Hydrolase that can remove 'Lys-48'-linked conjugated ubiquitin from proteins . Binds to polyubiquitin chains of different linkage types, including 'Lys-6', 'Lys-11', 'Lys-29', 'Lys-33', 'Lys-48' and 'Lys-63' . May play a regulatory role at the level of protein turnover .