Hsp90 as a capacitor for morphological evolution

The periplasmic Escherichia coli peptidylprolyl cis,trans-isomerase FkpA. I. Increased functional expression of antibody fragments with and without cis-prolines

The production of recombinant proteins in the periplasm of Escherichia coli can be limited by folding problems, leading to periplasmic aggregates. We used a selection system for periplasmic chaperones based on the coexpression of an E. coli library with a poorly expressing antibody single-chain Fv (scFv) fragment displayed on filamentous phage (Bothmann, H., and Plückthun, A. (1998) Nature Biotechnol. 16, 376-380). By selection for a functional antibody, the protein Skp had been enriched previously and shown to improve periplasmic expression of a wide range of scFv fragments. This selection strategy was now repeated with a library constructed from the genomic DNA of an skp-deficient strain, leading to enrichment of the periplasmic peptidylprolyl cis,trans-isomerase (PPIase) FkpA. Coexpression of FkpA increased the amount of fusion protein displayed on the phage and dramatically improved functional periplasmic expression even of scFv fragments not containing cis-prolines. In contrast, the coexpression of the periplasmic PPIases PpiA and SurA showed no increase in the functional scFv fragment level in the periplasm or displayed on phage. Together with the in vitro data in the accompanying paper (Ramm, K., and Plückthun, A. (2000) J. Biol. Chem. 275, 17106-17113), we conclude that the effect of FkpA is independent of its PPIase activity.

Engineering stability in gene networks by autoregulation

The genetic and biochemical networks which underlie such things as homeostasis in metabolism and the developmental programs of living cells, must withstand considerable variations and random perturbations of biochemical parameters. These occur as transient changes in, for example, transcription, translation, and RNA and protein degradation. The intensity and duration of these perturbations differ between cells in a population. The unique state of cells, and thus the diversity in a population, is owing to the different environmental stimuli the individual cells experience and the inherent stochastic nature of biochemical processes (for example, refs 5 and 6). It has been proposed, but not demonstrated, that autoregulatory, negative feedback loops in gene circuits provide stability, thereby limiting the range over which the concentrations of network components fluctuate. Here we have designed and constructed simple gene circuits consisting of a regulator and transcriptional repressor modules in Escherichia coli and we show the gain of stability produced by negative feedback.

Ancestors and variants: tales from the cryptic

Those who work at the interface of development and evolution are united by the conviction that developmental comparisons can shed light on both the evolution of specific morphologies and the macroevolutionary process itself. In practice, however, the field comprises a diversity of approaches. As the field grows and practitioners attempt to digest a growing mountain of comparative data, the various approaches of "Evo Devo" have themselves evolved. A meeting organized by the authors and held at the University of Chicago in the Spring of 1999 illustrated some of these changes. This review will draw on its content to discuss recent developments in two areas: the reconstruction of common ancestors and the developmental basis of evolutionary change.

Magnetic field exposure stimulates transposition through the induction of DnaK/J synthesis

Like some naturally occurring environmental stress factors such as heat shock and UV irradiation, magnetic field exposure is also stimulatory to transposition activity. This feature could be illustrated by a bacterial conjugation study using an Escherichia coli strain that carries the transposable element Tn5 as the donor. When the donor cultures were exposed to a low-frequency (50 Hz) magnetic field of 1.2 mT, Tn5 located on the bacterial chromosome was stimulated to transpose and settled on the extrachromosomal episome, and eventually transferred to the recipient cell through conjugation. Such transposition activity stimulation was mediated by the induced synthesis and accumulation of the heat shock proteins DnaK/J.

Large accumulation of mRNA and DNA point modifications in a plant senescent tissue

Although nucleic acids are the paradigm of genetic information conservation, they are inherently unstable molecules that suffer intrinsic and environmental damage. Oxidative stress has been related to senescence and aging and, recently, it has been shown that mutations accumulate at high frequency in mitochondrial DNA with age. We investigated RNA and DNA modifications in cork, a senescent plant tissue under high endogenous oxidative stress conditions. When compared to normally growing young tissue, cork revealed an unexpected high frequency of point modifications in both cDNA (Pn = 1/1784) and nuclear DNA (Pn = 1/1520). Cork should be viewed as a mosaic of genetically heterogeneous cells. This has biological implications: it supports somatic mutation models for aging and challenges 'single cDNA clone' as descriptor for the molecular genetics of senescent tissues.

