Protein turnover refers to the continual process of protein synthesis and degradation. Ubiquitin, a small protein present in all eukaryotic cells, tags proteins for degradation; the proteins are then transported to the proteasome for degradation. Three enzymes participate in the attachment of ubiquitin to each protein: ubiquitin-activating enzyme (E1), ubiquitin-conjugating enzyme (E2) and ubiquitin-protein ligase (E3).