Influenza virus hemagglutinin (HA) mediates virus entry by binding to cell surface receptors and fusing the viral and endosomal
membranes following uptake by endocytosis. The acidic environment of endosomes triggers a large-scale conformational change
in the transmembrane subunit of HA (HA2) involving a loop (B loop)-to-helix transition, which releases the fusion peptide at
the HA2 N terminus from an interior pocket within the HA trimer. Subsequent insertion of the fusion peptide into the endo-
somal membrane initiates fusion. The acid stability of HA is influenced by residues in the fusion peptide, fusion peptide pocket,
coiled-coil regions of HA2, and interactions between the surface (HA1) and HA2 subunits, but details are not fully understood
and vary among strains. Current evidence suggests that the HA from the circulating pandemic 2009 H1N1 influenza A virus
[A(H1N1)pdm09] is less stable than the HAs from other seasonal influenza virus strains. Here we show that residue 205 in HA1
and residue 399 in the B loop of HA2 (residue 72, HA2 numbering) in different monomers of the trimeric A(H1N1)pdm09 HA
are involved in functionally important intermolecular interactions and that a conserved histidine in this pair helps regulate HA
stability. An arginine-lysine pair at this location destabilizes HA at acidic pH and mediates fusion at a higher pH, while a gluta-
mate-lysine pair enhances HA stability and requires a lower pH to induce fusion. Our findings identify key residues in HA1 and
HA2 that interact to help regulate H1N1 HA stability and virus infectivity.