Has an ATP-independent isopeptidase activity, cleaving at the C-terminus of the ubiquitin moiety in natural or engineered linear fusion proteins, irrespective of their size or the presence of an N-terminal extension to ubiquitin. Removes ubiquitin chains that initiate proteolysis of FZO1 and inhibit mitochondrial fusion. 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). Forms a ternary complex with RSP5 and RUP1. Interacts with FZO1. Present with 2420 molecules/cell in log phase SD medium. Belongs to the peptidase C19 family.