Volume 21, Issue 2
April 1996, pages 101-257
pp 101-102 April 1996
pp 103-121 April 1996
A variety of physical and chemical stimuli were known to cause specific stress response in all organisms. Research done over the past 25 years has shown that there is a striking uniformity in the manner in which organisms respond to different forms of stress. Expression of stress responsive genes was shown to be regulated by two or more specific transcription factors present in the cell prior to stress. These specific genes were also activated during development. In this review, the role of heat shock transcription factors in stress response, during development and during cell cycle is described.
pp 123-132 April 1996
The highly conserved heat shock proteins (HSP) belong to a subset of cellular proteins that localize to the nucleus. HSPs are atypical nuclear proteins in that they localize to the nucleus selectively, rather than invariably. Nuclear localization of HSPs is associated with cell stress and cell growth. This aspect of HSPs is highly conserved with nuclear localization occurring in response to a wide variety of cell stresses. Nuclear localization is likely important for at least some of the heat shock proteins’ protective functions; little is known about the function of the heat shock proteins in the nucleus. Nuclear localization is signalled by the presence of a basic nuclear localization sequence (NLS) within a protein. Though most is known about HSP 72’s nuclear localization, the NLS(s) has not been definitively identified for any of the heat shock proteins. Likely more is involved than presence of a NLS; since the heat shock proteins only localize to the nucleus under selective conditions, nuclear localization must be regulated. HSPs also function as chaperons of nuclear transport, facilitating the movement of other macromolecules across the nuclear membrane. The mechanisms involved in chaperoning of proteins by HSPs into the nucleus are still being identified.
pp 133-148 April 1996
A detailed study of the heat shock response and its regulation has only been carried out inEscherichia coli and to some extent inBacillus subtilis. InE. coli, the induction of heat shock genes is obtained through two different alternativeσ factors, σ-32 and σ-24 which modify the promoter recognition specificity of the RNA polymerase to enable the expression of heat shock genes. Whereas σ-32 controls the expression of some 31 heat shock genes, σ-24 activates at least ten different genes of which three have been identified so far. In contrast, inBacillus subtilis, three classes of heat shock genes have been described, all three regulated at the level of transcription. Whereas class I genes are regulated by a repressor, class II are under the control of an alternate sigma factor, and the regulation mechanism of class III heat shock genes is unknown. The repressor protein of class I genes interacts with an operator sequence designated CIRCE which is widespread among eubacteria. This novel heat shock element has been described more than 50 times occurring in 28 different euBacteriol species suggesting that the underlying regulation mechanism is also widespread. Preliminary results suggest that there are Bacterial species which express both the σ-32 and the CIRCE mechanism.
pp 149-160 April 1996
Bacterial pathogens survive under two entirely different conditions, namely, their natural environment and in their hosts. Response of these pathogens to stresses encountered during transition from the natural environment to human hosts has been described. The virulence determinants of pathogenic bacteria are under the control of transcriptional activators which respond to fluctuations in growth temperature, osmolarity, metal ion concentration and oxygen tension of the environment. The regulation of stress induced genes may occur at the level of transcription or translation or by post-translational modifications. Under certain stress conditions local changes in the superhelicity of DNA induce or repress genes. In addition to their role in survival of bacteria under stressful situations, the stress induced proteins are also implicated in the manifestation of pathogenicity of bacterial pathogensin vivo.
pp 161-177 April 1996
Most stress-inducible polypeptides are members of broader protein families that function either as molecular chaperones or constituents of proteolytic pathways. These systems control many aspects of protein structure and function throughout the cell under all types of growth regimes. The Clp/HSP1 00 protein family is a recently characterized representative, with constitutive and stress-inducible members found in many different organisms and various intracellular locations. Besides being regulators of energy-dependent proteolysis, Clp proteins may also function as molecular chaperones. Constitutive Clp proteins are involved foremost in cellular protein maintenance and repair, in cooperation with other chaperone and proteolytic systems. At high temperatures, additional Clp proteins are induced in response to rising levels of inactive polypeptides, resulting from either biosynthetic errors, thermal denaturation and aggregation. Clp proteins presumably help to stabilize selected polypeptides during severe thermal stress and enable resolubilization of non-functional protein aggregates, as well as promoting the degradation of irreversibly damaged polypeptides. The union of chaperone and proteolytic regulatory functions in one molecule suggests that certain Clp proteins play a decisive role in determining the destiny of proteins, not only during normal growth but also under conditions of extreme stress. This review briefly covers recent findings on the diversity of Clp proteins and their potential importance within the cell.
pp 179-190 April 1996
The 90 kDa heat shock protein (HSP90) is an ATP-binding molecular chaperone with an associated ATPase activity having nucleoplasmin and HSP70-binding homology domains and containing Ca-binding EF-hands and a nuclear localization signal. Here we characterize the HSP90-associated ATPase and show that it is (i) a P-type ATPase inhibited by molybdate and vanadate, (ii) able to hydrolyze methylfluorescein phosphate with a 5–6-fold higher affinity, (iii) a 3-times better GTPase than ATPase in the presence of calcium and (iv) HSP27 and F-actin, but not HSP10 can “convert” the HSP90-associated ATPase activity to HSP90 autokinase activity. The HSP90-associated ATP/GTPase may participate in the regulation of complex formation of HSP90 with other proteins, such as F-actin, tubulin and heat shock proteins.
