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A Ciechanover

Publications and source records attributed to A Ciechanover.

122 records · Page 7Linked to original sources
PubMed1982

Immunochemical analysis of the turnover of ubiquitin-protein conjugates in intact cells. Relationship to the breakdown of abnormal proteins.

Previous studies in a cell-free proteolytic system from reticulocytes indicated that the conjugation of ubiquitin with proteins plays a role in protein breakdown. To examine some of the physiological functions of the ubiquitin conjugation system, and immunochemical method was developed for the isolation of ubiquitin-protein conjugates from intact cells. A specific antiserum was raised against ubiquitin and purified by affinity chromatography on ubiquitin-Sepharose. When cells are labeled with tryptophan (which is missing from ubiquitin), labeled immunoreactive material isolated by the antibody is derived from the protein moiety of ubiquitin-protein conjugates. There is a marked increase in the labeling of ubiquitin-protein conjugates during the formation of abnormal proteins in reticulocytes (induced by the incorporation of amino acid analogs), suggesting that proteins with abnormal structure are more readily conjugated to ubiquitin than most normal proteins. Essentially similar, although less marked, effects of amino acid analogs were observed in Ehrlich ascites cells. When further protein synthesis was blocked with cycloheximide, ubiquitin conjugates decayed more extensively than the corresponding average labeled cellular proteins. This is consistent with the interpretation that a considerable part of ubiquitin conjugates is derived from a pool of rapidly degradable proteins.

Animals↗
PubMed1982

"Covalent affinity" purification of ubiquitin-activating enzyme.

We have previously described an enzyme that activates ubiquitin, the heat-stable polypeptide of the ATP-dependent proteolytic system from reticulocytes (Ciechanover, A., Heller, H., Katz-Etzion, R., and Hershko, A. (1981) Proc. Natl. Acad. Sci. U. S. A. 78, 761-765). It carries out ubiquitin-dependent PPi-ATP and AMP-ATP exchange reactions and binds to the activated polypeptide by a thiolester linkage. We describe here a procedure for the purification of this enzyme by its binding to ubiquitin-Sepharose. Binding of the enzyme to the affinity column requires ATP and Mg2+, and bound enzyme cannot be displaced by high salt but can be eluted by raising the pH, by increased concentrations of a thiol compound, or by the joint supplementation of AMP and pyrophosphate. Another form of the enzyme that cannot carry out AMP-ATP exchange (but catalyzes ubiquitin-dependent PPi-ATP exchange) does not bind to the affinity column. These data indicate that a covalent, possibly thiolester intermediate, is formed between the activating enzyme and Sepharose-bound ubiquitin. It is suggested designating this procedure of enzyme isolation "covalent affinity" chromatography. The purified enzyme has an apparent Mr = 210,000 and appears to be composed of two subunits of Mr = 105, 000. ATP-dependent binding of ubiquitin to the purified enzyme and to its subunit is demonstrated.

Animals↗
PubMed1981

Identification of the active amino acid residue of the polypeptide of ATP-dependent protein breakdown.

The heat-stable polypeptide of the ATP-dependent proteolytic system was previously found to form covalent conjugates with proteins and to be activated by ATP in an adenylylation mechanism. To identify the functional amino acid of the polypeptide, the activated residue was specifically labeled by the reductive cleavage of the intermediate with [3H]borohydride. Following acid hydrolysis, the reduced labeled derivative was found to be completely oxidizable by periodate with formation of [3H]formaldehyde, and was identified as ethanolamine by thin layer chromatography, electrophoresis, and amino acid analyzer chromatography. These results indicate that the activated amino acid residue of the polypeptide is COOH-terminal glycine.

Adenosine Triphosphate↗
PubMed1981

Activation of the heat-stable polypeptide of the ATP-dependent proteolytic system.

