Heparan sulfate promotes autoactivation of procathepsin K by destabilizing the propeptide-catalytic domain interaction.

Procathepsin K (pro-CtsK) is the zymogen of cathepsin K (CtsK), a collagenase that is essential for bone resorption. Pro-CtsK is known to bind heparan sulfate (HS), but the biological significance of the interaction remains unclear. Here, we report that HS accelerates the autoprocessing of pro-CtsK in a manner dependent on both sulfation pattern and oligosaccharide length. We discovered a previously unknown electrostatic interaction between the propeptide and the catalytic domain, which stabilizes the conformation of the propeptide and prevents it from intermolecular proteolytic activation. HS accelerates autoprocessing of pro-CtsK by disrupting this critical electrostatic interaction. Mechanistically, HS competes with two glutamic acids in the propeptide for binding to three basic residues on the catalytic domain, thereby substantially altering the conformation of the propeptide and making it more labile for autoprocessing. We further discovered that HS is highly enriched in secretory lysosomes of osteoclasts and might be directly involved in autoactivation of CtsK.