Ubiquitin, proteins tagged with

Multiprotein complex that catalyses ATP-dependent degradation of proteins tagged with ubiquitin. 

Proteasomes (Figure 20.10) are cytosolic particles involved in proteolytic digestion. The come in two sizes - 20S and 26S. Structurally the proteasome shows remarkable similrity to the GroEL chaperonin. Proteasomes degrade cytosolic proteins tagged with ubiquitin.. 

Figure 8.2 The tagging of a protein molecule with ubiquitin to identify it for degradation. Three enzymes are involved (1) Ei is activated by binding to ubiquitin (ii) E2 displaces Ei (iii) E3 catalyses the transfer of ubiquitin from E2 to the doomed protein more than one molecule is transferred to produce multi-ubiquitinated protein, which is the ideal substrate for the proteolytic enzyme.

Protein destined for degradation is tagged with molecules of ubiquitin. 

And cytosolic proteins that are damaged or otherwise destined for degradation are enzymically tagged with the activated ubiquitin. 

Another protein modification, which occurs through a three-enzyme complex similar to that required for ubiquitin addition, is SUMOylation. SUMO stands for small ubiqutin-like modifier, and when proteins are tagged with SUMO their activites are altered (either positively or negatively, depending on the protein). SUMOylation presents yet another means of fine-tuning existing regulatory systems. 

Breakdown of selected proteins by a stmcture called the proteasome. This is like a small machine for degrading proteins, containing an assemblage of protein-digesting enzyme activities. It only admits proteins that have been specifically labelled for breakdown by tagging with a small protein called ubiquitin. 

Fig. 13.4 Diagrammatic representation of the proteasome and its role in protein degradation via the uhiquitin-proteasome pathway. After being tagged with ubiquitin and unfolded for degradation on the 19S regulatory particles which aid the opening of a proteolytic gate in the 20S cote, proteins ate degraded into small peptides in the barrel-shaped core where pi caspase-, p2 trypsin-, and p5 chymo-trypsin-like activities reside. Regulatory particles are...

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