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.     

Expression and variability of molecular chaperones in the sugarcane expressome

Molecular chaperones perform folding assistance in newly synthesized polypeptides preventing aggregation processes, recovering proteins from aggregates, among other important cellular functions. Thus their study presents great biotechnological importance. The present work discusses the mining for chaperone-related sequences within the sugarcane EST genome project database, which resulted in approximately 300 different sequences. Since molecular chaperones are highly conserved in most organisms studied so far, the number of sequences related to these proteins in sugarcane was very similar to the number found in the Arabidopsis thaliana genome. The Hsp70 family was the main molecular chaperone system present in the sugarcane expressome. However, many other relevant molecular chaperones systems were also present. A digital RNA blot analysis showed that 5'ESTs from all molecular chaperones were found in every sugarcane library, despite their heterogeneous expression profiles. The results presented here suggest the importance of molecular chaperones to polypeptide metabolism in sugarcane cells, based on their abundance and variability. Finally, these data have being used to guide more in deep analysis, permitting the choice of specific targets to study.     

Molecular chaperones regulate p53 and suppress senescence programs

Many types of cancer cells constitutively express major molecular chaperones at high levels. Recent findings demonstrate that specific depletion of individual chaperones, including various members of the Hsp70 family, small heat shock proteins, or VCP/p97, leads to activation of p53 pathway and subsequently triggers cellular senescence. Here, we discuss a possibility that in cancer cells high levels of chaperones serve to keep the p53 signaling under control, thus allowing cancer cells to evade the default senescence and form tumors.     

The activities and function of molecular chaperones in the endoplasmic reticulum

Most proteins in the secretory pathway are translated, folded, and subjected to quality control at the endoplasmic reticulum (ER). These processes must be flexible enough to process diverse protein conformations, yet specific enough to recognize when a protein should be degraded. Molecular chaperones are responsible for this decision making process. ER associated chaperones assist in polypeptide translocation, protein folding, and ER associated degradation (ERAD). Nevertheless, we are only beginning to understand how chaperones function, how they are recruited to specific substrates and assist in folding/degradation, and how unique chaperone classes make quality control "decisions".     

Apoptosis,necrosis and cellular senescence: chaperone occupancy as a potential switch

Chaperone function plays a key role in repairing proteotoxic damage and in the maintenance of cell survival. Here we compare the regulatory role of molecular chaperones (heat shock proteins, stress proteins) in cellular senescence, apoptosis and necrosis. We also review the current data on chaperone level and function in aging cells, and list some possible therapeutic interventions. Finally, we postulate a hypothesis, that increasing chaperone occupancy might be an important event which forces cells out of the normal cell cycle towards senescence. In the case of severe stress, this may lead to apoptosis or, following lethal stress, to cell necrosis.     

Heat shock proteins,cellular chaperones that modulate mitochondrial cell death pathways

Stress or heat shock proteins (HSPs) are ubiquitous and highly conserved proteins whose expression is induced in response to a wide variety of physiological and environmental insults. They allow the cells to survive to otherwise lethal conditions. Various mechanisms have been proposed to account for the cytoprotective functions of HSPs. These proteins play an essential role in intracellular "house-keeping" by assisting the correct folding of nascent and stress-accumulated misfolded proteins and preventing their aggregation. Several HSPs have also demonstrated to directly interact with various components of the tightly regulated programmed cell death machinery, upstream, and downstream of the mitochondrial events. Finally, HSPs could play a role in the proteasome-mediated degradation of selected proteins under stress conditions. Altogether, these properties could make HSPs appropriate targets for modulating cell death pathways.     

Molecular chaperones and the biogenesis of mitochondria and peroxisomes

Summary— A review of the proteinaceous machinery involved in protein sorting pathways and protein folding and assembly in mitochondria and peroxisomes is presented. After considering the various sorting pathways and targeting signals of mitochondrial and peroxisomal proteins, we make a comparative dissection of the protein factors involved in: i) the stabilization of cytosolic precursor proteins in a translocation competent conformation; ii) the membrane import apparatus of mitochondria and peroxisomes; iii) the processing of mitochondrial precursor proteins, and the eventual processing of certain peroxisomal precursor, in the interior of the organelles; and iv) the requirement of molecular chaperones for appropriate folding and assembly of imported proteins in the matrix of both organelles. Those aspects of mitochondrial biogenesis that have developed rapidly during the last few years, such as the requirement of molecular chaperones, are stressed in order to stimulate further parallel investigations aimed to understand the origin, biochemistry, molecular biology and pathology of peroxisomes. In this regard, a brief review of findings from our group and others is presented in which the role of the F₁-ATPase α-subunit is pointed out as a molecular chaperone of mitochondria and chloroplasts. In addition, data are presented that could question our previous indication that the immunoreactive protein found in the rat liver peroxisomes is due to the presence of the F₁-ATPase α-subunit.     

