Zinc metalloprotease that specifically cleaves 'Lys-63'-linked polyubiquitin chains. Does not cleave 'Lys-48'-linked polyubiquitin chains (By similarity). Functions at the endosome and is able to oppose the ubiquitin-dependent sorting of receptors to lysosomes (By similarity). Binds 2 Zn(2+) ions per subunit. The JAMM motif is essential for the protease activity. Belongs to the peptidase M67C family.