STRUCTURE OF RAT PROCATHEPSIN-B - MODEL FOR INHIBITION OF CYSTEINE PROTEASE ACTIVITY BY THE PROREGION

Background: Cysteine proteases of the papain superfamily are synthesiz ed as inactive precursors with a 60-110 residue N-terminal prosegment, The propeptides are potent inhibitors of their parent proteases, Alth ough the proregion binding mode has been elucidated for all other prot ease classes, that of the cysteine proteases remained elusive. Results : We report the three-dimensional structure of rat procathepsin B, det ermined at 2.8 Angstrom resolution. The 62-residue proregion does not form a globular structure on its own, but folds along the surface of m ature cathepsin B. The N-terminal part of the proregion packs against a surface loop, with Trp24p (p indicating the proregion) playing a piv otal role in these interactions, Inhibition occurs by blocking access to the active site: part of the proregion enters the substrate-binding cleft in a similar manner to a natural substrate, but in a reverse or ientation. Conclusions: The structure of procathepsin B provides the f irst insight into the mode of interaction between a mature cysteine pr otease from the papain superfamily and its prosegment. Maturation resu lts in only one loop of cathepsin B changing conformation significantl y, replacing contacts lost by removal of the prosegment. Contrary to m any other proproteases, no rearrangement of the N terminus occurs foll owing activation. Binding of the prosegment involves interactions with regions of the enzyme remote from the substrate-binding cleft and sug gests a novel strategy for inhibitor design, The region of the prosegm ent where the activating cleavage occurs makes little contact with the enzyme, leading to speculation on the activation mechanism.