STRUCTURE OF HUMAN PROCATHEPSIN-L REVEALS THE MOLECULAR-BASIS OF INHIBITION BY THE PROSEGMENT

Cathepsin L is a member of the papain superfamily of cysteine protease s and, like many other proteases, it is synthesized as an inactive pro enzyme, Its prosegment shows little homology to that of procathepsin B , whose structure, the first for a cysteine protease proenzyme, has be en determined recently. We report here the 3-D structure of a mutant o f human procathepsin L determined at 2.2 Angstrom resolution, describe the mode of binding employed by the prosegment and discuss the molecu lar basis for other possible roles of the prosegment. The N-terminal p art of the prosegment is globular and contains three alpha-helices wit h a small hydrophobic core built around aromatic side chains, This dom ain packs against a loop on the enzyme's surface, with the aromatic si de chain from the prosegment being located in the center of this loop and providing a large contact area, The C-terminal portion of the pros egment assumes an extended conformation and follows along the substrat e binding cleft toward the N-terminus of the mature enzyme, The direct ion of the prosegment in the substrate binding cleft is opposite to th at of substrates, The previously described role of the prosegment in t he interactions with membranes is supported by the structure of its N- terminal domain, The fold of the prosegment and the mechanism by which it inhibits the enzymatic activity of procathepsin L is similar to th at observed in procathepsin B despite differences in length and sequen ce, suggesting that this mode of inhibition is common to all enzymes f rom the papain superfamily.