Small Stress Proteins

Studies have identified important families of proteins (denoted: heat shock or stress proteins, Hsps) which display an enhanced expression in response to heat shock or other physiological stresses. Besides the characterization of the genes encoding Hsp and the mechanisms of their induction, recent studies have concentrated on the function of these proteins. It was shown that the expression of Hsp protects the cell against different types of aggressions. In addition,Hsp can regulate essential biochemical processes in unstressed cells. For example, members of the Hsp60 and Hsp70 families act as ATP-binding proteins allowing the folding of nascent or denatured proteins as well as the assembly or disassembly of protein complexes. These observations have led to the discovery of the molecular chaperone concept (Ellis and Hemmingsen 1989). Amongst the proteins whose expression is up-regulated by heat shock or other types of stresses are the small stress proteins also denoted (sHsps, sHsp or sHSP). Small stress proteins encompass a large numbers of related proteins which are represented in virtually all organisms, including prokary­ otes. These polypeptides share a structural domain, often referred to as the a-crystallin domain, common to the lens protein alpha-crystallin (Ingolia and Craig 1982;Wistow 1985). In addition to being increased in response to several types of stresses, the Hsp level is also upregulated during development and correlates with the differentiation and oncogenic status of the cell. In spite of the fact that sHsp can confer cellular protection against stresses,their molecular function has remained enigmatic for years.