Disruption of hsp90 function results in degradation of the death domain kinase, receptor-interacting protein (RIP), and blockage of tumor necrosis factor-induced nuclear factor-kappaB activation

The death domain kinase, receptor interacting protein (RIP), is one of the major components of the tumor necrosis factor receptor 1 (TNFR1) complex and plays an essential role in tumor necrosis factor (TNF)-mediated nuclear factor kappaB (NF-kappaB) activation. The activation of NF-kappaB protects cells against TNF-induced apoptosis. Heat-shock proteins (Hsps) are chaperone molecules that confer protein stability and help to restore protein native folding following heat shock and other stresses. The most abundant Hsp, Hsp90, is also involved in regulating the stability and function of a number of cell-signaling molecules. Here we report that RIP is a novel Hsp90-associated kinase and that disruption of Hsp90 function by its specific inhibitor, geldanamycin (GA), selectively causes RIP degradation and the subsequent inhibition of TNF-mediated IkappaB kinase and NF-kappaB activation. MG-132, a specific proteasome inhibitor, abrogated GA-induced degradation of RIP but failed to restore the activation of IkappaB kinase by TNF, perhaps because, in the presence of GA and MG-132, RIP accumulated in a detergent-insoluble subcellular fraction. Most importantly, the degradation of RIP sensitizes cells to TNF-induced apoptosis. These data indicate that Hsp90 plays an important role in TNF-mediated NF-kappaB activation by modulating the stability and solubility of RIP. Thus, inhibition of NF-kappaB activation by GA may be a critical component of the anti-tumor activity of this drug.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Canalization in evolutionary genetics: a stabilizing theory?

Canalization is an elusive concept. The notion that biological systems ought to evolve to a state of higher stability against mutational and environmental perturbations seems simple enough, but has been exceedingly difficult to prove. Part of the problem has been the lack of a definition of canalization that incorporates an evolutionary genetic perspective and provides a framework for both mathematical and empirical study. After briefly reviewing the importance of canalization in studies of evolution and development, we aim, with this essay, to outline a research program that builds upon the definition of canalization as the reduction in variability of a trait, and uses molecular genetic approaches to shed light on the problems of canalization.

A critical role for the proteasome activator PA28 in the Hsp90-dependent protein refolding

The 90-kDa heat shock protein, Hsp90, was previously shown to capture firefly luciferase during thermal inactivation and prevent it from undergoing an irreversible off-pathway aggregation, thereby maintaining it in a folding-competent state. While Hsp90 by itself was not sufficient to refold the denatured luciferase, addition of rabbit reticulocyte lysate remarkably restored the luciferase activity. Here we demonstrate that Hsc70, Hsp40, and the 20 S proteasome activator PA28 are the effective components in reticulocyte lysate. Purified Hsc70, Hsp40, and PA28 were necessary and sufficient to fully reconstitute Hsp90-initiated refolding. Kinetics of substrate binding support the idea that PA28 acts as the molecular link between the Hsp90-dependent capture of unfolded proteins and the Hsc70- and ATP-dependent refolding process.

Mechanisms of mRNA surveillance in eukaryotes

A conserved mRNA degradation system, referred to as mRNA surveillance, exists in eukaryotic cells to degrade aberrant mRNAs. A defining aspect of aberrant transcripts is that the spatial relationship between the termination codon and specific downstream sequence information has been altered. A key, yet unknown, feature of the mRNA surveillance system is how this spatial relationship is assessed in individual transcripts. Two views have emerged to describe how discrimination between proper and improper termination might occur. In the first view, a surveillance complex assembles onto the mRNA after translation termination, and scans the mRNA in a 3' to 5' direction for a limited distance. If specific downstream sequence information is encountered during this scanning, then the surveillance complex targets the transcript for rapid decay. An alternate view suggests that the downstream sequence information influences how translation termination occurs. This view encompasses several ideas including: (a) The architecture of the mRNP can alter the rate of key steps in translation termination; (b) the discrimination between a proper and improper termination occurs via an internal, Upf1-dependent, timing mechanism; and (c) proper termination results in the restructuring of the mRNP to a form that promotes mRNA stability. This proposed model for mRNA surveillance is similar to other systems of kinetic proofreading that monitor the accuracy of other biogenic processes such as translation and spliceosome assembly.