Modular assembly of genes and the evolution of new functions

Modular assembly of novel genes from existing genes has long been thought to be an important source of evolutionary novelty. Thanks to major advances in genomic studies it has now become clear that this mechanism contributed significantly to the evolution of novel biological functions in different evolutionary lineages. Analyses of completely sequenced bacterial, archaeal and eukaryotic genomes has revealed that modular assembly of novel constituents of various eukaryotic intracellular signalling pathways played a major role in the evolution of eukaryotes. Comparison of the genomes of single-celled eukaryotes, multicellular plants and animals has also shown that the evolution of multicellularity was accompanied by the assembly of numerous novel extracellular matrix proteins and extracellular signalling proteins that are absolutely essential for multicellularity. There is now strong evidence that exon-shuffling played a general role in the assembly of the modular proteins involved in extracellular communications of metazoa. Although some of these proteins seem to be shared by all major groups of metazoa, others are restricted to certain evolutionary lineages. The genomic features of the chordates appear to have favoured intronic recombination as evidenced by the fact that exon-shuffling continued to be a major source of evolutionary novelty during vertebrate evolution.     

Thematic Review Series: Recent Advances in the Treatment of Lysosomal Storage Diseases: Lysosomal storage diseases and the heat shock response: convergences and therapeutic opportunities

Lysosomes play a vital role in the maintenance of cellular homeostasis through the recycling of cell constituents, a key metabolic function which is highly dependent on the correct function of the lysosomal hydrolases and membrane proteins, as well as correct membrane lipid stoichiometry and composition. The critical role of lysosomal functionality is evident from the severity of the diseases in which the primary lesion is a genetically defined loss-of-function of lysosomal hydrolases or membrane proteins. This group of diseases, known as lysosomal storage diseases (LSDs), number more than 50 and are associated with severe neurodegeneration, systemic disease, and early death, with only a handful of the diseases having a therapeutic option. Another key homeostatic system is the metabolic stress response or heat shock response (HSR), which is induced in response to a number of physiological and pathological stresses, such as protein misfolding and aggregation, endoplasmic reticulum stress, oxidative stress, nutrient deprivation, elevated temperature, viral infections, and various acute traumas. Importantly, the HSR and its cardinal members of the heat shock protein 70 family has been shown to protect against a number of degenerative diseases, including severe diseases of the nervous system. The cytoprotective actions of the HSR also include processes involving the lysosomal system, such as cell death, autophagy, and protection against lysosomal membrane permeabilization, and have shown promise in a number of LSDs. This review seeks to describe the emerging understanding of the interplay between these two essential metabolic systems, the lysosomes and the HSR, with a particular focus on their potential as a therapeutic target for LSDs.     

光激活荧光蛋白及其在植物分子细胞生物学研究中的应用

光激活荧光蛋白是指用特定光照射时,其荧光特性发生显著改变的一类荧光蛋白。借助光激活荧光蛋白的这种特性,可以实现对活细胞、细胞器或胞内分子的时空标记和追踪。该文介绍了目前光激活荧光蛋白的性质,并从多个方面对其应用进行了概括,包括分子标记与动态分析、蛋白质相互作用、细胞器及细胞组分动态研究、细胞追踪以及在光激活定位显微镜中的应用等,且对目前光激活荧光蛋白在植物分子细胞生物学中的应用进行了详细介绍。     

Extracellular chaperone networks and the export of J-domain proteins

An extracellular network of molecular chaperones protects a diverse array of proteins that reside in or pass through extracellular spaces. Proteins in the extracellular milieu face numerous challenges that can lead to protein misfolding and aggregation. As a checkpoint for proteins that move between cells, extracellular chaperone networks are of growing clinical relevance. J-domain proteins (JDPs) are ubiquitous molecular chaperones that are known for their essential roles in a wide array of fundamental cellular processes through their regulation of heat shock protein 70s. As the largest molecular chaperone family, JDPs have long been recognized for their diverse functions within cells. Some JDPs are elegantly selective for their “client proteins,” some do not discriminate among substrates and others act cooperatively on the same target. The realization that JDPs are exported through both classical and unconventional secretory pathways has fueled investigation into the roles that JDPs play in protein quality control and intercellular communication. The proposed functions of exported JDPs are diverse. Studies suggest that export of DnaJB11 enhances extracellular proteostasis, that intercellular movement of DnaJB1 or DnaJB6 enhances the proteostasis capacity in recipient cells, whereas the import of DnaJB8 increases resistance to chemotherapy in recipient cancer cells. In addition, the export of DnaJC5 and concurrent DnaJC5-dependent ejection of dysfunctional and aggregation-prone proteins are implicated in the prevention of neurodegeneration. This review provides a brief overview of the current understanding of the extracellular chaperone networks and outlines the first wave of studies describing the cellular export of JDPs.     

