Heat shock proteins as molecular chaperones

Exposure to different conditions or agents that destabilize cell homeostasis often alters protein folding. Depending on stress intensity irreversible protein aggregation and cell death can occur. Cells have developed a conserved defense mechanism aimed at reducing the deleterious effects induced by protein folding alteration. This mechanism is characterized by the expression of a small number of genes encoding specific proteins, named Hsps. Several of these proteins act as molecular chaperones through their ability to refold polypeptides with an altered conformation. Moreover, constitutive Hsps homologues have been characterized that participate in the folding of newly made polypeptides, in the assembly of protein complexes in the endoplasmic reticulum, in the translocation of polypeptides through membranes or in masking mutations that alter protein folding. Neurodegeneratives and cancereous diseases are discussed as examples where high levels of Hsp expression can be either beneficial or deleterious to the cells.