Swine ANP32A Supports Avian Influenza Virus Polymerase

Abstract

Avian influenza viruses occasionally infect and adapt to mammals, including humans. Swine are often described as “mixing vessels,” being susceptible to both avian- and human-origin viruses, which allows the emergence of novel reassortants, such as the precursor to the 2009 H1N1 pandemic. ANP32 proteins are host factors that act as influenza virus polymerase cofactors. In this study, we describe how swine ANP32A, uniquely among the mammalian ANP32 proteins tested, supports the activity of avian-origin influenza virus polymerases and avian influenza virus replication. We further show that after the swine-origin influenza virus emerged in humans and caused the 2009 pandemic, it evolved polymerase gene mutations that enabled it to more efficiently use human ANP32 proteins. We map the enhanced proviral activity of swine ANP32A to a pair of amino acids, 106 and 156, in the leucine-rich repeat and central domains and show these mutations enhance binding to influenza virus trimeric polymerase. These findings help elucidate the molecular basis for the mixing vessel trait of swine and further our understanding of the evolution and ecology of viruses in this host. IMPORTANCE Avian influenza viruses can jump from wild birds and poultry into mammalian species such as humans or swine, but they only continue to transmit if they accumulate mammalian adapting mutations. Pigs appear uniquely susceptible to both avian and human strains of influenza and are often described as virus “mixing vessels.” In this study, we describe how a host factor responsible for regulating virus replication, ANP32A, is different between swine and humans. Swine ANP32A allows a greater range of influenza viruses, specifically those from birds, to replicate. It does this by binding the virus polymerase more tightly than the human version of the protein. This work helps to explain the unique properties of swine as mixing vessels.

Mammalian ANP32A and ANP32B proteins drive alternative avian influenza virus polymerase adaptations

“…Across different mammalian species, there is variation in the ability of ANP32A or ANP32B to support influenza virus polymerase. For example, in humans and mice ANP32B is the more potent polymerase co-factor, whilst the ANP32A protein of pigs, horses, dogs, seals and bats more efficiently support polymerase activity (Supplementary Figure S2A,B) (Peacock et al, 2020, Zhang et al, 2019. This dominance is maintained across a range of different ratios of these proteins (Supplementary Figure S2C).…”

“…These subtle variations are due to polymorphisms between the ANP32 orthologues in different species. For example, the potent pro-viral activity of swine ANP32A is due to polymorphisms at positions 106 and 156 (Peacock et al, 2020, Zhang et al, 2020. Similarly, the weak pro-viral activity of dog (as well as bat and seal) ANP32B is attributed to residue 153 that is glutamine in the human ANP32B, but arginine in the canine orthologue (Supplementary Figure S3A-C)…”

“…As well as differences in avian and mammalian ANP32 length, there also exists differences in the level of redundancy to support influenza virus polymerase in different vertebrate hosts. ANP32A is the sole ANP32 family member that supports influenza polymerase in birds, while in humans and most other mammalian influenza hosts ANP32A and ANP32B can both support polymerase activity to varying levels (Long, Idoko-Akoh et al, 2019a, Peacock, Swann et al, 2020, Zhang et al, 2019. One exception is mice in which only ANP32B can efficiently support influenza polymerase due to a polymorphism in murine ANP32A at position 130 -a residue critical for the interaction between ANP32 proteins and viral polymerase (Beck, Zickler et al, 2020, Zhang et al, 2019.…”

“…One exception is mice in which only ANP32B can efficiently support influenza polymerase due to a polymorphism in murine ANP32A at position 130 -a residue critical for the interaction between ANP32 proteins and viral polymerase (Beck, Zickler et al, 2020, Zhang et al, 2019. It has recently been shown that, although both swine ANP32A and ANP32B can support mammalian-adapted polymerases, swine ANP32A has the more potent pro-viral function and can even partially support avian polymerases (Peacock et al, 2020, Zhang, Li et al, 2020. Similarly in horses, dogs, seals and bats, ANP32A appears to be a more potent pro-viral factor than ANP32B (Peacock et al, 2020).…”

“…It has recently been shown that, although both swine ANP32A and ANP32B can support mammalian-adapted polymerases, swine ANP32A has the more potent pro-viral function and can even partially support avian polymerases (Peacock et al, 2020, Zhang, Li et al, 2020. Similarly in horses, dogs, seals and bats, ANP32A appears to be a more potent pro-viral factor than ANP32B (Peacock et al, 2020).…”