Cleavage of the human respiratory syncytial virus fusion protein at two distinct sites is required for activation of membrane fusion

Preparations of purified full-length fusion (F) protein of human respiratory syncytial virus (HRSV) expressed in recombinant vaccinia-F infected cells, or of an anchorless mutant ([F.sub.TM-]) lacking the C-terminal 50 amino acids secreted from vaccinia-[F.sub.TM-]-infected cells contain a minor polypeptide that is an intermediate product of proteolytic processing of the F protein precursor FO. N-terminal sequencing of the intermediate demonstrated that it is generated by cleavage at a furin-motif, residues 106-109 of the F sequence. By contrast, the F1 N terminus derives from cleavage at residue 137 of FO which is also C-terminal to a furin recognition site at residues 131-136. Site-directed mutagenesis indicates that processing of FO protein involves independent cleavage at both sites. Both cleavages are required for the F protein to be active in membrane fusion as judged by syncytia formation, and they allow changes in F structure from cone- to lollipop-shaped spikes and the formation of rosettes by anchorless F.