Beyond transcription--new mechanisms for the regulation of molecular chaperones

Molecular chaperones are an essential part of the universal heat shock response that allows organisms to survive stress conditions that cause intracellular protein unfolding. During the past few years, two new mechanisms have been found to control the activity of several chaperones under stress conditions-the regulation of chaperone activity by the redox state and by the temperature of the environment. Hsp33, for example, is redox-regulated. Hsp33 is specifically activated by disulfide bond formation during oxidative stress, where it becomes a highly efficient chaperone holdase that binds tightly to unfolding proteins. Certain small heat shock proteins, such as Hsp26 and Hsp16.9, on the other hand, are temperature regulated. Exposure to heat shock temperatures causes these oligomeric proteins to disassemble, thereby changing them into highly efficient chaperones. The ATP-dependent chaperone folding system DnaK/DnaJ/GrpE also appears to be temperature regulated, switching from a folding to a holding mode during heat stress. Both of these novel post-translational regulatory strategies appear to have one ultimate goal: to significantly increase the substrate binding affinity of the affected chaperones under exactly those stress conditions that require their highest chaperone activity. This ensures that protein folding intermediates remain bound to the chaperones under stress conditions and are released only after the cells return to non-stress conditions.     

Heat induced stress proteins and the concept of molecular chaperones

Heat stress proteins can be assigned to eleven protein families conserved among bacteria, plants and animals. Most of them aid other proteins to maintain or regain their native conformation by stabilizing partially unfolded states. Hence, they are called molecular chaperones. Experimental data indicate that many of them form heterooligomeric complexes, so-called chaperone machines, interacting with each other to generate a network for maturation, assembly and intracellular targeting of proteins. In this review we summarize the essential information on the structure and function of chaperone and chaperone complexes. In addition we present a compilation ofin vivo andin vivo test systems used in the preceding ten years of chaperone research.     

Dynamics of the response of cyanobacteria to salt stress: Deciphering the molecular events

Cyanobacteria, the only prokaryotes performing oxygemc photosynthesis and probable ancestors of chloroplasts, constitute valuable models for the study of the molecular mechanisms involved in tolerance to high salinity, or to its corollary, drought, a major agricultural problem. The critical demands of cyanobacteria exposed to high salinity, i.e., accumulation of osmoprotectors and extrusion of sodium ions, are met through immediate activation and/or long term (protein synthesis-dependent) adaptation of various processes: (1) uptake and endogenous biosynthesis of osmotica, the nature and amount of which are strain- and salt concentration-dependent; (2) enhancement of P-ATPase activity and active extrusion of sodium ions; (3) probable modifications of membrane lipid composition: and (4) increased energetic capacity, at the level of cyclic electron flow around photosystem I (through routes induced under these conditions) and cytochrome c oxidase. The processes involved highlight similarities with general stress responses and with salt stress responses in plants. Deciphering the molecular and genetic events regulating these coordinated responses is presently starting in cyanobacteria.     

c-Abl与应激损伤调控

非受体酪氨酸激酶c-Abl广泛表达于人和哺乳动物等的细胞中并受到严格调控,通过蛋白之间相互作用、与DNA相互作用及其酪氨酸激酶活性在一系列的重要生命活动中发挥调节作用。在应激损伤反应如DNA损伤反应中．c-Abl的Ser^465被ATM和DNA-PK磷酸化而激活,通过与Rad51、p53和p73等分子的相互作用参与DNA重组修复、细胞周期和细胞凋亡等的调控,不同信号途径之间的平衡决定细胞的生存和死亡。     

Inactivation of parkin by oxidative stress and C-terminal truncations: a protective role of molecular chaperones

Loss of parkin function is linked to autosomal recessive juvenile parkinsonism. Here we show that proteotoxic stress and short C-terminal truncations induce misfolding of parkin. As a consequence, wild-type parkin was depleted from a high molecular weight complex and inactivated by aggregation. Similarly, the pathogenic parkin mutant W453Stop, characterized by a C-terminal deletion of 13 amino acids, spontaneously adopted a misfolded conformation. Mutational analysis indicated that C-terminal truncations exceeding 3 amino acids abolished formation of detergent-soluble parkin. In the cytosol scattered aggregates of misfolded parkin contained the molecular chaperone Hsp70. Moreover, increased expression of chaperones prevented aggregation of wild-type parkin and promoted folding of the W453Stop mutant. Analyzing parkin folding in vitro indicated that parkin is aggregation-prone and that its folding is dependent on chaperones. Our study demonstrates that C-terminal truncations impede parkin folding and reveal a new mechanism for inactivation of parkin.     

RNA-dependent regulation of the cell wall stress response

Stress response requires the precise modulation of gene expression in response to changes in growth conditions. This report demonstrates that selective nuclear mRNA degradation is required for both the cell wall stress response and the regulation of the cell wall integrity checkpoint. More specifically, the deletion of the yeast nuclear dsRNA-specific ribonuclease III (Rnt1p) increased the expression of the mRNAs associated with both the morphogenesis checkpoint and the cell wall integrity pathway, leading to an attenuation of the stress response. The over-expression of selected Rnt1p substrates, including the stress associated morphogenesis protein kinase Hsl1p, in wild-type cells mimicked the effect of RNT1 deletion on cell wall integrity, and their mRNAs were directly cleaved by the recombinant enzyme in vitro. The data supports a model for gene regulation in which nuclear mRNA degradation optimizes the cell response to stress and links it to the cell cycle.     

