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.
The E3 molecule is responsible for binding the target protein substrate and transferring the ubiquitin from the E2 cysteine to a lysine residue on the target protein.
[1] A particular cell usually contains only a few types of E1 molecule, a greater diversity of E2s, and a very large variety of E3s.
Since the ubiquitylation system is shared across all organisms, studies can use modified E2 proteins in order to understand the overall system for how all organisms process proteins.