Comparison of the positions and efficiency of cleavage activation of fusion protein precursors of virulent and avirulent strains of Newcastle disease virus: insights into the specificities of activating proteases.

The F1- and F2-polypeptide components of in ovo activated fusion proteins of one virulent (AV or Australia-Victoria) strain, one low-virulence (EG or Eaves-Grimes) strain, and two avirulent (V4 or Queensland and WA2116) strains of Newcastle disease virus (NDV) were isolated and subjected to structural analysis. This included complementary application of amino acid analysis, fast atom bombardment-mass spectrometry, and N-terminal sequence analysis to fragments isolated from AspN protease digests of the F2-polypeptides using HPLC. As a result, the complete sequences of the F2-polypeptides were determined, including documentation of glycosylation of asparagine 54. The sequence of the cleavage-activation site of the WA2116 F0-protein was found to be distinctly different from this site in any other NDV F0-protein. Cleavage activation at the C termini of the F2-polypeptide regions was found to have occurred to approximately equivalent extents at arginines 82 and 85 of the AV and EG strains, but was restricted largely to arginine 85 of the V4 strain and completely to arginine 85 of the WA2116 strain. In each case cleavage activation was apparently succeeded by trimming of the basic residues from the newly formed C termini. Immunochemical analysis with antipeptide antisera showed that the extent of cleavage was influenced by amino acids adjacent to these arginines. These data provide insight into the substrate specificities of the enzymes involved in cleavage activation of the fusion protein precursors.