Cells which are subject to stress such as starvation, heat-shock, chemical insult or mutation respond by increasing the rates of proteolysis . One function of this enhanced proteolysis is to salvage amino acids from non-essential proteins. These amino acids can then be reutilized in the synthesis of essential proteins or metabolized directly to provide energy. Another function is in the repair of damage caused by the stress. For example, oxidative stress has been shown to damage a variety of proteins and cause them to be rapidly degraded.
Abnormal proteins serve as eukaryotic stress signals and trigger the activation of heat shock genes.
The mechanisms by which proteins are selected for degradation by the cell are poorly understood. In general, large unstable proteins are degraded more rapidly suggesting that damage or local denaturation may play an important role. There are also signal sequences which target proteins to degradation. PEST sequences are signals for rapid intracellular proteolysis. Identification of C-terminal extensions that protect proteins from intracellular proteolysis.