Subcloning and Expression of Functional Human Cathepsin B and K in E. coli: Characterization and Inhibition by Flavonoids

1.1 Cathepsins Cathepsins, originally identified as lysosomal proteases, play a fundamental role in intracellular protein turnover in lysosomes. However, several cathepsins and variants of cathepsins can also be found on the cell membrane, in the cytosol, nucleus, mitochondria, and extracellular space. These cathepsins are involved in a variety of important physiological and pathological processes [reviewed in: (Brix et al., 2008; Frlan and Gobec, 2006; Lutgens et al., 2007; Mohamed and Sloane, 2006; Nomura and Katunuma, 2005; Obermajer et al., 2008; Reiser et al., 2010; Stoka et al., 2005; Turk et al., 2001; Vasiljeva et al., 2007; Victor and Sloane, 2007)]. Cathepsins are classified mechanistically into groups which include serine (cathepsins A and G), aspartic (cathepsins D and E), and cysteine cathepsins (cathepsins B, C, F, H, L, K, O, S, V, W, and X). This classification is based on the nucleophilic residues present on their active sites responsible for proteolytic cleavage (Rawlings et al., 2006; Turk et al., 2001). Cathepsins are synthesized as zymogens composed of a signal peptide, a propeptide, and mature protein of distinct length and substrate specificity for individual cathepsins (Rawlings et al., 2006). The signal peptide is cleaved in the Endoplasmic Reticulum and the pro-protein is activated by proteolytic removal of the N-terminal pro-peptide either by autocatalysis in acidic environments, or by other proteases. The pro-peptide region of the cathepsin plays multiple roles. It can act as an inhibitor to block access to the active site that regulates cathepsin activity. In addition the propeptide can act as an intramolecular chaperone that assists in protein folding, or as a trafficking signal that targets the protein to its destination (Turk et al., 2002). Cathepsins exhibit a broad range of functions and tissue expression (Brix et al., 2008; Turk et al., 2001). Some of the cathepsins are ubiquitously expressed and others are tissue or cell-type specific. Cathepsins have been shown to be involved in the process of tumor invasion and metastasis (Bialas and Kafarski, 2009; Lindeman et al., 2004; Nomura and Katunuma, 2005; Obermajer