**2. Cell stress or heat shock proteins**

All organisms respond to potentially harmful environmental factors by an up-regulation of heat shock protein expression. Cell stress or heat shock proteins were first discovered in 1962 by Ritossa who observed a pattern of Drosophila salivary gland chromosome puffs induced under transient exposure to high temperature. It was subsequently described that these highly conserved group of proteins could be induced by many other stress factors. Mammalian heat shock proteins are classified into two groups according to their size: high molecular weight heat shock proteins (HSP) and small HSP - sHSP. The first group includes three major families: HSP90, HSP70 and HSP60. Some of these proteins are constitutively expressed (in-house chaperones), whereas the expression of the others is induced by stressful conditions. High molecular weight stress protein are ATP-dependent chaperones and require co-chaperones to perform their ATP-binding and modulate their conformation. In contrast sHSP are ATP-independent. Heat shock proteins are expressed in both normal and stress conditions and are responsible for:

1) facilitating the proper folding of nascent proteins in cytosol, endoplasmatic reticulum, mitochondria; 2) import of proteins into cellular transport; 3) prevention of protein aggregates, refolding of denatured proteins; 4) degradation of unstable proteins; 5) control of apoptosis.

HSP also participates in the intracellular transport and have been implicated in the loading of immunogenic peptides in histocompatibility complexes (MHC) in the T-cell presentation.
