Intracellular proteolytic processing of fusion glycoprotein precursors (F) of paramyxoviruses, i.e. a virulent strain of Newcastle disease virus (NDV), parainfluenza virus type 3 (PIV3) and simian virus 5 (SV5), was examined in NALM6 and BSC40 cells and compared with that in LLCMK cells to investigate the distribution of the virus-activating protease(s) among the cells and its substrate specificity. BSC40 cells lack a processing endoprotease of the neuropeptide precursor, pro-opiomelanocortin (POMC), which possesses multiple cleavage sites at pairs of basic residues, Lys-Arg and Arg-Arg, a motif similar to that found in the cleavage site of the F proteins. In NALM6 cells, only small amounts of the F protein of virulent NDV was cleaved whereas those of PIV3 and SV5 were efficiently cleaved. In BSC40 cells the F proteins of these three viruses were cleaved normally as well as in LLCMK cells. The processing inhibitors monensin, chloroquine and A23187 suppressed the F cleavage in the three cell types. These results indicate that both NALM6 and BSC40 cells possess virus-activating proteases similar to that of LLCMK cells, but suggest that the enzyme of NALM6 may be slightly different in its substrate specificity from those of BSC40 and LLCMK. The results also suggest that the virus-activating proteases are different in their distribution and substrate specificity from the processing enzyme of POMC.


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