Characterization of cathepsin S exosites that govern its elastolytic activity.

We have previously determined that the elastolytic activities of cathepsins (Cat) K and V require two exosites sharing the same structural localization on both enzymes. The structural features involved in the elastolytic activity of CatS have not yet been identified. We first mutated the analogous CatK and V putative exosites of CatS into the elastolytically inactive CatL counterparts. The modification of the exosite 1 did not affect the elastase activity of CatS whilst mutation of the Y118 of exosite 2 decreased the cleavage of elastin by ∼70% without affecting the degradation of other macromolecular substrates (gelatin, thyroglobulin). T06, an ectosteric inhibitor that disrupt the elastolytic activity of CatK, blocked ∼80% of the elastolytic activity of CatS without blocking the cleavage of gelatin and thyroglobulin. Docking studies showed that T06 preferentially interacts with a binding site located on the Right domain of the enzyme, outside of the active site. The structural examination of this binding site showed that the loop spanning the L174N175G176K177 residues of CatS is considerably different from that of CatL. Mutation of this loop into the CatL-like equivalent decreased elastin degradation by ∼70% and adding the Y118 mutation brought down the loss of elastolysis to ∼80%. In addition, the Y118 mutation selectively reduced the cleavage of the basement membrane component laminin by ∼50%. In summary, our data show that the degradation of elastin by CatS requires two exosites where one of them is distinct from those of CatK and V whilst the cleavage of laminin requires only one exosite.