An overview on the small heat shock proteins
In the last 25 years, a huge amount of literature has been accumulated describing the cell’s response to different kinds of environmental stress conditions, such as high temperatures, altered pH, exposure of the cell to toxins, starvation, oxygen, and water deprivation, among others. Heat shock proteins (HSPs) are one of the main expressed products of the cell in response to stresses. HSPs can be classified into six structurally conserved classes according to their molecular weight namely, HSP100, HSP90, HSP70, HSP60, small heat shock proteins (sHSPs) and ubiquitin (8.5 kDa). In eukaryotes, different heat shock genes are expressed uncoordinatedly, whereas in prokaryote, heat shock genes form a regulon and appear simultaneously. sHSPs are associated with nuclei, cytoskeleton and membranes. They bind partially to denatured proteins, preventing irreversible protein aggregation during stress. In animals, only one sHSP gene has been located in yeast cells, ten in mammalian, two in birds and four genes have been found in Drosophila. However, in plants more than 20 sHSPs have been reported and they can be divided into 6 classes, of which, 3 classes (CI, CII and CIII) are in the cytosole or in the nucleus and the other three (CIV, CV and CVI) in the plastids, endoplasmic reticulum and mitochondria. Mitochondrial and chloroplast sHSPs protect electron transport chain. During development in animals, sHSP genes are normally regulated at late neurula and early tailbud stage and in plants during pollen development, seed maturation, seed imbibition and germination. Transcriptional regulation of sHSPs depends on particular activation of heat shock factors (HSF) which recognize the highly conserved heat-shock elements (HSEs). After the heat stress has been released, the sHSPs are quite stable, suggesting that sHSPs may be important for recovery as well.