Membrane fusion machines of paramyxoviruses: capture of intermediates of fusion

Peptides derived from heptad repeat regions adjacent to the fusion peptide and transmembrane domains of many viral fusion proteins form stable helical bundles and inhibit fusion specifically. Paramyxovirus SV5 fusion (F) prot ein-mediated fusion and its inhibition by the peptides N-1 and C-1 were ana lyzed. The temperature dependence of fusion by F suggests that thermal ener gy, destabilizing praline residues and receptor binding by the hemagglutini n-neuraminidase (HN) protein collectively contribute to F activation from a metastable native state. F-mediated fusion was reversibly arrested by low temperature or membrane-incorporated lipids, and the resulting F intermedia tes were characterized. N-1 inhibited an earlier F intermediate than C-1. C o-expression of HN with F lowered the temperature required to attain the N- 1-inhibited intermediate, consistent with HN binding to its receptor stimul ating a conformational change in F. C-1 bound and inhibited an intermediate of F that could be detected until a point directly preceding membrane merg er. The data are consistent with C-1 binding a pre-hairpin intermediate of F and with helical bundle formation being coupled directly to membrane fusi on.