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12.7Proteasomes and Ubiquitination

Fig. 12-15 Protein degradation by ubiquitin and proteasomes

The inside of the cell contains a huge protein complex called a proteasome. The function of the proteasome is to selectively degrade proteins. The proteins to be degraded by the proteasome are tagged by a protein called ubiquitin. Multiple ubiquitins bind to one protein (ubiquitination), which is then degraded by the proteasome. ATP is required for both ubiquitination and degradation by the proteasome.

Degradation of materials in the cell is performed by a lysosome using acid hydrolase (described earlier) or by selectively degrading only a specific protein. In the latter case, Multiple ubiquitins, which are small-sized proteins, bind to several specific proteins that are to be degraded (ubiquitination). In this case, ubiquitin binding occurs through the action of ubiquitin ligase. Furthermore, ubiquitinated proteins are recognized by a large-sized protein degradation enzyme called proteasome (composed of a complex of proteins) and then degraded using energy from ATP (Fig. 12-15). This type of selective protein degradation by proteasomes has a very important role in various cellular functions, such as in protein quality control and the cell cycle.

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