pp 191-205 April 1996
Among the various heat shock proteins (HSPs), members of the HSP70 and HSP90 families have drawn particular attention due to their heat shock-unrelated functions. HSP90, an ubiquitous and abundant member of the HSP90 family has been shown to be associated with a large array of protein factors. These proteins reside in the nucleus as well as in the cytoplasm and are involved in various physiological processes, such as, regulation of chromatin structure, cell cycle, cytoskelelal architecture, protein trafficking and protein synthesis. In this article, we focus our interest on the role of HSP90 in protein synthesis. Recent data obtained from a few laboratories strongly suggest that HSP90 interacts with the heme-regulated eukaryotic initiation factor 2α (elF-2α) kinase, also called the heme-regulated inhibitor, and causes its activation which leads to inhibition of protein synthesis. On the basis of data reported from various laboratories, including our own, we propose a possible model on the mechanism of HSP90-mediated activation of heme-regulated inhibitor and regulation of protein synthesis.
pp 207-219 April 1996
Thehrsω or the 93D heat shock locus ofDrosophila melanogaster, which does not code for any protein, has an important role in development since nullosomy of this locus in transheterozygotes for two overlapping deficiencies, viz.,Df(3R)eGp4 (eGp4) andDf(3R)GC14 (GC14), is known to cause a high (∼ 80%) mortality with the small number of escapee nullosomic flies being sterile, weak and surviving for only a few days. We now show that a majority of thehsrω-nulosomics die as embryo and that the 20% escapee embryos develop slower compared to their sibs carrying either one or two copies of thehsrω locus but after hatching survive to pupal/imago stage. Most interestingly, we further show that when onehsp83 mutant allele (hsp83e4A) is introduced ineGp4/GC14 trans-heterozygotes, practically none of thehsrω-nullosomic embryos develop beyond the 1st instar larval stage. The specificity of this interaction betweenhsp83 andhsrω genes was further confirmed by examining the effect of thehsp83 mutant allele on other mutations in the 93D cytogenetic region. Therefore, we conclude that thehsp83 mutation acts as a dominant enhancer of the lethality associated with nullosomy for thehsrω gene. The observed genetic interaction between these two members of the heat shock gene family during normal embryonic development ofDrosophila reveals novel aspects of their biological functions.
pp 221-234 April 1996
The α-crystallin-related stress protein HSP27, which promotes cellular resistance to different types of stress, is constitutively expressed during the growth of several primate tissue culture cells. Here, we report an analysis of the cellular localization of this protein in CV-1 monkey cells. Following cell lysis and fractionation in the absence of detergent about 2 5 % of the cellular content of HSP27 was recovered in the particu late fractions while the remaining of this protein was in the soluble cytoplasmic fraction. This association of HSP27 with particulate fractions was no more observed when cells were lysed in the presence of non-ionic detergent or when cells were pretreated with drugs, such as monensin and colcemid, that disrupt cytoskeletal architecture. Immunofluorescence analysis revealed that HSP27 is concentrated in a polarized perinuclear zone of CV-1 cells from where microtubules radiate. The particular locale of HSP27 was investigated in cells exposed to drugs or treatments, such as monensin, colcemid, cold stess and serum starvation, that disrupt the cellular architecture of microtubules. A correlation was observed between HSP27 cellular locale and microtubules integrity. Our results suggest a possible interaction of a fraction of HSP27 with cytoplasmic organelles or structures, different from the Golgi apparatus, whose distribution depends upon the organization of microtubules.
pp 235-246 April 1996
The selective inducibility ofhsrω gene by heat shock and several chemical agents and its selective non-inducibility by heat shock under certain conditions led to suggestion that this locus is subject to multiple controls at the level of transcription. With a view to delimit these different control elements, transgenic lines horbouringhsrω 5’ promoter deletion variants tagged to thelacZ reporter gene were used. Three different assays, viz., staining forβ-galactosidase activity in different larval tissues using chromogenic X-gal substrate, [3H] uridine labelling of polytene nuclei andin situ DNA-DNA hybridization with a non-radioactive probe to polytene chrmosome spreads for checking the puffing status of the resident and the transgene in larval salivary glands, were applied to monitor the activiy of the reporter gene following different treatments. Our results showed that the − 844 bp to +107 bp sequence was sufficient for heat shock induction of the transgene in all tissues. An analysis of the base sequence of thehsrω promoter revealed the presence of three consensus heat shock elements at − 466, − 250 and at − 57 bp and of two GAGA factor binding sites at − 496 and at − 68bp within the − 844 bp region. Germline transformants carrying the − 346 bp to − 844 bp region of thehsrω promoter showed only a very weak heat shock inducibility of the reporter gene in agreement with the presence of only one of the three putative heat shock elements and one of the two GAGA factor binding sites in this region. Interestingly, neither of the transformed lines (carrying the − 844 bp to + 107 bp or the − 844 bp to −346 bp of thehsrω promoter region) showed any response of the transgene to benzamide or colchicine treatments. These results showed that while the heat shock response elements of thehsrω are included within the − 844 bp region the response elements for benzamide and colchicine treatments are outside this region.
pp 247-257 April 1996
We summarize the most remarkable features of the heat shock inducible large telomeric puffs (T-BRs)in polytene chromosomes ofChironomus thummi.Kinetic aspects of formation ofT-BRsas well as their transcriptional behaviour clearly support the view thatT-BRsare components of the heat shock response inChironomus.Available molecular data indicateT-BRsto include long arrays of 176 bp tandem repeats. A large transcript (>10 kb) encompassing the telomere associated repeat has been detected. Several other similarities betweenT-BRs ofChironomus and thehsrω genes ofDrosophila suggest theT-BRs to behsrω counterpart inChironomus.