It had been shown previously that the heat-stable polypeptide of the ATP-dependent proteolytic system of reticulocytes, designated APF-1, forms covalent conjugates with protein substrates in an ATP-requiring process. We now describe an enzyme that carries out the activation by ATP of the polypeptide with pyrophosphate displacement. The formation of AMP-polypeptide and transfer of the polypeptide to a secondary acceptor are suggested by an APF-1 requirement for ATP-PPi and ATP-AMP exchange reactions, respectively. With radiolabeled polypeptide, an ATP-dependent labeling of the enzyme was shown to be by a linkage that is acid stable but is labile to treatment with mild alkali, hydroxylamine, borohydride, or mercuric salts. It therefore appears that the AMP-polypeptide undergoes attack by an -SH group of the enzyme to form a thiolester.

Adenosine Triphosphate↗
PubMed1980

Characterization of the heat-stable polypeptide of the ATP-dependent proteolytic system from reticulocytes.

A heat-stable polypeptide from rabbit reticulocytes has been previously shown to be required for ATP-dependent protein breakdown (Ciechanover, A., Hod, Y., and Hershko, A. (1978) Biochem. Biophys. Res Commun. 81, 1100-1105) and to form covalent conjugates with proteins in an ATP-requiring reaction (Ciechanover, A., Heller, H., Elias, S., Haas, A.L., and Hershko, A. (1980) Proc. Natl. Acad. Sci. U.S.A. 77, 1365-1368). We now describe its purification, characterization, and tissue distribution. Its presence in erythrocytes at high level makes this a possible preferred source in the future.

Amino Acids↗
PubMed1980

ATP-dependent conjugation of reticulocyte proteins with the polypeptide required for protein degradation.

The heat-stable polypeptide (APF-1) required for ATP-dependent proteolysis in reticulocytes enters into high molecular weight conjugates upon incubation with the fraction of reticulocytes that is retained by DEAE-cellulose. Conjugate formation requires ATP and Mg2+ and its inhibited by N-ethylmaleimide. UTP and GTP are inactive. These properties are identical to those of ATP-dependent protein breakdown in the same system, suggesting that the conjugates are intermediates in this process. The APF-1 conjugates are stable in sodium dodecyl sulfate/polyacrylamide gel electrophoresis and Sephadex G-75 isolation and are resistant to mild acid, alkali, heat denaturation, and reduction; the conjugates are therefore covalent.

Adenosine Triphosphate↗
PubMed1980

Proposed role of ATP in protein breakdown: conjugation of protein with multiple chains of the polypeptide of ATP-dependent proteolysis.

The heat-stable polypeptide ATP-dependent proteolysis factor 1 (APF-1) of the reticulocyte proteolytic system forms covalent compounds with proteins in an ATP-requiring reaction. APF-1 and lysozyme, a good substrate for ATP-dependent proteolysis, form multiple conjugates, as was shown by comigration of label from each upon gel electrophoresis. Multiple bands were also seen with other substrates of the ATP-dependent proteolytic system, such as globin or alpha-lactalbumin. Analysis of the ratio of APF-1 to lysozyme radioactivities and of the molecular weights of the bands indicated that they consist of increasing numbers of the APF-1 polypeptide bound to one molecule of lysozyme. The covalent linkage is probably of an isopeptide nature, because it is stable to hydroxylamine and alkali, and polylysine is able to give conjugates of APF-1. Removal of ATP after formation of the 125I-labeled APF-1 conjugates with endogenous proteins caused the regeneration of APF-1, indicating presence of an amidase. This reaction is thought to compete with proteases that may act on APF-1-protein conjugates, especially those containing several APF-1 ligands. A sequence of reactions in which the linkage of APF-1 to the substrate is followed by the proteolytic breakdown of the substrate is proposed to explain the role of ATP.

Adenosine Triphosphate↗
PubMed1979

Resolution of the ATP-dependent proteolytic system from reticulocytes: a component that interacts with ATP.

The ATP-dependent proteolytic cell-free system from reticulocytes has been resolved into three components, each of which is absolutely required for acid solubilization of 125I-labeled bovine serum albumin radioactivity. In addition to the previously reported heat-stable polypeptide [Ciechanover, A., Hod, Y. & Hershko, A. (1978) Biochem. Biophys. Res Commun. 81, 1100-1105], we now describe a protein of high molecular weight (approximately 450,000) that is labile at 42 degrees C. The extremely heat-labile factors is remarkably stabilized by ATP. GTP and CTP, which do not stimulate protolysis, do not stabilize this factor. Adenylate nucleotides such as ADP or the nonhydrolyzable beta,gamma imido or methylene analogues of ATP cause stabilization although they do not activate proteolysis. A third protein component of the protease system, stable at 42 degrees C, has been separated from the heat-labile species by salt precipitation. All three components are required with ATP for proteolytic activity, but thus far only the heat-labile factor has been shown to interact directly with ATP.

