Evolution of the NS genes of the influenza A viruses. II. Characteristics of the amino acid changes in the NS1 proteins of the influenza A viruses.

We compared the amino acid sequences of the NS1 proteins of human, equine, and avian influenza viruses. The ratios of the amino acid substitutions per nucleotide substitutions in the NS1 proteins were about 27-45%, suggesting the existence of constraints on the amino acid changes of the NS1 protein in evolution. As a measure of constraints exerted on the regions of a protein, a changeability index is proposed. There was a highly conserved region between amino acid residues 30 and 50. The C-terminal region of amino acid residue 165 was a continuously changeable region. We have either introduced several nucleotide substitutions to the NS cDNA of the A/Udorn/72 virus in vitro or constructed the recombinant NS cDNAs between the A/Udorn/72 and A/chick/Japan/24 viruses, and then expressed them in animal cells. We have found that the amino acid substitutions introduced to the low-conserved region of the NS1 protein affected the stability and nuclear localization of the NS1 protein. One of the chimeric proteins between the A/Udorn/72 and A/chick/Japan/24 viruses did not move to the nucleus of the cell and remained in the cytoplasm.