Regulation of NF-B occurs through phosphorylation-dependent ubiquitination of IB␣, which is degradedNF-B is a transcription factor required for inducible expression of a number of proinflammatory mediators including cytokines, chemokines, and leukocyte adhesion molecules (6). In addition, NF-B regulates the expression of survival genes which prevent cell death in response to tumor necrosis factor alpha (TNF-␣) (7,37,59,62). NF-B is a member of the Rel family of proteins and is typically a heterodimer composed of p50 and p65 subunits. In quiescent cells, NF-B is retained in the cytosol bound to IB, a family of inhibitory proteins which mask the nuclear localization and DNA binding sequences on NF-B (5, 22). Stimulation of these cells with various cytokines, lipopolysaccharide, viruses, antigens, or oxidants triggers signaling events that ultimately lead to the phosphorylation and degradation of IB, allowing NF-B to translocate into the nucleus and activate target genes (3,21,38,54).Phosphorylation of Ser 32 and Ser 36 has been shown to target IB for ubiquitination and subsequent proteolysis by the ubiquitin-proteasome pathway (UPP) of protein degradation (2,8,45,49). The UPP is the principal pathway for intracellular protein turnover, including regulatory proteins (9). Protein substrates that enter the UPP are first marked by the covalent ligation of polyubiquitin chains mediated by a cascade of enzymes called E1 (ubiquitin activation enzyme), E2 (ubiquitinconjugating enzyme), and E3 (ubiquitin ligase) (9). In a reaction requiring ATP, ubiquitin is activated by E1 and charged onto an E2 through a thioester formed between the active-site cysteine residue in the E2 and the C-terminal glycine of ubiquitin. The E3 then directs the transfer of ubiquitin from the E2 onto lysine residues within specific substrate proteins, ultimately resulting in the formation of a ubiquitin-protein conjugate. Polyubiquitinated proteins are then recognized and degraded by the 26S proteasome complex to yield small peptides and monomeric ubiquitin.Recently, the receptor component of the IB E3 was identified as a member of the ␤TrCP (beta-transducin repeatcontaining protein) family of proteins called E3RS

Structural information from the adduct of ArsC with its substrate (arsenate) and with its product (arsenite) together with functional information from mutational and biochemical studies on ArsC suggest a plausible mechanism for the reaction. The exceptionally well-defined water structure indicates that this crystal system has precise long-range order within the crystal and that the upper limit for the number of bound waters in crystal structures is underestimated by the structures in the Protein Data Bank.

Temporal control of p27 Kip1 (p27) degradation imposes periodicity in its activity during cell cycle progression and its accumulation during cell cycle exit. Degradation of p27 is initiated by phosphorylation of p27 at Thr-187, which marks the protein for ubiquitination by SCF Skp2 and subsequent proteolysis by the 26S proteasome. Here we show that the p27 ubiquitination activity in cell extracts depends on the presence of the ubiquitin-like protein Nedd8 and enzymes that catalyze Nedd8 conjugation to proteins. Moreover, we show that reconstitution of the p27 ubiquitination activity of recombinant SCF Skp2 also requires Nedd8 conjugation pathway components. Inactivation of the Nedd8 conjugation pathway by a dominant negative mutant of the Nedd8-conjugating enzyme Nce1͞Ubc12 blocks the ubiquitination and degradation of p27 in cell extracts. Consistent with a role in cell-cycle progression, Nedd8 is expressed in proliferating cells and is itself down-regulated upon cellular differentiation. These results suggest that the Nedd8 conjugation pathway may regulate the turnover of p27 Kip1 , independently of p27 phosphorylation, and further establishes the identity of protein components involved in p27 ubiquitination. Finally, these findings provide a direct demonstration of a function for Nedd8 in a biological process.

Resistance to toxic oxyanions in Escherichia coli is conferred by the ars operon carried on plasmid R773. The gene products of this operon catalyze extrusion of antimonials and arsenicals from cells of E. coli, thus providing resistance to those toxic oxyanions. In addition, resistance to arsenate is conferred by the product of the arsC gene. In this report, purified ArsC protein was shown to catalyze reduction of arsenate to arsenite. The enzymatic activity of the ArsC protein required glutaredoxin as a source of reducing equivalents. Other reductants, including glutathione and thioredoxin, were not effective electron donors. A spectrophotometric assay was devised in which arsenate reduction was coupled to NADPH oxidation. The results obtained with the coupled assay corresponded to those found by direct reduction of radioactive arsenate to arsenite. The only substrate of the reaction was arsenate (Km = 8 mM); other oxyanions including phosphate, sulfate, and antimonate were not reduced. Phosphate and sulfate were weak inhibitors, while the product, arsenite, was a stronger inhibitor (Ki = 0.1 mM). Arsenate reductase activity exhibited a pH optimum of 6.3-6.8. These results indicate that the ArsC protein is a novel reductase, and elucidation of its enzymatic mechanism should be of interest.