Ubiquitin-protein hydrolase involved in the release of ubiquitin attached via both peptide and isopeptide linkages. Able to cleave 'Lys-48'-linked polyubiquitin chains. Involved in the direct or indirect regulation of AUX/IAA proteins stability (PubMed:17559514). Acts as a linker between the TREX-2 complex and 26S proteasome (PubMed:22951400). 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). Heterodimer (PubMed:22951400). Interacts with UCH1, EER5, DSS1(V), DSS1(I), RPN3A, RPN3B, RPN12A, RPN12B, SAC3B, CML20 and NUP1 (PubMed:22951400). Expressed in leaves, stems, sepals, stamens, petals, roots, hypocotyls and cotyledons. No visible phenotype. Uch1 and uch2 double mutants are less fertile, accumulated less chlorophyll and have slightly fewer and shorter cauline branches. UCH1 and UCH2 are not integral polypeptides of the 26S proteasome, unlike their S.pombe and animal orthologs (PubMed:17559514). However, they interact with the 26S proteasome lid complex as well as to the TREX-2 complex (PubMed:22951400). Belongs to the peptidase C12 family.