Genetic analysis of speciation by means of introgression into Drosophila melanogaster

In the last decade, the genetic basis of reproductive isolation has been shown to be surprisingly polygenic, and yet even the most efficient system currently in use could lend itself to molecular analysis only in highly selected cases. By extending the recent discovery of fertility rescue between Drosophila melanogaster and Drosophila simulans, we show that this hybridization can permit systematic and precise delineation of the genetic and molecular basis of speciation. In a region of 5% of the D. simulans genome introgressed into D. melanogaster, we discover at least six genes of hybrid male sterility and none for female sterility by deficiency mapping. A single case of hybrid inviability has been tracked down to a 3-Kb element that was inserted into the Cyclin E locus during species hybridization. The extent of interspecific genetic divergence underlying hybrid male sterility, especially in contrast with the low degree of inviability and female sterility, is far greater than expected from previous studies.

Independence between developmental stability and canalization in the skull of the house mouse

The relationship between the two components of developmental homeostasis, that is canalization and developmental stability (DS), is currently debated. To appraise this relationship, the levels and morphological patterns of interindividual variation and fluctuating asymmetry were assessed using a geometric morphometric approach applied to the skulls of laboratory samples of the house mouse. These three samples correspond to two random-bred strains of the two European subspecies of the house mouse and their F1 hybrids. The inter- and intraindividual variation levels were found to be smaller in the hybrid group compared to the parental ones, suggesting a common heterotic effect on skull canalization and DS. Both buffering mechanisms might then depend on the same genetic condition, i.e. the level of heterozygosity. However, related morphological patterns did not exhibit any congruence. In contradiction with previous studies on insect wing traits, we therefore suggest that canalization and DS may not act on the same morphological characters. The fact that this discrepancy could be related to the functional importance of the symmetry of the characters under consideration is discussed in the light of our knowledge of the genetic bases of both components of developmental homeostasis.

Macroevolution is more than repeated rounds of microevolution

Arguments over macroevolution versus microevolution have waxed and waned through most of the twentieth century. Initially, paleontologists and other evolutionary biologists advanced a variety of non-Darwinian evolutionary processes as explanations for patterns found in the fossil record, emphasizing macroevolution as a source of morphologic novelty. Later, paleontologists, from Simpson to Gould, Stanley, and others, accepted the primacy of natural selection but argued that rapid speciation produced a discontinuity between micro- and macroevolution. This second phase emphasizes the sorting of innovations between species. Other discontinuities appear in the persistence of trends (differential success of species within clades), including species sorting, in the differential success between clades and in the origination and establishment of evolutionary novelties. These discontinuities impose a hierarchical structure to evolution and discredit any smooth extrapolation from allelic substitution to large-scale evolutionary patterns. Recent developments in comparative developmental biology suggest a need to reconsider the possibility that some macroevolutionary discontinuites may be associated with the origination of evolutionary innovation. The attractiveness of macroevolution reflects the exhaustive documentation of large-scale patterns which reveal a richness to evolution unexplained by microevolution. If the goal of evolutionary biology is to understand the history of life, rather than simply document experimental analysis of evolution, studies from paleontology, phylogenetics, developmental biology, and other fields demand the deeper view provided by macroevolution.

Hormesis: an adaptive expectation with emphasis on ionizing radiation

Non-linear fitness gradients with maxima between extremes are expected for any environmental variable to which free-living populations are exposed. For exceedingly toxic agents, including ionizing radiation, such deviations from linearity are close to zero exposure and are conventionally called hormesis. Accordingly, hormesis is an extreme version of the non-linear fitness gradients for general environmental stresses such as temperature fluctuations, for which maximum fitness occurs at the moderate temperature fluctuations to which free-living populations are most commonly exposed. Some metabolic reserves should occur under moderate temperature stresses because of the need for pre-adaptation enabling survival during exposure to occasional periods of more extreme stress, especially at species borders where selection for stress resistance is likely to be most intense. Because heat shock proteins are induced by all stresses, adaptation to extreme temperatures should translate into adaptation to other stresses. Consequently, metabolic reserves from adaptation to extreme temperatures in the past should translate into protection from correlated abiotic stresses, especially in human populations where modern cultural processes are now ameliorating exposure to extreme stresses. Ambient and man-made radiations of non-catastrophic dimensions should therefore lead to stress-derived radiation hormesis. Other stresses can, in principle, be incorporated into this model. Accordingly, evolutionary and ecological considerations suggest two components of hormesis in relation to ionizing radiation: background radiation hormesis based upon the background exposure to which all organisms on earth are subjected; and stress-derived radiation hormesis. Exposure under stress-derived radiation hormesis is considerably larger than under background radiation hormesis, so significant deleterious effects from non-catastrophic radiation normally may be impossible to detect. Suggestions are provided for testing such postulated deviations from the commonly assumed linear no-threshold (LNT) hypothesis for the biological consequences of exposure to radiation.

