Single amino acid change at the cleavage site of the fusion protein is responsible for both enhanced chymotrypsin sensitivity and trypsin resistance of a Sendai virus mutant, TR-5.
A trypsin-resistant mutant of Sendai virus, TR-5, which was obtained by passaging the wild-type (wt) virus in the presence of chymotrypsin followed by plaque purification, also had an enhanced susceptibility to activation by chymotrypsin. A trypsin-sensitive revertant, TSrev-58, derived from TR-5 had a decreased susceptibility to chymotrypsin and was susceptible to trypsin to the same degree as the wt virus. Based on the facts that TR-5 had an amino acid change (Arg----Ile) at residue 116 of the fusion (F) protein, which is the cleavage site for trypsin in the wt virus F protein, and that this change had reverted in TSrev-58, it was concluded that the above amino acid change was responsible for both the enhanced chymotrypsin sensitivity and the trypsin resistance of TR-5. In this paper, the cleavage site of the F protein of TR-5 by chymotrypsin was determined in comparison with those of the wt virus and TSrev-58. The cleavage sites were not different among these viruses, being located between residues 114 (Gln) and 115 (Ser), the two residues at the N-terminal side of the trypsin cleavage site. A possible reason for the increased susceptibility of TR-5 to chymotrypsin is discussed.