A point mutation in the F1 subunit of human respiratory syncytial virus fusion glycoprotein blocks its cell surface transport at an early stage of the exocytic pathway.

Vaccinia virus recombinants expressing either wild-type or mutant forms of human respiratory syncytial (RS) virus (Long strain) fusion (F) glycoprotein were obtained. Proteolytic processing of the precursor, F0, and cell surface transport of the F glycoprotein were unaffected in the recombinants, except in those that contained the replacement Phe --> Ser at position 237 of the F1 subunit. In recombinants containing this mutation, either alone or in combination with others, the traffic of the F molecule was arrested at some intermediate step of its transport to the cell surface and, consequently, the endoproteolytic cleavage of the F0 precursor was inhibited. Immunofluorescence staining of infected cells and endoglycosidase H (Endo-H) sensitivity assays indicated that the arrest occurred before the mid-Golgi compartment. Dimerization and folding of the F protein were also affected by the Phe237 --> Ser substitution. Other amino acid replacements at positions 236 or 237 of the F1 subunit had various effects upon F0 maturation. These results are discussed in terms of the maturation requirements for the RS virus F molecule.