UCSF

Antiviral restriction factors such as MxA (myxovirus resistance protein A) inhibit many viruses. Viral escape drives restriction factors to evolve rapidly at virus-binding interfaces to regain defense. Here, we explore how antiviral proteins balance restricting many viruses with evolving specificity against individual viruses. Human MxA uses its rapidly evolving loop L4 as the specificity determinant for orthomyxoviruses such as thogotovirus (THOV) and influenza (IAV). Previous combinatorial mutagenesis of rapidly evolving residues in human MxA loop L4 identified THOV "super-restrictors" and suggested an antiviral breadth-specificity trade-off. Using a modified combinatorial mutagenesis strategy, we find super-restrictor MxA variants specific to H5N1 IAV. A single L4 residue underlies the MxA breadth-specificity trade-off. However, rare "generalist" super-restrictors or a heterozygous combination of more common "specialist" super-restrictors can overcome the breadth-specificity trade-off. Our findings suggest that at least two strategies enable restriction factors such as MxA to increase their restriction of diverse viruses to overcome breadth-specificity trade-offs, which might be pervasive in host-virus conflicts.