Dr. Hershko is a Distinguished Professor at the Technion-Israel Institute of Technology. He has made seminal contributions to the fields of protein degradation and cell cycle. In particular, he discovered the role of ubiquitin in intracellular protein degradation and has defined the biochemical system that carries out ubiquitin-protein ligation.
In recognition of his work, he has been the recipient of several distinguished awards including the Lasker Basic Medical Research award, the Gairdner award, the General Motors Award, the Massry prize, the American Society for Biochemistry and Molecular Biology/Merk award, the Weizman Prize for Sciences, the Israel Prize in Biochemistry, the Wolf award and the Louisa Gross Horwitz Prize. Dr. Hershko was elected to the National Academy of Sciences as a foreign member in 2003 and in 2004 he was awarded the Nobel Prize in Chemistry.
Following is a more detailed history of Dr. Hershko's discoveries.
The dynamic turnover of cellular proteins was discovered in the pioneering studies of Rudolf Schoenheimer in the 1930s, when he first used isotopically labeled compounds for biological studies. In 1960-1970 it became evident that protein degradation in animal cells is highly selective, and plays important roles in the control of the levels of specific enzymes. However, the molecular mechanisms responsible for this process remained unknown.
Dr. Hershko became interested in the mechanisms of intracellular protein breakdown when he was a post-doctoral fellow in the laboratory of Gordon Tomkins more than 30 years ago. At that time, the main subject in this laboratory was the mechanism by which corticosteroid hormones cause the increased synthesis of the enzyme tyrosine aminotransferase (TAT). Dr. Hershko found work on this subject a bit crowded, so he chose to study a different process that also regulates TAT levels, the degradation of this enzyme. He found that the degradation of TAT in cultured hepatoma cells is completely arrested by inhibitors of cellular energy production, such as fluoride or azide.
Dr. Hershko was intrigued by the energy-dependence of intracellular protein breakdown, because proteolysis per se is an exergonic process that does not require energy. He assumed that there is an as yet unknown proteolytic system that utilizes energy for the high selectivity of protein degradation. Following his return to Israel in 1971 and the setting up of his laboratory at the Technion, his major goal was to identify the energy-dependent system that is responsible for the degradation of cellular proteins. He decided to reproduce ATP-dependent protein breakdown in a cell-free system and then to fractionate this system in order to find out the mode of action of its components.
An ATP-dependent proteolytic system from reticulocytes was first described by Etlinger and Goldberg and then was analyzed by Dr. Hershko's biochemical fractionation-reconstitution studies. In this work, Dr. Hershko was helped by his graduate student Aaron Ciechanover and by Irwin Rose, who hosted him in his laboratory for a sabbatical year in 1977-78 and many times afterwards. Initially, reticulocyte lysates were fractionated into two crude fractions: Fraction 1, which was not adsorbed, and Fraction 2, which contained all proteins adsorbed to the resin and eluted with high salt. The original aim of his fractionation had been to remove hemoglobin (present in Fraction 1), but they found that Fraction 2 lost most of its ATP-dependent proteolytic activity. The active component in Fraction 1 was a small protein that was first called APF-1, for ATP-dependent Proteolysis Factor 1. The identity of APF-1 as ubiquitin was later found by Wilkinson and co-workers, following Dr. Hershko's discovery of its ligation to proteins, as described below. Ubiquitin was first thought to be a thymic hormone but subsequently was found to be present in many tissues and organisms, hence its name.
At first Dr. Hershko thought that ubiquitin could be an activator, a regulatory subunit of a protease present in Fraction 2. He was surprised to find, however, that ubiquitin was ligated to numerous high-molecular-weight proteins via a covalent amide linkage. He began to suspect that ubiquitin may be linked to protein substrates, rather to an enzyme. In support of this interpretation, he found that proteins that are good substrates for ATP-dependent proteolysis form multiple conjugates with ubiquitin. Based on these findings, Dr. Hershko proposed that multiple molecules of APF-1/ubiquitin are linked to lysine epsilon-NH2 groups of the protein substrate by an "APF-1-protein amide synthetase". Proteins ligated to multiple ubiquitins were proposed to be broken down to free amino acids by a specific protease that recognizes such conjugates.
Comparison of the original model proposed by Dr. Hershko with our current knowledge of the reactions of the ubiquitin pathway shows that the original model was essentially correct, but much additional detail provides explanation for the high selectivity of ubiquitin-mediated protein degradation. Thus, Dr. Hershko found that APF-1-protein amide synthetase is actually composed of three types of enzymes: a ubiquitin-activating enzyme E1, a ubiquitin-carrier protein E2 and a ubiquitin-protein ligase E3. Specific E3 enzymes recognize structural features in specific protein substrates and thus account for substrate selectivity. Proteins ligated to multi-ubiquitin chains are degraded by a 26S proteasome complex discovered by Rechsteiner and coworkers.