Proteins translation, heat shock response

A remarkable variety of mechanisms has been proposed to explain the heat shock response in different cells. Lindquist et al. (1982) and Di Domenico et al. (1982) suggest that, in Drosophila and yeast cells, heat shock proteins repress their own synthesis. The response in Xenopus oocytes differs from that in somatic cells in that only translational control appears to be involved heat shock mRNA is masked and inactive in translation at normal temperatures, but is unmasked by high temperature (Bienz and Gurdon, 1982). At that temperature, translation of normal mRNA is inhibited by another mechanism. Lowering the temperature results in reactivation of normal mRNA and remasking of heat shock mRNA. 

Storti, R.V., Scott, M.P., Rich, A. Pardue, M.L. (1980). Translational control of protein synthesis in response to heat shock in D. melanogaster cells. Cell, 22,825-34. 

Both the heat and cold shock response are universal and have been studied extensively. The major heat shock proteins (HSPs) are highly conserved. They are involved in the homeostatic adaptation of cells to harsh environmental conditions. Some act as molecular chaperones for protein folding, while others are involved in the processing of denatured polypeptides whose accumulation would be deleterious. The cold shock results in the transient induction of cold shock proteins (CSPs), which include a family of small acidic proteins carrying the cold shock domain. The CSPs appear to be involved in various cellular functions such as transcription, translation and DNA recombination. 

Exposure of cells to elevated temperatures, or heat shock, induces the transcription and translation of a set of proteins known as the heat-shock proteins (HSPs) or the stress proteins. This event usually occurs with the concomitant inhibition of biosynthesis of other cellular components. The response is believed to be an attempt by the cells to protect themselves from injury, and there is some evidence to indicate that it may also be linked to oxidative stress. Evidence in favour of this idea comes from observations such as the following ... 

Initiation factor phosphorylation. The phosphorylation of eIF-2 in response to certain circumstances (e.g., heat shock, viral infections, and growth factor deprivation) has been observed to decrease protein synthesis generally. However, the translation of certain mRNA increases. For example, hsp (heat shock protein) synthesis increases in response to heat shock and other stressful conditions. The specific mechanisms are unknown. 

Reiter T, Penman S (1983) Prompt heat shock proteins Translationally regulated synthesis of new proteins associated with the nuclear matrix-intermediate filaments as an early response to heat shock. Proc Natl Acad Sci USA 80 4737-4741... 

Post translation unfolded protein, are bound by heat shock proteins (HSPs), such as HSP70 and HSP90 binds which facilitate their proper folding in cytoplasm (Pratt and Morishima 2010). However, not all proteins are folded properly, and high acciunulations of unfolded or mis-folded are reported to cause intracellular toxicity and to contribute to cell death (iageretal. 2012 Loewen and Feany 2010). These misfolded or unfolded proteins are trapped in cytoplasm or the ER and induce the expression of stress response proteins, such as HSPs, IRE, PERK, and ATF-6 (Badiola et a/. 2011 Gorman eto/. 2012). 

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