The hsp90-related protein TRAP1 is a mitochondrial protein with distinct functional properties

The hsp90 family of molecular chaperones was expanded recently due to the cloning of TRAP1 and hsp75 by yeast two-hybrid screens. Careful analysis of the human TRAP1 and hsp75 sequences revealed that they are identical, and we have cloned a similar protein from Drosophila. Immunofluorescence data show that human TRAP1 is localized to mitochondria. This mitochondrial localization is supported by the existence of mitochondrial localization sequences in the amino termini of both the human and Drosophila proteins. Due to the striking homology of TRAP1 to hsp90, we tested the ability of TRAP1 to function as an hsp90-like chaperone. TRAP1 did not form stable complexes with the classic hsp90 co-chaperones p23 and Hop (p60). Consistent with these observations, TRAP1 had no effect on the hsp90-dependent reconstitution of hormone binding to the progesterone receptor in vitro, nor could it substitute for hsp90 to promote maturation of the receptor to its hormone-binding state. However, TRAP1 is sufficiently conserved with hsp90 such that it bound ATP, and this binding was sensitive to the hsp90 inhibitor geldanamycin. In addition, TRAP1 exhibited ATPase activity that was inhibited by both geldanamycin and radicicol. Thus, TRAP1 has functions that are distinct from those of hsp90.

The evolution of genetic canalization under fluctuating selection

If the direction of selection changes from generation to generation, the ability to respond to selection is maladaptive: the response to selection in one generation leads to reduced fitness in the next. Because the response is determined by the amount of genetic variance expressed at the phenotypic level, rapidly fluctuating selection should favor modifier genes that reduce the phenotypic effect of alleles segregating at structural loci underlying the trait. Such reduction in phenotypic expression of genetic variation has been named "genetic canalization." I support this argument with a series of two- and multilocus models with alternating linear selection and Gaussian selection with fluctuating optimum. A canalizing modifier gene affects the fitness of its carriers in three ways: (1) it reduces the phenotypic consequences of genetic response to previous selection; (2) it reduces the genetic response to selection, which is manifested as linkage disequilibrium between the modifier and structural loci; and (3) it reduces the phenotypic variance. The first two effects reduce fitness under directional selection sustained for several generations, but improve fitness when the direction of selection has just been reversed. The net effect tends to favor a canalizing modifier under rapidly fluctuating selection regimes (period of eight generations or less). The third effect improves fitness of the modifier allele if the fitness function is convex and reduces it if the function is concave. Under fluctuating Gaussian selection, the population is more likely to experience the concave portion of the fitness function when selection is stronger. Therefore, only weak to moderately strong fluctuating Gaussian selection favors genetic canalization. This paper considerably broadens the conditions that favor genetic canalization, which so far has only been postulated to evolve under long-term stabilizing selection.

Structure and in vivo function of Hsp90

Until recently, Hsp90 was one of the least well understood of the molecular chaperones, but considerable progress is now being made in unravelling its biochemistry. Hsp90 has now been shown to possess an inherent ATPase that is essential for the activation of authentic 'client' proteins in vivo and in vitro. The molecular detail of Hsp90's interactions with co-chaperones is also becoming clearer and the identification of key roles in assembling regulatory and signalling pathways has made it a target for anticancer drug development. Despite this, a clear understanding of how Hsp90 contributes to the folding and/or activation of its client proteins remains some way off.

Function of 90-kDa heat shock protein in cellular differentiation of human embryonal carcinoma cells

Heat shock proteins (HSPs) have been recognized as molecules that maintain cellular homeostasis during changes in the environment. Here we report that HSP90 functions not only in stress responses but also in certain aspects of cellular differentiation. We found that HSP90 showed remarkably high expression in undifferentiated human embryonal carcinoma (EC) cells, which were subsequently dramatically down-regulated during in vitro cellular differentiation, following retinoic acid (RA) treatment, at the protein level. Surprisingly, heat shock treatment also triggered the down-regulation of HSP90 within 48 h at the protein level. Furthermore, the heat treatment induced cellular differentiation into neural cells. This down-regulation of HSP90 by heat treatment was shifted to an up-regulation pattern after cellular differentiation in response to RA treatment. In order to clarify the functions of HSP90 in cellular differentiation, we conducted various experiments, including overexpression of HSP90 via gene transfer. We showed that the RA-induced differentiation of EC cells into a neural cell lineage was inhibited by overexpression of the HSP90alpha or -beta isoform via the gene transfer method. On the other hand, the overexpression of HSP90beta alone impaired cellular differentiation into trophoectoderm. These results show that down-regulation of HSP90 is a physiologically critical event in the differentiation of human EC cells and that specific HSP90 isoforms may be involved in differentiation into specific cell lineages.

