminal low complexity acidic region (LCAR). Here we show that ANP32 is recruited to the replication complex (replicase-ANP32-encapsidase) by first acting as an electrostatic chaperone to stabilise the encapsidase moiety within apo-polymerase symmetric dimers that are distinct for influenza A and B polymerases. The encapsidase, with ANP32, then forms an asymmetric complex with the replicase. Cryo-EM structures of the influenza A and B replication complexes give new insight into the mutations known to adapt avian strain polymerases to use the distinct ANP32 in mammalian cells. The cryo-EM map of the FluPolB complex shows extra density attributable to the ANP32 LCAR wrapping around and stabilising the apo-encapsidase conformation. These results suggest a functional requirement for three polymerases for replication.