FIG 7 .

Implications for fusion triggering and HN-F interactions. Observation of viral surfaces in the absence of receptor engagement include HN tetramers in the heads-down configuration in arrays on the virus surface. These heads-down tetramers were not observed in association with F. On these virions without receptor engagement, some prefusion F is interspersed with HN in an extended configuration with the globular heads towering above the level of the F consistent with a heads-up but monomeric or dimeric form. F alone is also observed in both the prefusion and postfusion forms. The data support the notion that F and HN are clustered with each other prior to receptor engagement by HN and that the presence of a heads-up form of HN in proximity to prefusion F is not sufficient to induce F activation. (B) Since tetrameric, heads-up HN was not observed either alone or in association with F, the possibility exists that oligomerization induced by receptor engagement (not studied here) is important for triggering F.