Heat shock factor and the heat shock response

Abstract

Peter K. Sorger Department of Microbiology and Immunology University of California San Francisco, California 94143-0502 The induction of eukaryotic heat shock genes in response to a temperature upshift is mediated by the binding of a transcriptional activator, heat shock factor, to a short highly conserved DNA sequence known as the heat shock element. This review will discuss progress in several or- ganisms in characterizing heat shock factor and will sug- gest preliminary answers to the following questions. How is the DNA-binding activity and transcriptional potency of heat shock factor regulated? How is autoregulation of heat shock protein (hsp) synthesis achieved? What is the mech- anism by which heat shock is sensed? Heat Shock Elements and Their Interaction with Heat Shock Factor Heat shock elements are best described as contiguous arrays of variable numbers of the 5 bp sequence nGAAn arranged in alternating orientation (n denotes less strongly conserved nucleotides that nevertheless may be involved in important DNA-protein interactions; Figure 1; Xiao and Lis, 1988; Amin et al., 1988). At least two nGAAn units are needed for high affinity binding of heat shock factor in vitro, and these may be arranged either head-to-head (nGAAnnTTCn) or tail-to-tail (nTTCnnGAAn; Perisic et al., 1989). How can heat shock factor bind to these structurally distinct sites as well as to heat shock elements with larger numbers of 5 bp units? The answer may lie in the oligo- merit nature of the heat shock factor protein (Figure 2). Heat shock factor from both Saccharomyces cerevisiae (SC-HSF) and Drosophila (D-HSF) associates to form pro- tein trimers in solution and when bound to DNA (Perisic et al., 1989; Sorger and Nelson, 1989). It is not clear, how- ever, whether heat shock factor exists in vivo primarily as a trimeric, hexameric, or possibly even larger complex. Each subunit of a D-HSF multimer is thought to bind to a single nGAAn unit, and the binding to successive units is highly cooperative (Xiao et al., 1991). The coiled-coil structure that has been proposed to form the interface between heat shock factor monomers is inherently three- fold symmetric (see below). Thus, the binding of heat shock factor trimers to DNA (which is inherently two-fold symmetric) would require a flexible hinge between the tri- merization and DNA-binding surfaces of individual mono- mers. Flexibility at this hinge may also be exploited in the binding of subunits to differently oriented nGAAn units. The binding of trimers to adjacent sites is also highly cooperative (Shuey and Parker, 1986): in vitro, the dissoci- ation rate from DNA of a complex of two DNA-bound tri- mers is more than three orders of magnitude lower than that of a single trimer (Xiao et al., 1991). Thus, even when the molarity of sites exceeds that of the heat shock factor protein, heat shock factor preferentially forms large com-