Whither genomics?

The flood of data from genome-wide analysis is transforming biology. We need to develop new, interdisciplinary approaches to convert these data into information about the components and structures of individual biological pathways and to use the resulting information to yield knowledge about general principles that explain the functions and evolution of life.

Empirical laws of survival and evolution: their universality and implications

Presented analysis of human and fly life tables proves that with the specified accuracy their entire survival and mortality curves are uniquely determined by a single point (e.g., by the birth mortality q(0)), according to the law, which is universal for species as remote as humans and flies. Mortality at any age decreases with the birth mortality q(0). According to life tables, in the narrow vicinity of a certain q(0) value (which is the same for all animals of a given species, independent of their living conditions), the curves change very rapidly and nearly simultaneously for an entire population of different ages. The change is the largest in old age. Because probability to survive to the mean reproductive age quantifies biological fitness and evolution, its universal rapid change with q(0) (which changes with living conditions) manifests a new kind of an evolutionary spurt of an entire population. Agreement between theoretical and life table data is explicitly seen in the figures. Analysis of the data on basic metabolism reduces it to the maximal mean lifespan (for animals from invertebrates to mammals), or to the maximal mean fission time (for bacteria), and universally scales them with the total number of body atoms only. Phenomenological origin of this unification and universality of metabolism, survival, and evolution is suggested. Their implications and challenges are discussed.

Environmentally constrained mutation and adaptive evolution in Salmonella

The relationship between environment and mutation is complex [1]. Claims of Lamarkian mutation [2] have proved unfounded [3-5]; it is apparent, however, that the external environment can influence the generation of heritable variation, through either direct effects on DNA sequence [6] or DNA maintenance and copying mechanisms [7-10], or as a consequence of evolutionary processes [11-16]. The spectrum of mutational events subject to environmental influence is unknown [6] and precisely how environmental signals modulate mutation is unclear. Evidence from bacteria suggests that a transient recombination-dependent hypermutational state can be induced by starvation [5]. It is also apparent that changes in the mutability of specific loci can be influenced by alterations in DNA topology [10,17]. Here we describe a remarkable instance of adaptive evolution in Salmonella which is caused by a mutation that occurs in intermediate-strength osmotic environments. We show that the mutation is not 'directed' and describe its genetic basis. We also present compelling evidence in support of the hypothesis that the mutational event is constrained by signals transmitted from the external environment via changes in the activity of DNA gyrase.

The future of evolutionary developmental biology

Combining fields as diverse as comparative embryology, palaeontology, molecular phylogenetics and genome analysis, the new discipline of evolutionary developmental biology aims at explaining how developmental processes and mechanisms become modified during evolution, and how these modifications produce changes in animal morphology and body plans. In the next century this should give us far greater mechanistic insight into how evolution has produced the vast diversity of living organisms, past and present.

The Hsp90 of Candida albicans can confer Hsp90 functions in Saccharomyces cerevisiae: a potential model for the processes that generate immunogenic fragments of this molecular chaperone in C. albicans infections

During infections with a number of important eukaryotic pathogens the Hsp90 molecular chaperone of the pathogen is recognized as an immunodominant antigen by the host immune system. Yeast molecular genetics should allow study of the extent of sequence variation within conserved immunodominant epitopes on pathogen Hsp90s that is compatible with essential Hsp90 functions, as well as the processes that generate antigenic subfragments of these Hsp90s. The Hsp90 of the fungal pathogen Candida albicans was shown in this study to provide both essential and nonessential (pheromone signalling and mammalian steroid receptor activation) Hsp90 functions in Saccharomyces cerevisiae cells. Much of the C. albicans Hsp90 expressed in respiratory S. cerevisiae cells was shown to undergo a partial degradation in vivo, a degradation that closely resembles that of the native Hsp82 (one isoform of the homologous Hsp90) in S. cerevisiae. Allowing for the differences in the length of the charged linker region between the N- and C-terminal domains of C. albicans Hsp90 and S. cerevisiae Hsp82, these two proteins expressed in S. cerevisiae appear to give the same major degradation products. These Hsp90 fragments are similar to the products of incomplete Hsp90 degradation found in C. albicans cultures.

