There’s the Rub: a novel ubiquitin-like modification linked to cell cycle regulation (original) (raw)

  1. Mark Hochstrasser
  2. Department of Biochemistry and Molecular Biology, The University of Chicago, Chicago, Illinois 60637 USA

Covalent attachment of the polypeptide ubiquitin to intracellular proteins is achieved through an intricate and highly conserved enzymatic pathway that is beginning to be understood in some detail (for review, see Wilkinson 1995; Hochstrasser 1996; Pickart 1997). Ubiquitin is joined reversibly to proteins by an isopeptide linkage of the carboxy-terminal carboxyl group of ubiquitin to lysine side chains of the acceptor proteins (Fig. ). In the majority of cases examined, the modified protein, particularly when it is attached to multiple ubiquitin molecules, is targeted to a large, abundant intracellular protease called the proteasome, which degrades the substrate into small peptides but allows recycling of the ubiquitin moieties.

The ubiquitin–protein conjugation pathway. The ligation mechanisms of Rub1/NEDD8 and other Ubls are likely to be very similar. In the first step of the ubiquitin pathway, the carboxy-terminal carboxyl group of ubiquitin (Ub) is activated by E1 (Uba1 in yeast) by formation of a mixed anhydride with AMP. The ubiquitin–adenylate is then transferred to a cysteine residue of E1, resulting in a ubiquitin–E1 thiolester. (A second, tightly bound ubiquitin-adenylate is formed after this step but is not shown here.) Thereafter, the ubiquitin is transferred to a reactive cysteine of a second enzyme, E2 (Ubc). Although E2 enzymes can sometimes directly catalyze isopeptide bond formation between ubiquitin and substrate protein in vitro, this reaction generally requires the aid of at least one additional factor, called E3 or ubiquitin–protein ligase. Currently, there is evidence for two different E3-based mechanisms for substrate ubiquitination. For at least one structural subclass of E3s (Scheffner et al. 1995), ubiquitin is transferred from the E2 to a thiol group on the E3 protein and only then is conjugated to a lysine side chain of the substrate (pathway I). For other E3s, the E3 appears to function more as …