Abstract
Ubiquitin is a 76-amino acid polypeptide that is ubiquitously expressed and highly conserved in all eukaryotes. The best characterized function of ubiquitin is to mark a target protein for degradation by the proteasome (Hershko &Ciechan-over, 1998). This is achieved through covalent conjugation via an isopeptide bond between the C-terminus of ubiquitin and the ε-amino group of a lysine residue on the target protein. This conjugation, usually referred to as ubiquitination, is a three-step enzymatic process (Hershko &Ciechanover, 1992). In the first step, ubiquitin is activated by a ubiquitin-activating enzyme (El) in an ATP-dependent reaction to form a high-energy thioester bond between the C-terminal glycine of ubiquitin and the catalytic cysteine of El. In the second step, the activated ubiquitin is transferred to a ubiquitin-conjugating enzyme (E2). Finally, in the presence of a ubiquitin-protein ligase (E3), ubiquitin is conjugated to a lysine residue of the protein substrate through an isopeptide linkage. Ubiquitin itself has seven lysines. One of these lysines, typically Lys48, can be further conjugated by another ubiquitin to form a polyubiquitin chain through the processive action of the ubiquitination machinery (Chau et al., 1989). It is this polyubiquitin chain that is preferentially recognized by the proteasome, allowing the conjugated proteins to be degraded. Polyubiquitin chains linked through other lysines of ubiquitin also exist in cells, but the functions of these chains are largely unknown (Pickart, 2000).