Maintaining life and health by natural selection of protein molecules

A concept for a life and health-preserving principle is presented, with reference to evolutionary, medical, and biochemical observations. Life comprises two basic phenomena: it unfolds over longer periods at the population level, and is sustained for the duration of individual life spans. The evolution of life within populations by means of natural selection of individuals is central to Darwin's theory of evolution. An important component of maintaining individual life is proposed here to be the natural selection of molecular components-the proteins, a process of preferred removal of denatured and old, synonymous with the selection of younger, functional molecules. The proteins of the cell are committed to fulfilling all the tasks programmed by the genome while continuously maintaining all appropriate cellular functions, including protecting the DNA. Physiological and environmental influences accelerate the breakdown of aged protein molecules, driving this renewal process so that the cell can maintain its protein stock at high-performance levels. The principle of selection makes the incredible dynamics of continual protein turnover, and hence not only the preservation of life, but the maintenance of health in individual beings, comprehensible. Arguments are presented to counter the hypothesis that protein breakdown is a stochastic, random process governed by first-order kinetics.

Temperature-sensitive RB mutations linked to incomplete penetrance of familial retinoblastoma in 12 families

The tumor-suppressor activity of the retinoblastoma protein (RB) is encoded within a protein-binding ("pocket") domain that is targeted for mutations in all cases of familial retinoblastoma and in many common adult cancers. Although familial retinoblastoma is a paradigm for a highly penetrant, recessive model of tumorigenesis, the molecular basis for the phenotype of incomplete penetrance of familial retinoblastoma is undefined. We studied the RB pocket-binding properties of three independent, mutant RB alleles that are present in the germline of 12 kindreds with the phenotype of incomplete penetrance of familial retinoblastoma. Each arises from alterations of single codons within the RB pocket domain (designated "delta 480," "661W," or "712R"). Under the same conditions, we studied the properties of wild-type (WT) RB, an RB point mutant isolated from a lung carcinoma sample (706F) and an adjacent, in vitro-generated point mutant (707W). The delta 480, 661W, and 712R mutants lack pocket protein-binding activity in vitro but retain the WT ability to undergo cyclin-mediated phosphorylation in vivo. Each of the low-penetrant RB mutants exhibits marked enhancement of pocket protein binding when the cells are grown at reduced temperature. In contrast, in this temperature range, no change in binding activity is seen with WT RB, the 706F mutant, or the 707W mutant. We have demonstrated that many families with incomplete penetrance of familial retinoblastoma carry unstable, mutant RB alleles with temperature-sensitive pocket protein-binding activity. The variable frequency for tumor development in these families may result from reversible fluctuations in a threshold level of RB pocket-binding activity.

Protein misfolding and degradation in genetic diseases

Investigations of genetic diseases such as cystic fibrosis, alpha-1-antitrypsin deficiency, phenylketonuria, mitochondrial acyl-CoA dehydrogenase deficiencies, and many others have shown that enhanced proteolytic degradation of mutant proteins is a common molecular pathological mechanism. Detailed studies of the fate of mutant proteins in some of these diseases have revealed that impaired or aberrant folding of mutant polypeptides typically results in prolonged interaction with molecular chaperones and degradation by intracellular proteases before the functional conformation is acquired. This appears to be the case for many missense mutations and short in-frame deletions or insertions that represent a major fraction of the mutations detected in genetic diseases. In some diseases, or under some circumstances, the degradation system is not efficient. Instead, aberrant folding leads to accumulation of protein aggregates that damage the cell. Mechanisms by which misfolded proteins are selected for degradation have first been delineated for the endoplasmatic reticulum; this process has been termed "protein quality control." Similar mechanisms appear to be operative in all cellular compartments in which proteins fold. Within the context of genetic diseases, we review knowledge on the molecular processes underlying protein quality control in the various subcellular compartments. The important impact of such systems for variability of the expression of genetic deficiencies is emphasised.

Is Hsp90 a regulator of evolvability?