The relative efficiency of modular and non-modular networks of different size

Most biological networks are modular but previous work with small model networks has indicated that modularity does not necessarily lead to increased functional efficiency. Most biological networks are large, however, and here we examine the relative functional efficiency of modular and non-modular neural networks at a range of sizes. We conduct a detailed analysis of efficiency in networks of two size classes: ‘small’ and ‘large’, and a less detailed analysis across a range of network sizes. The former analysis reveals that while the modular network is less efficient than one of the two non-modular networks considered when networks are small, it is usually equally or more efficient than both non-modular networks when networks are large. The latter analysis shows that in networks of small to intermediate size, modular networks are much more efficient that non-modular networks of the same (low) connective density. If connective density must be kept low to reduce energy needs for example, this could promote modularity. We have shown how relative functionality/performance scales with network size, but the precise nature of evolutionary relationship between network size and prevalence of modularity will depend on the costs of connectivity.     

Chaperoning stem cells: a role for heat shock proteins in the modulation of stem cell self‐renewal and differentiation?

Self‐renewal and differentiation of stem cells are tightly regulated processes subject to intrinsic and extrinsic signals. Molecular chaperones and co‐chaperones, especially heat shock proteins (Hsp), are ubiquitous molecules involved in the modulation of protein conformational and complexation states. The function of Hsp, which are typically associated with stress response and tolerance, is well characterized in differentiated cells, while their role in stem cells remains unclear. It appears that embryonic stem cells exhibit increased stress tolerance and concomitant high levels of chaperone expression. This review critically evaluates stem cell research from a molecular chaperone perspective. Furthermore, we propose a model of chaperone‐modulated self‐renewal in mouse embryonic stem cells.     

Coevolution is a short-distance force at the protein interaction level and correlates with the modular organization of protein networks

We investigated what roles coevolution plays in shaping yeast protein interaction network (PIN). We found that the extent of coevolution between two proteins decreases rapidly as their interacting distance on the PIN increases, suggesting coevolutionary constraint is a short-distance force at the molecular level. We also found that protein-protein interactions (PPIs) with strong coevolution tend to be enriched in interconnected clusters, whereas PPIs with weak coevolution are more frequently present at inter-cluster region. The findings indicate the close relationship between coevolution and modular organization of PINs, and may provide insights into evolution and modularity of cellular networks.     

The Hsp70 chaperones of the Tritryps are characterized by unusual features and novel members

Proteins belonging to the Hsp70 class of molecular chaperones are highly conserved and ubiquitous, performing an essential role in the maintenance of cellular homeostasis in almost all known organisms. Trypanosoma brucei, Trypanosoma cruzi and Leishmania major are human parasites collectively known as the Tritryps. The Tritryps undergo extensive morphological changes during their life cycles, largely triggered by the marked differences between conditions in their insect vector and human host. Hsp70s are synthesised in response to these marked changes in environment and are proposed to be required for these parasites to successfully transition between differentiation stages while remaining viable and infective. While the Tritryps Hsp70 complement consists of homologues of all the major eukaryotic Hsp70s, there are a number of novel members, and some unique structural features. This review critically evaluates the current knowledge on the Tritryps Hsp70 proteins with an emphasis on T. brucei, and highlights some novel and previously unstudied aspects of these multifaceted molecular chaperones.     

The heat shock proteins: Their roles as multi-component machines for protein folding

The roles played by heat shock proteins in fungi have been subjected to intense scrutiny. Presuming they only played roles in tolerance to stress gave way to the realization that many of them were essential for maintenance of cell physiology under all conditions. Recent progress has revealed their action as multi-component machines, playing roles in signalling and expansion of phenotypic plasticity, as well as their well-established function as molecular chaperones.     

The RGS proteins add to the diversity of soybean heterotrimeric G-protein signaling

Regulator of G-protein signaling (RGS) proteins are a family of highly diverse, multifunctional proteins that function primarily as GTPase accelerating proteins (GAPs). RGS proteins increase the rate of GTP hydrolysis by Gα proteins and essentially regulate the duration of active signaling. Recently, we have identified two chimeric RGS proteins from soybean and reported their distinct GAP activities on individual Gα proteins. A single amino acid substitution (Alanine 357 to Valine) of RGS2 is responsible for differential GAP activity. Surprisingly, most monocot plant genomes do not encode for a RGS protein homolog. Here we discuss the soybean RGS proteins in the context of their evolution in plants, their relatedness to non-plant RGS protein homologs and the effect they might have on the heterotrimeric G-protein signaling mechanisms. We also provide experimental evidence to show that the interaction interface between plant RGS and Gα proteins is different from what is predicted based on mammalian models.     

Molecular motors: from one motor many tails to one motor many tales

Kinesin and dynein molecular motor proteins generate the movement of a wide variety of materials in cells. Such movements are crucial for many different cellular and developmental functions, including organelle movement, localization of developmental determinants, mitosis, meiosis and possibly long-range signaling in neurons. Kinesins that control the dynamics of microtubules have also been discovered. Recent work has begun to identify processes in which defective molecular motor function can cause human disease.