Has no ubiquitin ligase activity on its own. The heterodimer with UBC catalyzes the synthesis of non-canonical poly-ubiquitin chains that are linked through 'Lys-63'. This type of poly-ubiquitination does not lead to protein degradation by the proteasome. Mediates transcriptional activation of target genes. May play a role in the control of progress through the cell cycle and differentiation. Probably not involved in the error-free DNA repair. Heterodimer with UBC35 or UBC36. Expressed in roots, shoots, leaves, stems and flowers, but not in pollen. Detected in seedlings 6 hours and 2 days post-germination. Plants do not display MMS sensitivity during seed germination. Belongs to the ubiquitin-conjugating enzyme family.