Ubiquitin-protein hydrolase involved both in the processing of ubiquitin precursors and of ubiquitinated proteins (Probable). This enzyme is a thiol protease that recognizes and hydrolyzes a peptide bond at the C-terminal glycine of ubiquitin (PubMed:12913066). Also binds to free monoubiquitin and may prevent its degradation in lysosomes (PubMed:12913066). The homodimer may have ATP-independent ubiquitin ligase activity (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). Monomer. Homodimer. Interacts with COPS5 and SNCA (By similarity). Expressed in brain, where it is found in neurons but not in oligodendrocytes or astrocytes. Found in the ganglion cell layer and the inner nuclear layer of the retina (at protein level). Expressed in brain and testis. In the brain, expression is at its lowest in replaceable neurons of hippocampus and olfactory bulb. Highly expressed in senescent pituitary. O-glycosylated. Mice show sensory ataxia at an early stage, followed by motor ataxia at a later stage. They have reduced levels of monoubiquitin in the nervous system, and increased resistance to retinal ischemia. In contrast to UCHL3, does not hydrolyze a peptide bond at the C-terminal glycine of NEDD8. Belongs to the peptidase C12 family. The homodimer may have ATP-independent ubiquitin ligase activity. However, in another study, UCHL1 was shown to lack ubiquitin ligase activity.