In a recent paper, Rutherford and Lindquist (1998. Nature 396:336-342) identified mutations in the Hsp90 protein that act to unmask hidden genetic variation with a variety of phenotypic effects. The Hsp90 protein has a number of properties that suggest a role in regulating the expression of genetic variation and therefore in adjusting the evolvability of the organism. In this paper we reflect upon the evolutionary feasibility of such mechanisms and suggest some possible ways of testing the adaptation-for-evolvability hypothesis in more detail. We conclude that Hsp90 holds promise as a molecular model system for the evolution of evolvability. J. Exp. Zool. ( Mol. Dev. Evol. ) 285:116-118, 1999.

Identification of conserved residues required for the binding of a tetratricopeptide repeat domain to heat shock protein 90

The sequential binding of heat shock protein 90 (hsp90) to a series of tetratricopeptide repeat (TPR) proteins is critical to its function as a molecular chaperone. We have used site-directed mutagenesis to clarify the structural basis for the binding of hsp90 to the TPR domain of phosphoprotein phosphatase 5 (PP5). This TPR domain was chosen for study because its three-dimensional structure is known. We examined co-immunoprecipitation of hsp90 with wild type and mutant TPR constructs from transfected cells. Only mutations located on one face of the TPR domain affected hsp90 binding. This allowed the identification of a binding groove. Three basic residues that are highly conserved in hsp90-binding TPR proteins extend prominently into this groove. Lys-97 and Arg-101 were absolutely required for hsp90 binding, while mutation of Arg-74 diminished, but did not abrogate, hsp90 binding. Mutation of Lys-32, another conserved basic residue in the binding groove, also blocked hsp90 binding. The TPR domain of PP5 bound specifically to a 12-kDa C-terminal fragment of hsp90. This binding was reduced by mutation of acidic residues in the hsp90 fragment. These data suggest conservation, among hsp90-binding TPR proteins, of a binding groove containing basic residues that interact with acidic residues near the C terminus of hsp90.

Hsp90's secrets unfold: new insights from structural and functional studies

Hsp90 is a molecular chaperone associated with the folding of signal-transducing proteins, such as steroid hormone receptors and protein kinases. Results from recent studies have shed light on the structure of Hsp90 and have demonstrated that it can bind to and hydrolyse ATP. Hsp90 forms several discrete subcomplexes, each containing distinct groups of co-chaperones that function in folding pathways. Although Hsp90 is not generally involved in the folding of nascent polypeptide chains, there is a growing list of proteins whose activity depends on its function, including heat-shock factor. This review addresses recent developments in our understanding of the structure and function of Hsp90.

The role of HSP90 in evolution

A mechanism by which morphological mutations are stored without expressing phenotypes was unraveled by Rutherford & Lindquist (1998) through genetic studies of Hsp83 (HSP90) in Drosophila. Cryptic mutations are essentially neutral and therefore evolve in the absence of selective constraint. A shift from neutral mutations to selective mutations is induced when flies are exposed to environmental stress. This is a step toward understanding macroevolution in molecular terms.

A genetic analysis of the relationship between life-history variation and heat-shock tolerance in Drosophila buzzatii

Although exposure to environmental stress is common in most populations, and the physiological effects of stress on individuals are well studied, the evolutionary importance of stress to populations is not well understood. To address multitrait responses to environmental change and potential constraints on character evolution, we analysed, in 100 isofemale lines of Drosophila buzzatii, the genetic relationships between resistance to a short heat shock and several life-history traits: survival in benign conditions, larval developmental time, fecundity and longevity. Estimates of heritability of larval thermotolerance were low, but significant, and all life-history traits varied significantly among isofemale lines. Several of these traits covaried significantly. Most correlations indicated positive life-history relationships, but males and females from lines where female fecundity was higher developed more slowly in the absence of stress, which is a negative life-history relationship. The stress reduced or negated many trait associations, and showed one additional relationship; more larvae from lines that developed fast at 25 degrees C survived to adult after stress than did larvae from slow developing lines. These shifts in fitness relationships, when a single stress bout is applied, suggest that even small increases in environmental stress can have profound effects on evolutionary relationships among life-history traits.

