Cracking the code of cathepsin S: Structural determinants of specificity switching

Abstract

Cathepsin S is a cysteine protease in the papain family that digests antigens as part of the adaptive immune response, activates receptors, and is associated with extracellular matrix degradation in autoimmune diseases and cancer. A comprehensive review of the literature revealed potential pH- and redox-dependent specificity switches in the proteolytic specificity of cathepsin S. These were investigated through the digestion of peptides across a variety of pH and redox conditions. These experiments confirmed both pH- and redox-dependent patterns of proteolytic specificity, with narrowed specificity in alkaline and oxidizing conditions. An analysis of publicly available structures of cathepsin S identified a lysine residue which descends into the S3 pocket of the active site above pH 7.0, acting as a pH-dependent gate. Energy minimization of crystal structures show disorder in the loops which make up the active site of the protein, which increases in disorder when the disulfide bonds on the surface of cathepsin S are reduced. This is explored as a potential mechanism for the redox-dependent specificity identified in the digest experiments. These specificity switches may contribute to pathological structural damage attributed to cathepsin S, as pH and redox dysregulation are features of several cathepsin S-associated diseases.