chondroitin sulfate promotes activation of cathepsin k cathepsin k catk a major lysosomal collagenase produced by osteoclasts plays an important role in bone resorption evidence exists that the collagenase activity of catk is promoted by chondroitin sulfate cs a sulfated glycosaminoglycan this study examines the role of cs in facilitating catk activation we have demonstrated that chondroitin 4sulfate c4s promotes autoprocessing of the prodomain of catk at ph 5 leading to a fully matured enzyme with collagenase and peptidase activities we present evidence to demonstrate this autoactivation process is a transactivation event that is efficiently inhibited by both the covalent cysteine protease inhibitor e64 and the reversible selective catk inhibitor l006235 during bone resorption catk and c4s are colocalized at the ruffled border between osteoclast bone interface supporting the proposal that catk activation is accomplished through the combined action of the acidic environment together with the presence of a high concentration of c4s formation of a multimeric complex between c4s and procatk has been speculated to accelerate catk autoactivation and promote efficient collagen degradation together these results demonstrate that cs plays an important role in contributing to the enhanced efficiency of catk collagenase activity in vivo