Avian Paramyxovirus Serotype-1: A Review of Disease Distribution, Clinical Symptoms, and Laboratory Diagnostics

Figure 1

(a) The prefusion form of F contains a globular head with the HRA region in 11 distinct sections, and the HRB region is in a three-helix bundle. The F TM domain is also represented as a three-helix bundle, consistent with the oxidative cross-linking data. (b) Upon HN binding to target cells (HN not shown for clarity), F is activated for fusion, and the HRB region separates, forming the open-stalk conformation where N-1 peptide can bind to HRB. At this open-stalk stage, the TM domain is still thought to be in a three-helix bundle because N-1-HAt can still bind to HRB after the addition of the oxidative cross-linker. (c) After formation of the open-stalk conformation, HRA rearranges to form the extended α-helical bundle, and the FP is inserted into the target cell membrane (the prehairpin intermediate). (d-e) Finally, the postfusion state occurs with the formation of the 6-HB. (d-e and f–i) Lipid intermediates in fusion with the F protein, removed for clarity. The two bilayers contain an inner and outer leaflets and are separated by the extracellular space. During the process of F refolding to form the postfusion form, water is excluded from the extracellular space and the outer leaflets initially merge to form the lipid stalk intermediate. The lipids of the bilayers mix, forming the hemifusion intermediate, and then the fusion pore forms. F domains: FP (red), HRA (green), globular head (yellow), HRB (blue), TM domain (orange), cytoplasmic tail (pink). (From Bissonnette et al., 2009 [44] with permission.)