A molecular mechanism for the low-pH stability of sialidase activity of influenza A virus N2 neuraminidases1<FN ID="FN1"><NO>1</NO>Nucleotide sequence data reported are available in the DDBJ/EMBL/GenBank databases under the accession numbers AB101671, AB101672, AB101673, AB101674 and AB101675.</FN>

Four human pandemic influenza A virus strains isolated in 1957 and 1968, but not most of the epidemic strains isolated after 1968, possess sialidase activity under low-pH conditions. Here, we used cell-expressed neuraminidases (NAs) to determine the region of the N2 NA that is associated with low-pH stability of sialidase activity. We found that consensus amino acid regions responsible for low-pH stability did not exist in pandemic NAs but that two amino acid substitutions in the low-pH-stable A/Hong Kong/1/68 (H3N2) NA and a single substitution in the low-pH-unstable A/Texas/68 (H2N2) NA resulted in significant change in low-pH stability. [Copyright &y& Elsevier]