Accepts the ubiquitin from the E1 complex and catalyzes its covalent attachment to other 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. Expressed in antipodal cells of developing seeds and in the vascular junction between the cotyledons or leaves and the hypocotyl. Not expressed in pollen. Not induced by heat shock. Belongs to the ubiquitin-conjugating enzyme family.