Regulated Protein Degradation

Cells need to regulate their levels of intracellular proteins according to the alterations in their state, such as environmental stimuli, or other metabolic or developmental changes. 

This regulation or control of protein concentrations inside a cell depends on their rate of synthesis as well as their degradation. 

In eukaryotes, intracellular protein degradation is mostly mediated by the ubiquitin-proteasome system. This system serves two primary functions inside the cells - the quality control of misfolded or damaged proteins, and the control of the levels of important cellular proteins. 

The specificity of the proteasomal degradation in the cell is regulated through various mechanisms. 

Eukaryotic cells have a class of proteins called E3 ligases - the main catalytic unit of ubiquitin ligase. Each of these unique E3 molecules can ubiquitinate only a specific target protein upon activation. 

Each E3 protein ligase can also only be activated through a specific signal such as phosphorylation, allosteric transition due to ligand binding, or protein subunit addition. This regulates the ubiquitination of the target protein leading to its degradation by the proteasome. 

For example, APC