IbpB-bound substrate release in living cells as revealed by unnatural amino acid-mediated photo-crosslinking

Small heat shock proteins (sHSPs) are known to bind non‐native substrates and prevent irreversible aggregation in an ATP‐independent manner. However, the dynamic interaction between sHSPs and their substrates in vivo is less studied. Here, by utilizing a genetically incorporated crosslinker, we characterized the interaction between sHSP IbpB and its endogenous substrates in living cells. Through photo‐crosslinking analysis of five Bpa variants of IbpB, we found that the substrate binding of IbpB in living cells is reversible upon short‐time exposure at 50 °C. Our data provide in vivo evidence that IbpB engages in dynamic substrate release under nonstress conditions and suggest that photo‐crosslinking may be a suitable method for investigating dynamic interaction between molecular chaperones and their substrates in living cells.

Conditional Disorder in Small Heat-shock Proteins

Small heat-shock proteins (sHSPs) are molecular chaperones that respond to cellular stresses to combat protein aggregation. HSP27 is a critical human sHSP that forms large, dynamic oligomers whose quaternary structures and chaperone activities depend on environmental factors. Upon exposure to cellular stresses, such as heat shock or acidosis, HSP27 oligomers can dissociate into dimers and monomers, which leads to significantly enhanced chaperone activity. The structured core of the protein, the α-crystallin domain (ACD), forms dimers and can prevent the aggregation of substrate proteins to a similar degree as the full-length protein. When the ACD dimer dissociates into monomers, it partially unfolds and exhibits enhanced activity. Here, we used solution-state NMR spectroscopy to characterize the structure and dynamics of the HSP27 ACD monomer. Web show that the monomer is stabilized at low pH and that its backbone chemical shifts, 15N relaxation rates, and 1H-15N residual dipolar couplings suggest structural changes and rapid motions in the region responsible for dimerization. By analyzing the solvent accessible and buried surface areas of sHSP structures in the context of a database of dimers that are known to dissociate into disordered monomers, we predict that ACD dimers from sHSPs across all kingdoms of life may partially unfold upon dissociation. We propose a general model in which conditional disorder-the partial unfolding of ACDs upon monomerization-is a common mechanism for sHSP activity.

Elucidation of the molecular mechanism of heat shock proteins and its correlation with K722Q mutations in Lon protease

Cells withstand the effects of temperature change with the help of small heat shock proteins IbpA and IbpB. The IbpAB protein complex interacts with Lon protease in their free form and gets degraded at physiological temperature when there is no temperature stress. However, the proteolytic degradation of IbpAB is diminished when Lon is mutated. The mutation K722Q in Lon brings about some structural changes so that the proteolytic interactions between the heat shock proteins with that of the mutated Lon protease are lost. However, the detailed molecular aspects of the interactions are not yet fully understood. In the present, we made an attempt to analyze the biochemical aspects of the interactions between the small heat shock proteins IbpAB with wild type and mutant Lon protease. We for the first time deciphered the molecular details of the mechanism of interaction of small heat shock proteins with Lon protease bearing K722Q mutation i.e. the interaction pattern of heat shock proteins with mutant Lon protease at physiological temperature in absence of proteolytic machinery. Our study may therefore be useful to elucidate the mechanistic details of the correlation with IbpA, IbpB and Lon protease.

Protein plasticity underlines activation and function of ATP-independent chaperones

One of the key issues in biology is to understand how cells cope with protein unfolding caused by changes in their environment. Self-protection is the natural immediate response to any sudden threat and for cells the critical issue is to prevent aggregation of existing proteins. Cellular response to stress is therefore indistinguishably linked to molecular chaperones, which are the first line of defense and function to efficiently recognize misfolded proteins and prevent their aggregation. One of the major protein families that act as cellular guards includes a group of ATP-independent chaperones, which facilitate protein folding without the consumption of ATP. This review will present fascinating insights into the diversity of ATP-independent chaperones, and the variety of mechanisms by which structural plasticity is utilized in the fine-tuning of chaperone activity, as well as in crosstalk within the proteostasis network. Research into this intriguing class of chaperones has introduced new concepts of stress response to a changing cellular environment, and paved the way to uncover how this environment affects protein folding.

Chaperone function and mechanism of small heat-shock proteins

Small heat-shock proteins (sHSPs) are ubiquitous ATP-independent molecular chaperones that play crucial roles in protein quality control in cells. They are able to prevent the aggregation and/or inactivation of various non-native substrate proteins and assist the refolding of these substrates independently or under the help of other ATP-dependent chaperones. Substrate recognition and binding by sHSPs are essential for their chaperone functions. This review focuses on what natural substrate proteins an sHSP protects and how it binds the substrates in cells under fluctuating conditions. It appears that sHSPs of prokaryotes, although being able to bind a wide range of cellular proteins, preferentially protect certain classes of functional proteins, such as translation-related proteins and metabolic enzymes, which may well explain why they could increase the resistance of host cells against various stresses. Mechanistically, the sHSPs of prokaryotes appear to possess numerous multi-type substrate-binding residues and are able to hierarchically activate these residues in a temperature-dependent manner, and thus act as temperature-regulated chaperones. The mechanism of hierarchical activation of substrate-binding residues is also discussed regarding its implication for eukaryotic sHSPs.

