Hydrolase that can remove conjugated ubiquitin from proteins in vitro and may therefore play an important regulatory role at the level of protein turnover by preventing degradation (PubMed:24659992). Cysteine protease with a preference for Met-1 and 'Lys-48' over 'Lys-63'-linked ubiquitin (UB) tetramers (e.g. Ub2, Ub3 and Ub4) as substrates (PubMed:24659992). Thiol-dependent hydrolysis of ester, thioester, amide, peptide and isopeptide bonds formed by the C-terminal Gly of ubiquitin (a 76-residue protein attached to proteins as an intracellular targeting signal). Cleavage activities for 'Lys-48'- and 'Lys-63'-linked ubiquitin (UB) tetramers is inhibited by UB aldehyde and N-ethylmaleimide but not by the metalloprotease inhibitors 1,10-phenanthroline and EDTA, and the serine protease inhibitor phenylmethylsulfonyl fluoride. Optimum pH is 7. Belongs to the peptidase C65 family.