| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
J. Biol. Chem., Vol. 281, Issue 40, 32, October 6, 2006
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Characterization of the Heat-stable Polypeptide of the ATP-dependent Proteolytic System from Reticulocytes
(Ciechanover, A., Elias, S., Heller, H., Ferber, S., and Hershko, A. (1980) J. Biol. Chem. 255, 7525–7528)
Immunochemical Analysis of the Turnover of Ubiquitin-Protein Conjugates in Intact Cells. Relationship to the Breakdown of Abnormal Proteins
(Hershko, A., Eytan, E., Ciechanover, A., and Haas, A. L. (1982) J. Biol. Chem. 257, 13964–13970)
Components of Ubiquitin-Protein Ligase System. Resolution, Affinity Purification, and Role in Protein Breakdown
(Hershko, A., Heller, H., Elias, S., and Ciechanover, A. (1983) J. Biol. Chem. 258, 8206–8214)
In a series of pioneering biochemical studies in the late 1970s and early 1980s, Avram Hershko, Aaron Ciechanover, and Irwin A. Rose discovered and characterized the ATP-dependent, ubiquitin-mediated protein degradation system. Part of this work was done during a series of sabbatical leaves when Hershko and Ciechanover worked in Rose's laboratory at the Fox Chase Cancer Center in Philadelphia.
Hershko received his M.D. (1965) and his Ph.D. (1969) from the "Hadassah" Faculty of Medicine of the Hebrew University in Jerusalem and then did a postdoctoral fellowship with Gordon Tomkins at the University of California in San Francisco. There, he studied the degradation of tyrosine aminotransferase. Hershko's results confirmed and extended earlier findings by Melvin Simpson on the energy dependence of the liberation of amino acids from proteins.
In 1971 Hershko returned to Israel and became Chairman of Biochemistry in the Faculty of Medicine of the Technion, the Israel Institute of Technology. He continued to pursue the problem of how proteins are degraded in cells, and why energy is required for this process. For a number of years, he tried to establish a cell-free system that reproduced energy-dependent protein degradation. Finally, he settled on a reticulocyte cell-free crude extract established by Etlinger and Goldberg (1). At that time, Ciechanover joined Hershko's laboratory as a graduate (D.Sc.) student, after earning a Master of Science in 1971 and an M.D. in 1974 from the "Hadassah" Medical School of the Hebrew University in Jerusalem.
In their initial attempt to characterize the ATP-dependent activity in the reticulocyte crude extract, Ciechanover and Hershko showed, already early on, that unlike "classical" proteases that digest their substrates directly, the reticulocyte "protease" contained at least two complementing fractions, I (the active component of which was shown to be a small heat-stable protein designated initially as APF-1-ATP-dependent proteolysis factor 1, later identified as ubiquitin) and II, which were required together to reconstitute the activity observed in the crude extract (2). This was an important observation not only because the initial characterization of ubiquitin started at that point but also because from that stage on classical biochemistry of reconstitution was employed each time the proteolytic activity was lost during a purification step, leading to the discovery of the two-step novel mode of action of the system—initial substrate tagging of the target substrate by ubiquitin, followed by its subsequent degradation with recycling of ubiquitin.
|
|
|
Hershko had a sabbatical year in 1977 and decided to spend it with Rose who had been at The Institute for Cancer Research at the Fox Chase Cancer Center since 1963. Hershko brought Ciechanover with him, and the three scientists started working on elucidating the protein degradation system.
Hershko, Ciechanover, and Rose reported that the second fraction of the reticulocyte lysate could be further subdivided into an ATP-stabilized protein of approximately 450 kDa (most probably containing the 26 S proteasome) and a fraction that contained the E1–E3 conjugating enzymes that were later isolated (4). All three fractions were required to degrade 125I-labeled albumin. Soon, they discovered that multiple APF-1 molecules are bound covalently to target substrates in the lysate and mark them for degradation by a protease that recognizes only conjugated proteins. APF-1 is recycled during the process (5, 6).
Ciechanover and Hershko subsequently purified and characterized APF-1 from rabbit reticulocytes. This is the subject of the first JBC Classic reprinted here. An amino acid analysis of APF-1, along with its known molecular mass and other general characteristics raised the strong suspicion that APF-1 was ubiquitin.
|
The connection between APF-1 and ubiquitin was made by three postdoctoral fellows at Fox Chase who worked there in parallel to Hershko and Ciechanover and followed the development of the system. Keith Wilkinson and Arthur Haas were from the Rose laboratory, and Mike Urban was from Martin Nemer's laboratory. Urban was working with chromatin and noted that similar to APF-1, ubiquitin was also a small protein that was linked covalently to other proteins, in this case histones. As described in the first JBC Classic, the size and amino acid composition of APF-l and ubiquitin were in agreement, suggesting that the two proteins were the same. This final identification was published in a paper (11) that appeared back to back with the Ciechanover and Hershko JBC Classic.