Multilevel structural characteristics for the natural substrate proteins of bacterial small heat shock proteins

Small heat shock proteins (sHSPs) are ubiquitous molecular chaperones that prevent the aggregation of various non-native proteins and play crucial roles for protein quality control in cells. It is poorly understood what natural substrate proteins, with respect to structural characteristics, are preferentially bound by sHSPs in cells. Here we compared the structural characteristics for the natural substrate proteins of Escherichia coli IbpB and Deinococcus radiodurans Hsp20.2 with the respective bacterial proteome at multiple levels, mainly by using bioinformatics analysis. Data indicate that both IbpB and Hsp20.2 preferentially bind to substrates of high molecular weight or moderate acidity. Surprisingly, their substrates contain abundant charged residues but not abundant hydrophobic residues, thus strongly indicating that ionic interactions other than hydrophobic interactions also play crucial roles for the substrate recognition and binding of sHSPs. Further, secondary structure prediction analysis indicates that the substrates of low percentage of β-sheets or coils but high percentage of α-helices are un-favored by both IbpB and Hsp20.2. In addition, IbpB preferentially interacts with multi-domain proteins but unfavorably with α + β proteins as revealed by SCOP analysis. Together, our data suggest that bacterial sHSPs, though having broad substrate spectrums, selectively bind to substrates of certain structural features. These structural characteristic elements may substantially participate in the sHSP-substrate interaction and/or increase the aggregation tendency of the substrates, thus making the substrates more preferentially bound by sHSPs.

In vivo substrate diversity and preference of small heat shock protein IbpB as revealed by using a genetically incorporated photo-cross-linker

Small heat shock proteins (sHSPs), as ubiquitous molecular chaperones found in all forms of life, are known to be able to protect cells against stresses and suppress the aggregation of a variety of model substrate proteins under in vitro conditions. Nevertheless, it is poorly understood what natural substrate proteins are protected by sHSPs in living cells. Here, by using a genetically incorporated photo-cross-linker (p-benzoyl-l-phenylalanine), we identified a total of 95 and 54 natural substrate proteins of IbpB (an sHSP from Escherichia coli) in living cells with and without heat shock, respectively. Functional profiling of these proteins (110 in total) suggests that IbpB, although binding to a wide range of cellular proteins, has a remarkable substrate preference for translation-related proteins (e.g. ribosomal proteins and amino-acyl tRNA synthetases) and moderate preference for metabolic enzymes. Furthermore, these two classes of proteins were found to be more prone to aggregation and/or inactivation in cells lacking IbpB under stress conditions (e.g. heat shock). Together, our in vivo data offer novel insights into the chaperone function of IbpB, or sHSPs in general, and suggest that the preferential protection on the protein synthesis machine and metabolic enzymes may dominantly contribute to the well known protective effect of sHSPs on cell survival against stresses.

Small heat shock protein IbpB acts as a robust chaperone in living cells by hierarchically activating its multi-type substrate-binding residues

As ubiquitous molecular chaperones, small heat shock proteins (sHSPs) are crucial for protein homeostasis. It is not clear why sHSPs are able to bind a wide spectrum of non-native substrate proteins and how such binding is enhanced by heat shock. Here, by utilizing a genetically incorporated photo-cross-linker (p-benzoyl-l-phenylalanine), we systematically characterized the substrate-binding residues in IbpB (a sHSP from Escherichia coli) in living cells over a wide spectrum of temperatures (from 20 to 50 °C). A total of 20 and 48 residues were identified at normal and heat shock temperatures, respectively. They are not necessarily hydrophobic and can be classified into three types: types I and II were activated at low and normal temperatures, respectively, and type III mediated oligomerization at low temperature but switched to substrate binding at heat shock temperature. In addition, substrate binding of IbpB in living cells began at temperatures as low as 25 °C and was further enhanced upon temperature elevation. Together, these in vivo data provide novel structural insights into the wide substrate spectrum of sHSPs and suggest that sHSP is able to hierarchically activate its multi-type substrate-binding residues and thus act as a robust chaperone in cells under fluctuating growth conditions.

