Has an ATP-independent isopeptidase activity, cleaving at the C-terminus of the ubiquitin moiety. Acts late in the proteolytic pathway in conjunction with the 26S proteasome. Plays a role in avoiding DNA overreplication (By similarity). 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). Interacts with sfp47. Belongs to the peptidase C19 family.