Prostaphopain B Structure:  A Comparison of Proregion-Mediated and Staphostatin-Mediated Protease Inhibition^†,‡

Prostaphopain B is the precursor of staphopain B, a papain-type secreted cysteine protease from the pathogen Staphylococcus aureus. Here, we describe the 2.5 Å crystal structure of the proenzyme. Its 21 kDa proregion is organized around a central half-barrel or barrel-sandwich hybrid and occludes primed, but not nonprimed, sites in the active site cleft of the protease. The structure of the mature part of the protease is similar to previously reported staphopain structures, and no distortion of the catalytic residues is apparent at 2.5 Å resolution. A comparison of prostaphopain B with the staphopain B−staphostatin B complex shows that the proregion and the inhibitor interact with largely nonoverlapping parts of the protease surface. In a modeled complex of prostaphopain B with staphostatin B, clashes occur both inside and outside the active site cleft, but involve mostly poorly ordered regions of the protein that may be mobile.