18 Heat Shock Proteins and Stress Tolerance

I. INTRODUCTION The capacity of different individuals of the same species to survive short exposures to extreme temperatures (thermotolerance) varies over a remarkable range, commonly over several orders of magnitude. Both differences in growth conditions and differences in genetic background contribute. Although the contributions of genetic background are just beginning to be deciphered, the effects of growth conditions have been the subject of detailed and intense scrutiny. Nutrient availability, oxygen tension, diurnal rhythms, and a host of other variables exert highly reproducible effects on thermotolerance. By far the most closely studied phenomenon, however, is the tolerance afforded by short pretreatments at moderately elevated temperature. When whole organisms or cultured cells are given such pretreatments, their resistance to killing by extreme heat increases dramatically. This increased resistance, known as induced thermotolerance (Fig. 1), is observed in virtually every organism studied. Remarkably, mild heat pretreatments elicit resistance not just to high temperatures, but to an extraordinary variety of other stresses. In addition, exposure to other mild stresses elicits protection not just against higher doses of those particular stresses, but against high temperatures as well. Such tolerance-inducing treatments generally also induce the synthesis of a small number of proteins known as the heat shock proteins (hsps; Fig. 2) (Li and Laszlo 1985; Lindquist 1986; Nagao et al. 1986; Subjeck and Shyy 1986; Sanchez and Lindquist 1990; Nover 1991; Sanchez et al. 1992). Historically, many observations have suggested that hsps play a vital part in induced tolerance. For example, it is striking that hsps...