UBE2E3 also known as Ubiquitin-conjugating enzyme E2 E3 is a member of the E2 ubiquitin-conjugating enzyme family. This enzyme is required for the destruction of mitotic cyclins and for cell cycle progression. The ubiquitination process covalently attaches ubiquitin, a short protein of 76 amino acids, to a lysine residue on the target protein. Once a protein has been tagged with one ubiquitin molecule, additional rounds of ubiquitination form a polyubiquitin chain that is recognized by the proteasome's 19S regulatory particle, triggering the ATP-dependent unfolding of the target protein that allows passage into the proteasome's 20S core particle, where proteases degrade the target into short peptide fragments for recycling by the cell. Recombinant human UBE2E3, fused to His-tag at N-terminus, was expressed in E.coli and purified by using conventional chromatography techniques.