Phosphorylated Hsp27 activates ATM-dependent p53 signaling and mediates the resistance of MCF-7 cells to doxorubicin-induced apoptosis

DNA damage activates p53 and its downstream target genes, which further leads to apoptosis or survival either by the cell cycle arrest or by DNA repair. In many tumors, the heat shock protein 27 (Hsp27) is expressed at high levels to provide protection against anticancer drugs. However, the roles of Hsp27 in p53-mediated cellular responses to DNA damage are controversial. Here, we investigated the interplay between the phosphorylation status of Hsp27 and p53 in kidney 293A (HEK293A) cells and found that over-expressing phosphorylated Hsp27 mimics (Hsp27-3D) activated p53/p21 in an ATM-dependent manner. In addition, incubation with doxorubicin (Dox), an anticancer drug, induced Hsp27 phosphorylation in human adenocarcinoma cells (MCF-7). In contrast, inhibition of Hsp27 phosphorylation retarded both p53 induction and p21 accumulation, and led to cell apoptosis. Furthermore, phosphorylated Hsp27 increased p53 nuclear importing and its downstream target gene expression such as p21 and MDM2, while de-phosphorylated Hsp27 impeded this procession. Taken together, our data suggest that Hsp27, in its phosphorylated or de-phosphorylated status, plays different roles in regulating p53 pathway and cell survival.

Overexpression of a heat shock protein (ThHSP18.3) from Tamarix hispida confers stress tolerance to yeast

It is well known that plant heat shock proteins (HSPs) play important roles both in response to adverse environmental conditions and in various developmental processes. However, among plant HSPs, the functions of tree plant HSPs are poorly characterized. To improve our understanding of tree HSPs, we cloned and characterized an HSP gene (ThHSP18.3) from Tamarix hispida. Sequence alignment reveals that ThHSP18.3 belongs to the class I small heat shock protein family. A transient expression assay showed that ThHSP18.3 protein was targeted to the cell nucleus. Treatment of Tamarix hispida with cold and heat shock highly induced ThHSP18.3 expression in all studied leaves, roots and stems, whereas, treatment of T. hispida with NaCl, NaHCO(3), and PEG induced ThHSP18.3 expression in leaves and decreased its expression in roots and stems. Further, to study the role of ThHSP18.3 in stress tolerance under different stress conditions, we cloned ThHSP18.3 into the pYES2 vector, transformed and expressed the vector in yeast Saccharomyces cerevisiae. Yeast cells transformed with an empty pYES2 vector were employed as a control. Compared to the control, yeast cells expressing ThHSP18.3 showed greater tolerance to salt, drought, heavy metals, and both low and high temperatures, indicating that ThHSP18.3 confers tolerance to these stress conditions. These results suggested that ThHSP18.3 is involved in tolerance to a variety of stress conditions in T. hispida.

Hsp27 protects adenocarcinoma cells from UV-induced apoptosis by Akt and p21-dependent pathways of survival

Transcriptional activation of p53 target genes, due to DNA damage, causes either apoptosis or survival by cell cycle arrest and DNA repair. However, the regulators of the choice between cell death and survival signaling have not been completely elucidated. Here, we report that human adenocarcinoma cells (MCF-7) survive UV-induced DNA damage by heat shock protein 27 (Hsp27)-assisted Akt/p21 phosphorylation/translocation. Protein levels of the p53 target genes, such as p21, Bcl-2, p38MAPK, and Akt, showed a positive correlation to Hsp27 level during 48 hours postirradiation, whereas p53 expression increased initially but started decreasing after 12 hours. Hsp27 prevented the G(1)-S phase cell cycle arrest, observed after 8 hours of post-UV irradiation, and PARP-1 cleavage was inhibited. Conversely, silencing Hsp27 enhanced G(1)-S arrest and cell death. Moreover, use of either Hsp27 or Akt small interference RNA reduced p21 phosphorylation and enhanced its retention in nuclei even after 48 hours postirradiation, resulting in enhanced cell death. Our results showed that Hsp27 expression and its direct chaperoning interaction increases Akt stability, and p21 phosphorylation and nuclear-to-cytoplasm translocation, both essential effects for the survival of UV-induced DNA-damaged cells. We conclude that the role of Hsp27 in cancer is not only for enhanced p53 proteolysis per se, rather it is also a critical determinant in p21 phosphorylation and translocation.

Posttranslational modulation on the biological activities of molecular chaperones

Molecular chaperones are a family of proteins that were first noticed to exist about 45 years ago from their increased transcription under heat shock conditions. As a result, the regulation of their encoding genes has been subject to extensive studies. Recent studies revealed that the biological activities of molecular chaperones can also be effectively modulated at the protein level. The ways of modulation so far elucidated include allosteric effect, covalent modification, protein-protein interaction, and conformational alteration induced by such macro-environmental conditions as temperature and pH. These latter aspects were reviewed here. Emphasized here is the importance of such immediate structural alterations that lead to an immediate activity increase, providing the immediate protection needed for the cells to survive the stress conditions.