Effect of stress on protein degradation: role of the ubiquitin system

Many factors which induce the stress response (heat shock protein synthesis) in eukaryotes also cause the formation of aberrant proteins. Such aberrant proteins are usually rapidly and selectively degraded in cells. Temperature step-up accelerates the degradation of a subset of normally stable proteins. This effect is transient and is confined to a narrow range of heat shock temperatures above which proteolysis is inhibited. The time course and extent of proteolysis elicited by a mild heat shock is consistent with data on the thermal transitions of cellular proteins. Biochemical and genetic evidence strongly supports the view that the ubiquitin system is primarily responsible for heat- or stress-damaged protein degradation in eukaryotic cells. It still remains to be determined how stress-damaged proteins are recognized by the ubiquitin system and selected for degradation. Ubiquitin-protein ligases (E3's) which attach multi-ubiquitin chains to proteins are thought to be responsible for the selection of proteins for degradation. Several species of E3 have recently been characterized. However, none of the known E3's seems to fulfil the role of selecting aberrant proteins for breakdown. Heat shock proteins which are thought to repair unfolded or misfolded proteins probably have a complementary function to the ubiquitin system which destroys damage proteins. The relationship between the ubiquitin system and the regulation of heat shock protein synthesis, which is still not understood, is discussed.