Fig. 1

The ubiquitin-proteasome system and PROTACs. The left part of the figure shows the relevant steps in the tagging of proteins for degradation by the ubiquitin-proteasome system. That process involves sequential steps catalyzed by three types of enzymes. The E1 activating enzyme catalyzes the activation of ubiquitin in an ATP-dependent process. The active site cysteine present in E1 established a bond with the carboxy-terminus of ubiquitin. In a second step, the thioesterified ubiquitin is transferred to the E2 ubiquitin conjugating enzyme. In a third step, the E3 ligase binds both the protein target and the E2-ubiquitin. The E3 ligases are the most numerous (more than 500) and are expected to contribute to the specificity in the degradation of the protein target. On the other side, only two E1 have been described and forty E2. The E3-E2-ubiquitin-Protein target multiprotein complex is then competent to transfer ubiquitin to lysine residues of the protein target. The ubiquitinated protein can then be targeted to the proteasome for degradation. Rpn receptors present in the 19S unit of the proteasome help degradation of tagged proteins acting as binding sites. Proteins entering the proteasome are then degraded by peptidases of the 20S region, resulting in the formation of fragmented proteins and the removal of the ubiquitin from the protein being degraded. The right part of the figure illustrates the mechanism of action of PROTACs. These molecules are heterobifunctional constructs consisting in a ligand that specifically binds the protein target and an E3 binding molecule. A linker is necessary to connect both the ligand and the E3 binding molecule. PROTACs act by stabilizing in close proximity the protein target and the E3-E2-Ubiquitin complex. That ternary complex (PROTAC+protein target+E3-E2-Ubiquitin) allows ubiquitination of the protein target, that is then recognized for degradation by the proteasome. PROTACs, therefore, take advantage of the protein degradation system to direct the removal or down regulation of a protein target that may play a pathophysiological role in a disease. In this respect, adequate engineering of a PROTAC may favor degradation of pathophysiological proteins in a cell or tissue-specific manner, for example, by directing degradation by E3 ligases specifically or mainly present in leukemic blasts or nervous tissue