Sequence features and phylogenetic analysis of the stress protein hsp90alpha in chinook salmon (Oncorhynchus tshawytscha), a poikilothermic vertebrate

We cloned and sequenced a chinook salmon Hsp90 cDNA; sequence analysis shows it to be Hsp90alpha. Phylogenetic analysis supports the hypothesis that alpha and beta paralogs of Hsp90 arose as a result of a gene duplication event and that they diverged early in the evolution of vertebrates, before tetrapods separated from the teleost lineage. Among several differences distinguishing poikilothermic Hsp90alpha sequences from their bird and mammal orthologs, the teleost versions specifically lack a characteristic QTQDQP phosphorylation site near the N-terminus. We used the cDNA to develop an RNA (Northern) blot to quantify cellular Hsp90 mRNA levels. Chinook salmon embryonic (CHSE-214) cells responded to heat shock with a rapid rise in Hsp90 mRNA through 4 h, followed by a gradual decline over the next 20 h. Hsp90 mRNA level may be useful as a stress indicator, especially in a laboratory setting or in response to acute heat stress.

Chance favors the prepared genome

Most descriptions of mutation have emphasized its negative consequences, and randomness with respect to biological function. This book seeks to balance the discussion by emphasizing mechanisms that both diversify the genome and increase the probability that a genome's descendants will survive. This chapter provides a framework for, and overview of, the diverse contributions to this book; these contributions will be stimulating companions, well into the 21st Century, as we work to comprehend the information contained in genomic databases. Genomes that encode "better" amino acid sequences are at a selective advantage. Genomes that generate diversity also are at an advantage to the extent that they can navigate efficiently through the space of possible sequence changes. Biochemical systems that tend to increase the ratio of useful to destructive genetic change may harness preexisting information (horizontal gene transfer, DNA translocation and/or DNA duplication), focus the location, timing, and extent of genetic change, adjust the dynamic range of a gene's activity, and/or sample regulatory connections between sites distributed across the genome. Rejecting entirely random genetic variation as the substrate of genome evolution is not a refutation, but rather provides a deeper understanding, of the theory of natural selection of Darwin and Wallace. The fittest molecular strategies survive, along with descendants of the genomes that encode them.

Heat shock protein 90 mediates macrophage activation by Taxol and bacterial lipopolysaccharide

Taxol, a plant-derived antitumor agent, stabilizes microtubules. Taxol also elicits cell signals in a manner indistinguishable from bacterial lipopolysaccharide (LPS). LPS-like actions of Taxol are controlled by the lps gene and are independent of binding to the known Taxol target, beta-tubulin. Using biotin-labeled Taxol, avidin-agarose affinity chromatography, and peptide mass fingerprinting, we identified two Taxol targets from mouse macrophages and brain as heat shock proteins (Hsps) of the 70- and 90-kDa families. Geldanamycin, a specific inhibitor of the Hsp 90 family, blocked the nuclear translocation of NF-kappaB and expression of tumor necrosis factor in macrophages treated with Taxol or with LPS. Geldanamycin did not block microtubule bundling by Taxol or macrophage activation by tumor necrosis factor. Thus, Taxol binds Hsps, and Hsp 90 helps mediate the activation of macrophages by Taxol and by LPS.

Molecular chaperones: the busy life of Hsp90

The heat shock protein Hsp90 is a molecular chaperone which assists the refolding of misfolded proteins, but also has highly selective functions in normal metabolism. These dual functions enable Hsp90 to connect environmental conditions with developmental processes and to buffer genetic changes.

Too late for the midwife toad: stress, variability and Hsp90

In the 1940s and 1950s, Waddington put forward his theories of canalization and genetic assimilation. These provided a genetic basis to account for the inheritance of some apparently 'acquired' characters. Rutherford and Lindquist have now provided a molecular framework for these theories. Their results are also relevant to observations from the 1950s concerning homozygosity and variability, with a bearing on current views concerning the use of inbred strains.

Hsp90 & Co. - a holding for folding

Hsp90 is an abundant molecular chaperone that is involved in the folding of a defined set of signalling molecules including steroid-hormone receptors and kinases. Recent in vitro experiments suggest that Hsp90 contains two different binding sites for non-native proteins, which allow it to combine the properties of a promiscuous chaperone with those of a dedicated folding-helper protein. Significant progress has been made in analysing co-chaperones, which form defined, substrate-dependent complexes with Hsp90 in vivo. Structural studies have identified the ATP-binding site in the N-terminal domain of Hsp90, which can be blocked by high-affinity inhibitors. Although a detailed understanding of the mechanism of Hsp90 action is still lacking, recent advances suggest that the protein is the centre of a dynamic, multifunctional and multicomponent chaperone machinery that extends the limits of protein folding in the cell.