Biochemical, biophysical and preliminary X‐ray crystallographic analyses of the fusion core of Sendai virus F protein

It is emerging that enveloped viruses may adopt a unique entry/fusion mechanism; in paramyxoviruses, including Sendai virus (SeV), the attachment protein HN (or its homologue H or G) binds a cellular receptor which triggers conformational changes of its fusion protein, F. There are at least three conformations of the F protein in the current fusion model: the pre‐fusion native conformation, the pre‐hairpin intermediate conformation and the post‐fusion coiled‐coil conformation. The fusion mechanism of SeV, a member of the Paramyxoviridae family, has been well established and several structural and functional domains or modules have been proposed from studies of its F protein. However, biochemical and biophysical studies of the heptad‐repeat (HR) regions (HR1 and HR2) have not been systematically carried out. HR1 and HR2 strongly interact with each other to form a stable six‐helix coiled‐coil bundle as the post‐fusion conformation. In this study, a single‐chain HR1–linker–HR2 protein of SeV was prepared in an Escherichia coli expression system and biochemical and biophysical analyses showed it to form a typical six‐helix coiled‐coil bundle; its trigonal crystals diffracted X‐rays to 2.5 Å resolution. The crystal structure will help to reveal the structural requirements of the post‐fusion coiled‐coil conformation of SeV F protein.