CHARACTERIZATION OF A CLEAVAGE MUTANT OF THE MEASLES-VIRUS FUSION PROTEIN DEFECTIVE IN SYNCYTIUM FORMATION

Membrane fusion caused by measles virus (MV) is a function of the fusi on (F) protein. This process is essential for penetration into the hos t cell and subsequent initiation of the virus replicative cycle. The b iological activity of the MV F protein is generated by endoproteolytic cleavage of a precursor protein (F-0) into a large F-1 subunit and a smaller F-2 subunit held together by disulfide bonds. The cleavage sit e consists of a cluster of five basic amino acids (amino acids 108 to 112) within the predicted primary structure of the F protein. To inves tigate the role of the arginine residue at the carboxy terminus of the F-2 subunit (arginine 112), site-directed mutagenesis was used to con struct a cleavage mutant of the MV F protein in which this arginine re sidue was changed to a leucine residue. The mutated F gene, encoding f our out of the five basic amino acids at the cleavage site, was insert ed into the genome of vaccinia virus. The resulting recombinant virus was used to study expression of the mutant F protein in infected cells . Analysis of the Leu-112 mutant protein made in infected cells demons trated that this single-amino-acid substitution resulted in a reduced rate of transport of the mutant protein to the cell surface, despite i ts efficient cleavage to yield F-1 and F-2 subunits. However, the elec trophoretic mobilities of the Leu-112 polypeptides suggested that the protein was cleaved incorrectly. This aberrant cleavage appears to hav e abolished the ability of the F protein to cause syncytium formation. The data indicate that the arginine 112 residue is critical for the c orrect proteolytic cleavage that is required for the membrane fusion a ctivity of the MV F protein.