Ubiquitin (Ub) and ubiquitin-like proteins (Ubls, e.g. SUMO, Nedd) are a group of approximately 15 proteins that have a molecular weight of around 8 kD. During the ubiquitination process, these are conjugated via activating (E1), conjugating (E2) and ligating (E3) enzymes to lysines of a target protein (1). Mammalian cells express over 600 potential ubiquitin ligases which exceeds that of the kinase superfamily of PTM proteins (2).nOne function of ubiquitination is to target proteins for proteosomal degradation. This role can range from a general housekeeping function that clears miss folded proteins from a cell to involvement in tightly regulated spatio-temporal cell signaling events (1). An emerging function of ubiquitination is its ability to activate proteins via the creation of unique protein:protein interactions (3). In common with many other PTMs, ubiquitination is reversible. Ubiquitin-specific proteases (USPs or DUBs) remove ubiquitins from target proteins (4). The reversible nature of ubiquitination further enhances the potential of this PTM to dynamically regulate protein function.