This ubiquitin tagging hypothesis received support when Hershko and Ciechanover observed a marked increase in the labeling of ubiquitin-protein conjugates during the formation of abnormal, rapidly degrading proteins in reticulocytes and nucleated eukaryotic cells. This is the subject of the second JBC Classic reprinted here. In this paper Hershko and Ciechanover developed an immunochemical method for isolating ubiquitin-protein conjugates from intact cells. Cells were incubated in the presence of amino acid analogues to generate unstable, rapidly degrading proteins and pulse-labeled with tryptophan, an amino acid that is missing in ubiquitin. Using an antibody against ubiquitin, they showed a direct correlation between the instability of the synthesized proteins and the level of the corresponding ubiquitin conjugates generated: degradation of the short lived proteins was accompanied by a corollary generation of a high level of ubiquitin adducts. Thus, it was clear that ubiquitination was not restricted to terminally differentiating reticulocytes but was also observed in the Ehrlich ascites cells used, indicating the universal nature of the system.
Between 1981 and 1983, Hershko and Ciechanover worked out the multistep ubiquitin-tagging hypothesis by isolating and characterizing the enzymes E1, E2, and E3. They first purified the ubiquitin-activating enzyme E1 by affinity chromatography using Sepharose-immobilized ubiquitin (12). E1 bound covalently to ubiquitin and was eluted by AMP and PPi, reversing the reaction of its activation and binding to ubiquitin. d1 Realizing that E1 could not form ubiquitin-protein conjugates by itself, Ciechanover and Hershko used their affinity chromatography strategy to purify the ubiquitin carrier protein E2 (by using dithiothreitol, reversing the mechanism of E2 binding to the immobilized ubiquitin via thiol ester formation) and the ubiquitin-protein ligase E3 (by using high salt or high pH). This purification and subsequent analysis of the roles of the three enzymes in the three-step cascade mechanism of ubiquitin activation and conjugation is reported in the third JBC Classic reprinted here.
Hershko and Ciechanover found that the binding of E2 to the immobilized ubiquitin required E1 and ATP, but the binding of E3 to the column required neither. Taken together, the results indicated that E2 bound covalently to the column like E1 did, whereas E3, which could be eluted by salt/high pH, bound non-covalently. Based on their observations, they hypothesized that E1 transferred ubiquitin to E2, which then transferred ubiquitin to the substrate in the presence of E3 (Fig. 4). Although they used a single E3 and a single model substrate, they predicted that the specificity of the E3 component determined which proteins in the cell were marked for destruction, determining the high specificity of the system toward its potential numerous substrates. Thus the "the multistep ubiquitin-tagging hypothesis" was born.
As a result of their pioneering studies on ubiquitin-mediated proteolysis, Hershko, Ciechanover, and Rose were awarded the 2004 Nobel Prize in Chemistry "for the discovery of ubiquitin-mediated protein degradation."
Hershko remained at the Israel Institute of Technology where he is currently a Distinguished Professor at the Rappaport Family Institute for Research in Medical Sciences as well as an Adjunct Professor of Pathology at New York University. In addition to the Nobel Prize, he has received many honors including the 1987 Weizmann Prize for Sciences, the 1994 Israel Prize in Biochemistry and Medicine, the 1999 Gairdner International Award, the 2000 Alfred P. Sloan Prize, the 2000 Albert Lasker Award for Basic Medical Research, the 2001 Merck Award from the American Society for Biochemistry and Molecular Biology, the 2001 Wolf Prize for Medicine, the 2002 E. B. Wilson Medal from the American Society of Cell Biology, and the 2005 Stein and Moore Award from the Protein Society.1
Ciechanover completed his doctorate in science (D.Sc.) in the Technion, after which he did a postdoctoral fellowship with Harvey F. Lodish in the Department of Biology and the Whitehead Institute at M.I.T. He then joined the Faculty of Medicine at the Technion, where he is currently a Distinguished Professor in the Cancer and Vascular Biology Research Center of the Rappaport Institute, and a Visiting Professor in the Department of Medicine in the Feinberg Medical School at Northwestern University in Chicago and the Department of Pediatrics at Washington University School of Medicine in St. Louis. In addition to the Nobel Prize, Ciechanover received the Austria Innsbruck University Wachter Prize in 1999, the Albert Lasker Award for Basic Medical Research in 2000, the Israel Prize for Biology in 2003, and the Japanese Society for Promotion of Science (JSPS) Distinguished Scientist Award in 2003.2
Rose remained at Fox Chase until his retirement in 1997. Following this, he moved to Laguna Woods in Southern California where he was able to continue his laboratory work using space and facilities at the University of California, Irvine. He is currently a Distinguished Professor-in-Residence at the Department of Physiology and Biophysics of the College of Medicine. Rose was elected to the National Academy of Sciences in 1979 and received the Philadelphia Section Award from the American Chemical Society in 1975, and the Washington State University's Regents' Distinguished Alumnus Award in 2005 (Rose attended Washington State University for 1 year prior to serving in the Navy during WWII).3
FOOTNOTES
1 Biographical information on Avram Hershko was taken from Refs. 13 and 14. 
2 Biographical information on Aaron Ciechanover was taken from Refs. 15 and 16.
3 Biographical information on Irwin Rose was taken from Refs. 17 and 18.
REFERENCES
1 and polypeptide 
1. J. Biol. Chem. 254, 987–995
CiteULike 
Complore 
Connotea 
Del.icio.us 
Digg 
Reddit 
Technorati What's this?
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
| All ASBMB Journals | Molecular and Cellular Proteomics |
| Journal of Lipid Research | ASBMB Today |