Adenosine Triphosphate↗
PubMedDate not supplied

Mechanisms of ubiquitin-mediated, limited processing of the NF-kappaB1 precursor protein p105.

In most cases, target proteins of the ubiquitin system are completely degraded. In several exceptions, such as the first step in the activation of the transcriptional regulator NF-kappaB, the substrate, the precursor protein p105, is processed in a limited manner to yield the active subunit p50. p50 is derived from the N-terminal domain of p105, whereas the C-terminal domain is degraded. The mechanisms involved in this unique process have remained elusive. We have shown that a Gly-rich region (GRR) at the C-terminal domain of p50 is one important processing signal and that it interferes with processing of the ubiquitinated precursor by the 26S proteasome. Also, amino acid residues 441-454 are important for processing under non-stimulated conditions. Lys 441 and 442 serve as ubiquitination targets, whereas residues 446-454 may serve as a ligase recognition motif. Following IkappaB kinase (IKK)-mediated phosphorylation, the C-terminal domain of p105, residues 918-934, recruits the SCF(beta-TrCP) ubiquitin ligase, and ubiquitination by this complex leads to accelerated processing. The two sites appear to be recognized under different physiological conditions by two different ligases, targeting two distinct recognition motifs. We have shown that ubiquitin conjugation and processing of a series of precursors of p105 that lack the C-terminal IKK phosphorylation/TrCP binding domain, is progressively inhibited with increasing number of ankyrin repeats. Inhibition is due to docking of active NF-kappaB subunits to the ankyrin repeat domain in the C-terminal half of p105 (IkappaBgamma). Inhibition is alleviated by phosphorylation of the C-terminal domain that leads to ubiquitin-mediated degradation of the ankyrin repeat domain and release of the anchored subunits. We propose a model that may explain the requirement for two sites: a) a basal site that may be involved in co-translational processing prior to the synthesis of the ankyrin repeat domain; and b) a signal-induced site that is involved in processing/degradation of the complete molecule following cell activation, with rapid release of stored, transcriptionally active subunits.

Amino Acid Motifs↗
PubMedDate not supplied

Role of arginine-tRNA in protein degradation by the ubiquitin pathway.

Degradation of intracellular proteins through the ubiquitin and ATP-dependent proteolysis pathway involves several steps. Initially, ubiquitin is covalently linked to the proteolytic substrate in an ATP-requiring reaction. Proteins marked by ubiquitin may then be selectively lysed in a reaction that also requires ATP (for reviews see refs 1-3). A major question concerns the structural features of a protein that make it a specific substrate for ubiquitin-mediated degradation. It was shown that a free alpha-NH2 group is one important feature of the protein structure recognized by the ubiquitin ligation system, and that the half-life in vivo of a protein with an exposed amino terminus depends on its amino terminal residue. We have previously demonstrated that transfer RNA (tRNA) is essential for conjugation of ubiquitin and for the subsequent degradation of proteins with acidic amino termini (aspartate or glutamate). We now show that tRNA is required for post-translational conjugation of arginine to acidic amino termini of proteins, a modification that is essential for their degradation by the ubiquitin pathway.

Adenosine Triphosphate↗
PubMedDate not supplied

The ubiquitin-mediated system for intracellular protein degradation.

Degradation of proteins by the ubiquitin system involves several discrete steps. Initially, multiple molecules of ubiquitin are covalently conjugated to the target substrate in an energy-requiring reaction. The protein thus marked is degraded by a specific ATP-dependent protease, and free and reutilizable ubiquitin is released. In this review we discuss the mechanisms involved in ubiquitin activation, selection of substrates for conjugation, and subsequent degradation of ubiquitin-conjugated proteins in the cell-free system. In addition, we summarize briefly what is currently known of the physiological roles of the ubiquitin system in vivo.

Animals↗