Recognizes and hydrolyzes the peptide bond at the C-terminal Gly of ubiquitin. Involved in the processing of poly-ubiquitin precursors as well as that of ubiquitinated proteins (Probable). Involved in the regulation of organ size. Acts as positive regulator of cell proliferation. Possesses deubiquitinating enzyme activity in vitro. The enzyme activity of UBP15 is required for its function in regulation of cell proliferation (PubMed:18485060). Functions antagonistically in a common pathway with DA1 to regulate seed size. Acts maternally to regulate seed size by promoting cell proliferation in the integuments of ovules and developing seeds. Functions independently of DA2 and BB (PubMed:24585836). 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 DA1. A number of isoforms are produced. According to EST sequences. Highly expressed in rosette leaves and inflorescence. Expressed at low levels in cotyledons, stems, cauline leaves and siliques. Narrow, serrated and flat rosette leaves, early flowering, weak apical dominance and reduced fertility, partially due to defect in cell proliferation (PubMed:18485060). Reduced seed weight (PubMed:24585836). Belongs to the peptidase C19 family.