The discovery of ubiquitin-dependent proteolysis

Abstract

In early 1980, Irwin A. Rose, Avram Hershko, and Aaron Ciechanover published two papers in PNAS that reported the astounding observation that energy-dependent intracellular proteolysis was far more complicated than the previously accepted models of lysosomal proteolysis or the action of ATP-dependent proteases such as bacterial lon. In fact, it has turned out to be even more complicated than they could have suspected. The general model of covalently attaching a small protein as a targeting signal has proved to be every bit as important to eukaryotic cells as the better understood modifications such as phosphorylation or acetylation. The key player in this modification, a small protein called ubiquitin (APF-1 in these papers), is the founding member of a large family of proteins containing the beta-grasp fold and is used as a posttranslational targeting signal to modify the structure, function, and/or localization of other proteins. The story of this discovery is a textbook example of the confluence of intellectual curiosity, unselfish collaboration, chance, luck, and preparation.

Fig. 1.

The laureates at the Karolinska Institute after their Nobel addresses. Shown are (left to right) Aaron Ciechanover, Irwin Rose, and Avram Hershko.

Fig. 2.

Formation of covalent compounds between APF-1 and lysozyme in an ATP-dependent reaction. Tracks 1–5, compound formation with ¹²⁵I-APF-1; track 1, without ATP; track 2, with ATP; tracks 3–5, with ATP and 5, 10, or 25 mg of unlabeled lysozyme, respectively; and tracks 6 and 7, compound formation with ¹²⁵I-lysozyme. Conditions were as detailed in Methods of ref. , except that 5 μgof ¹²⁵I-lysozyme (40,000 cpm) and 3 μgof unlabeled APF-1 were used. Track 6, ATP omitted; track 7, with 2 mM ATP. Contamination in ¹²⁵I-labeled lysozyme is indicated. [Reproduced with permission from ref. (Copyright 1980).]

Fig. 3.

Proposed sequence of events in ATP-dependent protein breakdown (see the text). 1, APF-1-protein amide synthetase (acting on lysine ε-NH₂ groups). 2, Amidase that allows correction when n = 1 or 2. 3, Peptidases that act strongly on (APF-1)n derivatives, when n > 1 or 2. 4, Amidase for APF-1-X; X is lysine or a small peptide. [Reproduced with permission from ref. (Copyright 1980).]