THE ROLES OF INDIVIDUAL CYSTEINE RESIDUES OF SENDAI VIRUS FUSION PROTEIN IN INTRACELLULAR-TRANSPORT

The role of intramolecular disulfide bonds in the fusion (F) protein o f Sendai virus was studied. The 10 cysteine residues were changed to s erine residues using site-directed mutagenesis. None of the cysteine m utant F proteins reacted with a monoclonal antibody specific for the m ature conformation of the F protein, but eight of ten mutants reacted with an immature conformation-specific monoclonal antibody, The transp ort of these mutant proteins to the cell surface was drastically reduc ed. All of the cysteine mutant F proteins remained sensitive to endogl ycosidase H (endo H) for 3 h after their synthesis. Moreover, cell sur face transport of the hemagglutinin-neuraminidase (HN) protein co-expr essed with each of these cysteine mutant F proteins was also reduced. These results suggest that all cysteine residues participate in the fo rmation of intramolecular disulfide bonds, that co-translational disul fide bond formation is crucial to the correct folding and intracellula r transport of the F protein, and that interaction of the F and HN pro teins takes place intracellulary.