Active Cathepsin D (CTSD)

Figure. The binding activity of CTSD with HSP90b1.
Cathepsin D (CSTD) is an aspartic protease that depends critically on protonation of its active site Asp residue. Along with Asp-protonation, lower pH also leads to conformational switch in cathepsin-D: the N-terminal segment of the protease moves out of the active site as pH drops. Similar to other aspartic protainases, cathepsin D accommodates up to 8 amino acid residues in the binding cleft of the active site. The main physiological functions of cathepsin D consist of metabolic degradation of intracellular proteins, activation and degradation of polypeptide hormones and growth factors, activation of enzymatic precursors, processing of enzyme activators and inhibitors, brain antigen processing and regulation of programmed cell death. Besides, Heat Shock Protein 90kDa Beta 1 (HSP90b1) has been identified as an interactor of CSTD, thus a binding ELISA assay was conducted to detect the interaction of recombinant human CSTD and recombinant human HSP90b1. Briefly, CSTD were diluted serially in PBS, with 0.01% BSA (pH 7.4). Duplicate samples of 100μL were then transferred to HSP90b1-coated microtiter wells and incubated for 2h at 37℃. Wells were washed with PBST and incubated for 1h with anti-CSTD pAb, then aspirated and washed 3 times. After incubation with HRP labelled secondary antibody, wells were aspirated and washed 3 times. With the addition of substrate solution, wells were incubated 15-25 minutes at 37℃. Finally, add 50µL stop solution to the wells and read at 450nm immediately. The binding activity of CSTD and HSP90b1 was shown in Figure 1, and this effect was in a dose dependent manner.