ASSIGNMENT OF DISULFIDE BRIDGES IN THE FUSION GLYCOPROTEIN OF SENDAI VIRUS

The mature fusion (F) glycoprotein of the paramyxovirus family consist s of two disulfide-linked subunits, the N-terminal F-2 and the C-termi nal F-1 subunits, and contains 10 cysteine residues which are highly c onserved at specific positions. The high level of conservation strongl y suggests that they are indeed disulfide linked and play important ro les in the folding and functioning of the molecule. However, it has no t even been clarified which cysteine residues link the F-2 and F-1 sub units. This report describes our assignment of the disulfide bridges i n purified Sendai virus F glycoprotein by fragmentation of the polypep tide and isolation of cystine-containing peptides and determination of their N-terminal sequences. The data demonstrate that all of the 10 c ysteine residues participate in disulfide bridges and that Cys-70, the only cysteine in F-2, and Cys-199, the most upstream cysteine in F-1, form the interchain bond. Of the remaining eight cysteine residues cl ustered near the transmembrane domain of F-1, the specific bridges ide ntified are Cys-338 to Cys-347 and Cys-362 to Cys-370. Although no exa ct pairings between the subsequent four residues were defined, it seem s likely that the most downstream, Cys-424, is linked to Cys-394, Cys- 399, or Cys-401. Thus, we conclude that the cysteine-rich domain indee d contributes to the formation of a bunched structure containing at le ast two tandem cystine loops.