Required for maintaining multiple types of adult stem cells, including male and female germline, epithelial follicle cell and intestinal stem cells. May function as a transcriptional repressor by continually deubiquiting histone H2B at the promoters of genes critical for cellular differentiation, thereby preventing histone H3 'Lys-4' trimethylation (H3K4). Controls selective autophagy activation by ubiquitinated proteins. 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 atms/PAF1, but not with CycT. Belongs to the peptidase C19 family.