Chemical cross-linking of proteins of the influenza virion. 2. Acid-induced irreversible conformational changes in HA1 and HA2.

Purified influenza virus (A/FPV/Rostock/34;H7N1) was exposed briefly to pH 5 before returning to an alkaline pH. Virus was then reacted with one of three chemical cross-linking reagents [dimethyl suberimidate (DMS), tartryl diazide (TDA), or formaldehyde which span 11, 6, and 2A, respectively]. Cross-linked polypeptides were analysed by SDS-polyacrylamide gel electrophoresis under reducing conditions and identified with monospecific antisera against HA1, HA2, NP and M1. Acidification resulted in changes in the cross-linking patterns for both HA1 and HA2 which could be detected with all three reagents. Most notable were the data with formaldehyde: under alkaline conditions cross-linking gave only HA1:HA2 heteropolymers but after brief acidification none of these were formed and in their place was a novel HA1 homodimer, an HA2 homotrimer and an HA2 of Mr 50k cross-linked to form a homodimer with another HA2 or to a heterodimer with M1. Although cross-linking by formaldehyde was much more affected by acidification of the virus than cross-linking by DMS or TDA, over half the polymers cross-linked by DMS were no longer formed after acidification. The patterns of cross-linking of NP and M1 were unchanged by low pH treatment.