Showing papers by "Susan Lindquist published in 1994"

Protein disaggregation mediated by heat-shock protein Hsp104.

Abstract: The heat-inducible members of the Hsp100 (or Clp) family of proteins share a common function in helping organisms to survive extreme stress, but the basic mechanism through which these proteins function is not understood. Hsp104 protects cells against a variety of stresses, under many physiological conditions, and its function has been evolutionarily conserved, at least from Saccharomyces cerevisiae to Arabidopsis thaliana. Homology with the Escherichia coli ClpA protein suggests that Hsp104 may provide stress tolerance by helping to rid the cell of heat-denatured proteins through proteolysis. But genetic analysis indicates that Hsp104 may function like Hsp70 as a molecular chaperone. Here we investigate the role of Hsp104 in vivo using a temperature-sensitive Vibrio harveyi luciferase-fusion protein as a test substrate. We find that Hsp104 does not protect luciferase from thermal denaturation, nor does it promote proteolysis of luciferase. Rather, Hsp104 functions in a manner not previously described for other heat-shock proteins: it mediates the resolubilization of heat-inactivated luciferase from insoluble aggregates.

18 Heat Shock Proteins and Stress Tolerance

Abstract: 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...

An Arabidopsis heat shock protein complements a thermotolerance defect in yeast.

Abstract: The heat shock protein Hsp104 of the yeast Saccharomyces cerevisiae plays a key role in promoting survival at extreme temperatures. We found that when diverse higher plant species are exposed to high temperatures they accumulate proteins that are antigenically related to Hsp104. We isolated a cDNA corresponding to one of these proteins from Arabidopsis. The protein, AtHSP101, is 43% identical to yeast Hsp104. DNA gel blot analysis indicated that AtHSP101 is encoded by a single- or low-copy number gene. AtHsp101 mRNA was undetectable in the absence of stress but accumulated to high levels during exposure to high temperatures. When AtHSP101 was expressed in yeast, it complemented the thermotolerance defect caused by a deletion of the HSP104 gene. The ability of AtHSP101 to protect yeast from severe heat stress strongly suggests that this HSP plays an important role in thermotolerance in higher plants.

Preferential deadenylation of Hsp70 mRNA plays a key role in regulating Hsp70 expression in Drosophila melanogaster.

Abstract: Following a standard heat shock, approximately 40% of Hsp70 transcripts in Drosophila melanogaster lack a poly(A) tail. Since heat shock disrupts other aspects of RNA processing, this observation suggested that heat might disrupt polyadenylation as well. We find, however, that as the temperature is increased a larger fraction of Hsp70 RNA is polyadenylated. Poly(A)-deficient Hsp70 RNAs arise not from a failure in polyadenylation but from the rapid and selective removal of poly(A) from previously adenylated transcripts. Poly(A) removal is highly regulated: poly(A) is (i) removed much more rapidly from Hsp70 RNAs than from Hsp23 RNAs, (ii) removed more rapidly after mild heat shocks than after severe heat shocks, and (iii) removed more rapidly after a severe heat shock if cells have first been conditioned by a mild heat treatment. Poly(A) seems to be removed by simple deadenylation rather than by endonucleolytic cleavage 5' of the adenylation site. During recovery from heat shock, deadenylation is rapidly followed by degradation. In cells maintained at high temperatures, however, the two processes are uncoupled and Hsp70 RNAs are deadenylated without being degraded. These deadenylated mRNAs are translated with low efficiency. Deadenylation therefore allows Hsp70 synthesis to be repressed even when degradation of the mRNA is blocked. Poly(A) tail shortening appears to play a key role in regulating Hsp70 expression.