Regulation of the Heat-Shock Response

Abstract

The heat-shock response is a conserved reaction of cells and organisms to elevated temperatures (heat shock or heat stress). Whereas severe heat stress leads to cellular damage and cell death, sublethal doses of heat stress induce a cellular response, the heat-shock response, which (a) protects cells and organisms from severe damage, (b) allows resumption of normal cellular and physiological activities, and (c) leads to a higher level of thermotolerance. Crucial to the survival of cells is the sensitivity of proteins and enzymes to heat inactivation and denaturation. Therefore, adaptive mechanisms exist that protect cells from the proteotoxic effects of heat stress. Owing to their sessile lifestyle, the acquisition of higher levels of environmental stress tolerance is of utmost importance to plants. It is not surprising that the heat-shock response is also linked to several other environmental stresses. Furthermore, an increasing number of studies indicate cross-protection between heat stress, dehydration/drought, cold/chilling/freezing, heavy-metal stress, and oxidative stress in plants. At the molecular level the heat-shock response is a transient reprogramming of cellular activities featured by the synthesis of HSPs, concomitant with a cessation of normal protein synthesis. HSPs seem to accumulate in a dosage-dependent manner to amounts sufficient to protect cells and to provide a higher level of thermotolerance. In most organisms, the major groups of stress proteins, HSP100, HSP90, HSP70, HSP60, and small HSPs, are represented by a few members of each class. HSPs are functionally linked to the large and diverse group of molecular chaperones that are defined by their capacity to recognize and to bind substrate proteins that are in an unstable, inactive state. All cellular proteins probably have to interact with molecular chaperones at least once in their lifetime, such as during synthesis, subcellular targeting, or degradation. Owing to heat denaturation, the fraction of potential targets for molecular chaperones seems to dramatically increase upon heat stress and, consequently, the cellular chaperone pool has to be replenished. It is not surprising that, except for small HSPs and HSP100, each class of HSPs is matched by one or several HSCs expressed at normal temperatures. Different HSPs may have different functional properties but common to all of them is their capacity to interact with other proteins and to act as molecular chaperones in vitro (for overview, see Boston et al., 1996;Schöffl et al., 1998a, 1998b). The in vivo chaperone function of plant HSPs was recently demonstrated by the protection and reactivation of a luciferase reporter in Arabidopsis cells (Forreiter et al., 1997). There is a striking correlation between the occurrence of HSPs and acquisition of thermotolerance, but there is little direct evidence for a causal relationship. Mutations would be required that result in a coordinate change in the expression of HSPs to study: (a) the signal pathway from stress to gene, (b) the mechanism of transcriptional regulation, and (c) the role of HSPs in thermotolerance. The effects of mutations in individual heat-shock genes have been investigated in different organisms. Analyses in yeast provided evidence for an important role of HSP104 and a minor, accessory role of HSP70 in thermotolerance (Sanchez et al., 1993). Mutations in Hsp26, the sole gene for a small HSP in yeast (Petko and Lindquist, 1986), overexpression of small HSPs, and antisense approaches in transgenic plants (Schöffl et al., 1987) had no obvious effects on the phenotype. The protective effect of HSPs is sometimes dependent on the physiological conditions of the cell, as has been shown shown by the disruption of a mitochondrial HSP30 gene in Neurospora crassa, which resulted in strains that were less thermotolerant under certain carbohydrate limitations (Plesovsky-Vig and Brambl, 1995). In other eukaryotes, other groups of HSPs seem to play important roles in thermotolerance; for example, HSP70 overexpression in mammalian cells and in Drosophila melanogaster (for overview, see Morimoto et al., 1990; Welte et al., 1993). Using genetic engineering of Arabidopsis as a model for higher plants, dominant regulatory mutations were generated that showed a constitutive synthesis of HSPs at normal temperatures (Lee et al., 1995;Prändl et al., 1998). In these transgenic plants the fundamental role of HSPs in stress tolerance is indicated by significantly higher levels of basal thermotolerance. However, it is not yet clear whether in addition to HSPs, other as-yet-unknown genes are involved in the generation of enhanced stress tolerance. The expression of the heat-shock genes encoding the different HSPs in plants is similar to the situation in other eukaryotes, that is, it is primarily regulated at the transcriptional level. The thermoinducibility is attributed to conserved cis-regulatory promoter elements (HSEs) located in the TATA-b