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. May play a role in the error-free DNA repair pathway and contributes to the survival of cells after DNA damage. Heterodimer with UBC35 or UBC36. Expressed in roots, shoots, leaves, stems and flowers, but not in pollen. Detected in seedlings 6 hours post-germination, but not 2 days post-germination. Not induced by stresses. May be due to a competing donor splice site. Belongs to the ubiquitin-conjugating enzyme family.