Influence of molecular and chemical chaperones on protein folding

Protein folding inside the cell involves the Participation of accessory components known as molecular chaperones. In addition to their active participation in the folding process, molecular chaperones serve as a type of ‘quality control system’, recognizing, retaining and targeting misfolded proteins for their eventual degradation. It is now known that a number of human diseases arise as a consequence of specific point mutations or deletions within genes encoding essential proteins. In many cases these mutations/deletions are not so sever as to totally destroy the biological activity of the particular gene product. Rather, the mutations often result in only subtle folding abnormalities which lead to the newly synthesized protein being retained at the endoplasmic reticulum by the actions of the cellylar quality control system. In this short review article we discuss our recent studies showing that the protein folding defect associated with the most common mutation in patients with cystic fibriosis can be overcome by a novel strategy. As shown in the paper by Brown et al in this issue (Brown et al 1996), a number of different low molecular weight compounds, all known to stabilize proteins in their native conformation, are effective in rescuing the processing defect of the mutant cystic fibrosis transmembrane conductance regulator protein. We then discuss how these same compounds, which we now call chemical chaperones, also may prove to be effective in correcting a number of other protein folding abnormalities which constitute the underlying basis of a large number of different human diseases.     

Cellular stress response pathways and ageing: intricate molecular relationships

Ageing is driven by the inexorable and stochastic accumulation of damage in biomolecules vital for proper cellular function. Although this process is fundamentally haphazard and uncontrollable, senescent decline and ageing is broadly influenced by genetic and extrinsic factors. Numerous gene mutations and treatments have been shown to extend the lifespan of diverse organisms ranging from the unicellular Saccharomyces cerevisiae to primates. It is becoming increasingly apparent that most such interventions ultimately interface with cellular stress response mechanisms, suggesting that longevity is intimately related to the ability of the organism to effectively cope with both intrinsic and extrinsic stress. Here, we survey the molecular mechanisms that link ageing to main stress response pathways, and mediate age-related changes in the effectiveness of the response to stress. We also discuss how each pathway contributes to modulate the ageing process. A better understanding of the dynamics and reciprocal interplay between stress responses and ageing is critical for the development of novel therapeutic strategies that exploit endogenous stress combat pathways against age-associated pathologies.     

Approaches to the isolation and characterization of molecular chaperones

Molecular chaperones are integral components of the cellular machinery involved in ensuring correct protein folding and the continued maintenance of protein structure. An understanding of these ubiquitous molecules is key to finding cures to protein misfolding diseases such as Alzheimer's and Creutzfeldt-Jacob diseases. In addition, further understanding of chaperones will enhance our comprehension of the way the body copes with the environmental stresses that humans encounter daily. Our laboratory and our collaborators specialize in the production and characterization of chaperones from a wide variety of sources in order to gain a fuller understanding of how chaperones function in the cell. In this review, we primarily use the Hsp70/Hsp40 chaperone pair as an example to discuss recent advances in technology and reductions in cost that lend themselves to chaperone purification from both native and recombinant sources. Common assays to assess purified chaperone activity are also discussed.     

Protein folding in the cell: molecular chaperones pave the way

In vitro, many unfolded polypeptides are able to fold to the native state spontaneously, indicating that the amino acid sequence of a protein contains all the information necessary to specify its three-dimensional conformation. It had been assumed that protein folding in vivo also generally occurs in a spontaneous process. This view has changed only recently due to the discovery of a number of proteins, now commonly called 'molecular chaperones', which are essential for cellular protein folding and occur ubiquitously in eubacteria, archaebacteria and in eukaryotic cells.     

Cyclin D1 and molecular chaperones: implications for tumorigenesis

We recently investigated the mechanisms of cyclin D1 action in human cancer using global analyses of gene expression. With an experimentally-determined expression signature for cyclin D1 overexpression, gene expression data from human tumors, and a novel data-mining method, we were able to reveal a previously unappreciated and apparently predominant functional interdependency between cyclin D1 and C/EBPbeta. Many of the genes we found to be affected by cyclin D1 overexpression are recognized as molecular chaperones or their regulators. Might this provide insights to the role of the cyclin D1-C/EBPbeta axis in carcinogenesis?     

Chaperonins are cell-signalling proteins: the unfolding biology of molecular chaperones

The chaperonins are a subgroup of oligomeric molecular chaperones; the best-studied examples are chaperonin 60 (GroEL) and chaperonin 10 (GroES), both from the bacterium Escherichia coli. At the end of the 20th century, the paradigm of chaperonins as protein folders had emerged, but it is likely that during the 21st century these proteins will come to be viewed as intercellular signals. Indeed, it is possible that the chaperonins were among the first intercellular signalling proteins to evolve. During the past few years, it has emerged that chaperonin 10 and chaperonin 60 can be found on the surface of various prokaryotic and eukaryotic cells, and can even be released from cells. Secreted chaperonins can interact with a variety of cell types, including leukocytes, vascular endothelial cells and epithelial cells, and activate key cellular activities such as the synthesis of cytokines and adhesion proteins. Much has been made of the high degree of sequence conservation among the chaperonins, particularly in terms of the immunogenicity of these proteins. However, different chaperonin 60 proteins can bind to different cell-surface receptors, including the Toll-like receptors, suggesting that this family of proteins cannot be treated as one biological entity and that several subfamilies may exist. Chaperonins have been implicated in human diseases on the basis of their immunogenicity. The finding that chaperonins can also induce tissue pathology suggests that they may play roles in infections and in idiopathic diseases such as atherosclerosis and arthritis.