THE BACULOVIRUS CYSTEINE PROTEASE HAS A CATHEPSIN B-LIKE S2-SUBSITE SPECIFICITY

Autographa californica nuclear polyhedrosis virus (AcNPV) encodes a functional cysteine protease of the papain family which is expressed after infection in Spodoptera frugiperda Sf9 cells. The protease displays an inhibition profile typical for cysteine proteases and is highly active against synthetic peptide substrates. The pH optimum of the bell-shaped pH-activity curve is between 5.0 and 5.5. The best substrate tested is Z-Arg-Arg-MCA which is specific for cathepsin B. The specificity constant (K-cat/K-m) of AcNPV protease for this substrate is approximately two times higher than for human cathepsin B. In contrast to human cathepsins, AcNPV protease does not exhibit a discriminating specificity towards neutral hydrophobic residues in the P-2 position. These substrates are hydrolysed at a ten-fold lower rate than the P-2 arginine containing substrate. The pH activity profile against the Z-Arg-Arg-MCA substrate reveals a pK of 5.35 which can be assigned to a glutamate residue in the S-2 subsite pocket. Like in cathepsin B, this residue facilitates the binding of positively charged P-2 residues in the primary binding pocket. In this respect, the AcNPV protease resembles cathepsin B more than cathepsins L and S.