In vitro validation of Cathepsin S docking models

Abstract

Abstract In dendritic cells, the dominant lysosomal protease involved in Ii chain degradation and antigen digestion is Cathepsin S, a cysteine protease in the papain family. Extracellularly, Cathepsin S also contributes to pathological processes in cardiovascular disease, cancer, and autoimmune arthritis: unlike most lysosomal proteases, it retains its activity at serum pH. In computational molecular docking studies comparing peptides from the literature, we found that Cathepsin S shows a pH-dependent specificity switch, with predicted dissociation constants for specific peptide sequences differing at lysosomal and serum pH. The aim of this study is to further characterize this specificity switch and to compare docking results to kinetic parameters of Cathepsin S digestion in vitro. We identified peptide sequences of interest from docking, including sequences from the Ii chain and elastin, and digested these peptides (LifeTein) using human Cathepsin S (Calbiochem), collecting samples for LC-MS analysis over 16 hour reaction times. We report the kinetic parameters of Cathepsin S catalysis at both lysosomal and serum pH. Digestion results are compared to docking models and the models most predictive of catalytic activity are identified. Further investigation is needed to determine whether pH-dependent catalytic activity and selectivity of Cathepsin S will enable the design of protein-based therapies and inhibitors optimized for their targeted microenvironments. This work was partially supported by the NIDA/NIH award UG3DA048775, the USDA National Institute of Food and Agriculture, AFRI project #2021-0858, and the department of Biological Systems Engineering at Virginia Tech.