Clusterin is a specific stabilizer and liberator of extracellular cathepsin K

The cysteine peptidase cathepsin K is a major player in extracellular proteolysis. Here we describe the identification of the multifunctional extracellular chaperone clusterin as a cathepsin K-binding protein. Clusterin increases the stability of cathepsin K in dilute solution and in the presence of high protein concentration. It does not alter the activity of the enzyme but acts as a liberator by preventing substrate inhibition. Kinetic measurements show that clusterin binds cathepsin K with high affinity (Kd = 0.5–0.6 nM). Altogether these results provide novel insights into the mechanisms involved in the fine-tuning of cysteine cathepsin activity in the extracellular space.

► We have identified clusterin as a cathepsin K-binding protein.
► Clusterin stabilizes cathepsin K at physiological plasma pH.
► Clusterin does not directly affect cathepsin K activity.
► Clusterin affects the conformational flexibility of cathepsin K.