Paramyxovirus fusion (F) protein: A conformational change on cleavage activation

The fusion (F) protein of the paramyxovirus SV5 promotes both virus-cell an d cell-cell fusion. Recently, the atomic structure at 1.4 Angstrom of an ex tremely thermostable six-helix bundle core complex consisting of two heptad repeat regions of the F protein has been described (K. A. Baker, R. E. Dut ch, R. A. Lamb, and T. S Jardetsky, Mol. Cell 3, 309-319, 1999). To analyze the conformations of the F protein at various stages of the membrane fusio n process and to understand further the role of formation of the six-helix bundle core complex in promotion of membrane fusion, antibodies to peptides corresponding to regions of the F protein were obtained. Major changes in F protein antibody recognition were found after cleavage of the precursor p rotein F-0 to the fusogenically active disulfide-linked heterodimer, F-1 F-2, and antibodies directed against the heptad repeat regions recognized o nly the uncleaved form. A monoclonal antibody directed against the F protei n showed increased recognition at the cell surface of the cleaved form of t he F protein as compared to uncleaved F protein, again indicating changes i n conformation between the uncleaved and cleaved forms of the F protein. An ti-peptide antibodies specific for the heptad repeat regions were unable to precipitate a synthetic protein that consisted of the heptad repeat region s separated only by a small spacer, suggesting that the antibodies are unab le to recognize their target regions when the heptad repeats are present in the six-helix bundle core complex. Taken together, these data indicate tha t the six-helix bundle core complex is not present in the precursor molecul e F-0 and that significant conformational changes occur subsequent to cleav age of the F protein, (C) 2001 Academic Press.