A core trimer of the paramyxovirus fusion protein: Parallels to influenza virus hemagglutinin and HIV-1 gp41

The paramyxovirus fusion (F) protein mediates membrane fusion. The biologically active F protein consists of a membrane distal subunit, F-2, and a membrane-anchored subunit, F-1. We have identified a highly stable structure composed of peptides derived from the F-1 heptad repeat A, which abuts the hydrophobic fusion peptide (peptide N-1), and the F-1 heptad repeat B, located 270 residues downstream and adjacent to the transmembrane domain (peptides C-1 and C-2). In isolation, peptide N-1 is 47% alpha-helical and peptide C-1 and C-2 are unfolded. When mixed together, peptides N1 + C1 form a thermostable (T-m >90 degrees C), 82% alpha-helical, discrete trimer of heterodimers (mass 31,300 M-r) that is resistant to denaturation by 2% SDS at 40 degrees C. We suggest that this alpha-helical trimeric complex represents the core most stable form of the FV protein that either is fusion competent or forms after fusion has occurred. Peptide C-1 is a potent inhibitor of both the lipid mixing and the aqueous content mixing fusion activity of the SV5 F protein. In contrast, peptides N-1 and N-2 inhibit cytoplasmic content mixing but not lipid mixing, leading to a stable hemifusion state. Thus, these peptides define functionally different steps in the fusion process. The parallels among both the fusion processes and the protein structures of paramyxovirus F proteins, HIV gp41, and influenza virus hemagglutinin are discussed, as the analogies are indicative of a conserved paradigm for fusion promotion among fusion proteins from widely disparate viruses. (C) 1998 Academic Press.