Purified influenza virus (A/FPV/Rostock/34; H7N1) was reacted with one of three chemical crosslinking reagents [dimethylsuberimidate (DMS), tartryl diazide (TDA) and formaldehyde] under conditions designed to give a ladder of crosslinked polypeptides (putative homo- and heteropolymers) when analysed by SDS-polyacrylamide gel electrophoresis under reducing conditions. The different virion polypeptides were identified by Western blotting with monospecific antisera against HA1, HA2, NP, and M1. When reacted with any crosslinker NP preferentially formed 2mer and 4mer homopolymers while M1 formed 2mers, 4mers, 6mers, and 8mers. 2mers and 3mers of HA1 were detected after crosslinking with TDA and DMS but homopolymers of HA2 could not be identified with certainty due to comigrating M1. One heteropolymer was clearly identified as 1NP:1M1 (with DMS and TDA) and others, as expected, as components of the haemagglutinin spike 1HA1:1HA2, 2HA1:2HA2, and 3HA1:3HA2. Formaldehyde gave rise only to HA1:HA2 polymers. The presence of other heteropolymers containing NP in conjunction with HA2 and HA1 seemed likely. Whenever HA2 ran with an Mr of about 50k it comigrated with M1 suggesting it may have formed (with DMS or TDA) a 1HA2:1M1 heterodimer. However it is possible that this band consisted of HA2 homodimers comigrating with M1 homodimers. Patterns of crosslinking with DMS and TDA were similar although not identical, but those obtained with formaldehyde were markedly different. All patterns were highly reproducible.

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.