Heat shock organizing protein

Before considering in detail the roles of heat-shock (stress) proteins in helping organisms cope with extremes of environmental temperature, it is important to examine more closely the... 

Cold stress may induce synthesis of heat-shock (stress) proteins. Exposure of cells to cold shock may lead to the induction of one or more of the classes of molecular chaperones that also are induced by heat shock. This is strong evidence that low temperature, like high temperature, can lead to non-native protein structures in vivo and, therefore, to the requirement for enhanced chaperoning activity. Induction of cold-induced protein chaperones has been seen in bacteria (Salotra et al., 1995), in whole organism studies of ectothermic animals (Petersen et al., 1990 Yocum et ah, 1991),... 

Heat shock proteins (HSPs) are synthesized by cells in response to an increase in temperature, as well to various other stressful stimuli. Their main function is to ensure intracellular protein homeostasis, thus preserving the cells viability in the presence of aggression. Current evidence points to a protective role for HSPs in several aspects of critical disease, such as ischemia-reperfusion, ARDS, and multiple organ failure. The increase of a few degrees Celsius above the normal environmental temperature of cells leads to the heat shock response 1) rapid expression of heat shock genes, 2) suppression of normal protein synthesis, and 3) the ability of cells to survive a second and otherwise lethal heat challenge (thermotolerance). 

Heat-shock proteins (Hsps) are proteins expressed virtually in all organisms as a response of exposure to a stress, such as elevated temperature (fever), protein degradation, mechanical or chemical stress. As chaperone proteins they are concerned with the intracellular folding and refolding, assembly and translocation of damaged proteins. 

Fig. 4.4. The principle of signal transduction by nuclear receptors. Nuclear receptors are ligand-controlled transcription factors that bind cognate DNA sequences, or hormone responsive elements (HRE). The hormone acts as a regulating ligand. Most nuclear receptors bind their cognate HREs, which tend to be symmetrically organized, as homo- or heterodimers. The DNA-bound, activated receptor stimulates transcription initiation via direct or indirect protein-protein interactions with the transcription initiation complex. The arrows demonstrate the different possible configurations of the HRE (see also 4.6). H hormone Hsp heat shock protein.

Sequence Comparisons Proteins called molecular chaperones (described in Chapter 4) assist in the process of protein folding. One class of chaperone found in organisms from bacteria to mammals is heat shock protein 90 (Hsp90). All Hsp90 chaperones contain a 10 amino acid signature sequence, which allows for ready identification of these proteins in sequence databases. Two representations of this signature sequence are shown below. 

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