Accepts the ubiquitin from the E1 complex and catalyzes its covalent attachment to other proteins. Mediates the selective degradation of short-lived and abnormal proteins. S-ubiquitinyl-[E1 ubiquitin-activating enzyme]-L-cysteine + [E2 ubiquitin-conjugating enzyme]-L-cysteine = [E1 ubiquitin-activating enzyme]-L-cysteine + S-ubiquitinyl-[E2 ubiquitin-conjugating enzyme]-L-cysteine. Protein modification; protein ubiquitination. Interacts with CIP8, CHIP, NLA and XERICO. Highest expression in young stems, old leaves. Lowest levels in floral buds, anthers and young leaves. Up-regulated during senescence, but not during the G0 to S phase transition. Not induced by heat shock or wounding. Belongs to the ubiquitin-conjugating enzyme family.