Complementation of an Escherichia coli DnaK defect by Hsc70-DnaK chimeric proteins

Human Hsp70 Substrate-Binding Domains Recognize Distinct Client Proteins

The 13 Hsp70 proteins in humans act on unique sets of substrates with diversity often being attributed to J-domain-containing protein (Hsp40 or JDP) cofactors. We were therefore surprised to find drastically different binding affinities for Hsp70-peptide substrates, leading us to probe substrate specificity among the 8 canonical Hsp70s from humans. We used peptide arrays to characterize Hsp70 binding and then mined these data using machine learning to develop an algorithm for isoform-specific prediction of Hsp70 binding sequences. The results of this algorithm revealed recognition patterns not predicted based on local sequence alignments. We then showed that none of the human isoforms can complement heat-shocked DnaK knockout Escherichia coli cells. However, chimeric Hsp70s consisting of the human nucleotide-binding domain and the substrate-binding domain of DnaK complement during heat shock, providing further evidence in vivo of the divergent function of the Hsp70 substrate-binding domains. We also demonstrated that the differences in heat shock complementation among the chimeras are not due to loss of DnaJ binding. Although we do not exclude JDPs as additional specificity factors, our data demonstrate substrate specificity among the Hsp70s, which has important implications for inhibitor development in cancer and neurodegeneration.

Insertion of GGMP repeat residues of Plasmodium falciparum Hsp70-1 in the lid of DnaK adversely impacts client recognition

Although Hsp70 is a conserved molecular chaperone, it exhibits some degree of functional specialisation across species. Features of Hsp70 regulating its functional specialisation remain to be fully established. We previously demonstrated that E. coli Hsp70 (DnaK) exhibits functional features that distinguishes it from PfHsp70-1, a canonical cytosolic Hsp70 of Plasmodium falciparum. One of the defining features of PfHsp70-1 is that it possesses GGMP repeat residues located in its C-terminal lid segment, while DnaK lacks this motif. Previously, we demonstrated that the insertion of GGMP repeat residues of PfHsp70-1 into E. coli DnaK abrogates the chaperone activity of DnaK. However, the role of the GGMP motif in regulating Hsp70 function remains to be fully understood. To explore the function of this motif, we expressed recombinant forms of wild type DnaK and its GGMP insertion motif, DnaK-G and systematically characterised the structure-function features of the two proteins using in silico analysis, biophysical approaches and an in cellulo complementation assay. Our findings demonstrated that the GGMP inserted in DnaK compromised various functional features such as nucleotide binding, allostery, substrate binding affinity and cellular proteome client selectivity. These findings thus, highlight the GGMP motif of Hsp70 as an important functional module.

Interdomain communication suppressing high intrinsic ATPase activity of Sse1 is essential for its co-disaggregase activity with Ssa1

In eukaryotes, Hsp110s are unambiguous cognates of the Hsp70 chaperones, in primary sequence, domain organization, and structure. Hsp110s function as nucleotide exchange factors (NEFs) for the Hsp70s although their apparent loss of Hsp70-like chaperone activity, nature of interdomain communication, and breadth of domain functions are still puzzling. Here, by combining single-molecule FRET, small angle X-ray scattering measurements (SAXS), and MD simulation, we show that yeast Hsp110, Sse1 lacks canonical Hsp70-like interdomain allostery. However, the protein exhibits unique noncanonical conformational changes within its domains. Sse1 maintains an open-lid substrate-binding domain (SBD) in close contact with its nucleotide-binding domain (NBD), irrespective of its ATP hydrolysis status. To further appreciate such ATP-hydrolysis-independent exhaustive interaction between two domains of Hsp110s, NBD-SBD chimera was constructed between Hsp110 (Sse1) and Hsp70 (Ssa1). In Sse1/Ssa1 chimera, we observed undocking of two domains leading to complete loss of NEF activity of Sse1. Interestingly, chimeric proteins exhibited significantly enhanced ATPase rate of Sse1-NBD compared to wild-type protein, implying that intrinsic ATPase activity of the protein remains mostly repressed. Apart from repressing the high ATPase activity of its NBD, interactions between two domains confer thermal stability to Sse1 and play critical role in the (co)chaperoning function of Sse1 in Ssa1-mediated disaggregation activity. Altogether, Sse1 exhibits a unique interdomain interaction, which is essential for its NEF activity, suppression of high intrinsic ATPase activity, co-chaperoning activity in disaggregase machinery, and stability of the protein.

DnaK protein alleviates toxicity induced by citrate-coated gold nanoparticles in Escherichia coli

A number of previously reported studies suggest that synthetic gold nanoparticles (AuNPs) are capable of stabilising proteins against heat stress in vitro. However, it remains to be understood if AuNPs confer stability to proteins against cellular stress in vivo. Heat shock proteins (Hsps) are conserved molecules whose main role is to facilitate folding of other proteins (chaperone function). Hsp70 (called DnaK in prokaryotes) is one of the most prominent molecular chaperones. Since gold nanoparticles exhibit chaperone-like function in vitro, we investigated the effect of citrate-coated gold nanoparticles on the growth of E. coli BB1553 cells that possess a deleted dnaK gene. We further investigated the effects of the AuNPs on the solubility of the E. coli BB1553 proteome. E. coli BB1553 cells exposed to AuNPs exhibited cellular defects such as filamentation and plasma membranes pulled off the cell wall. The toxic effects of the AuNPs were alleviated by transforming the E. coli BB1553 cells with a construct expressing DnaK. We also noted that cells in which DnaK was restored exhibited distinct zones to which the nanoparticles were restricted. Our study suggests a role for DnaK in alleviating nanoparticle induced stress in E. coli.

Comprehensive review on the HSC70 functions, interactions with related molecules and involvement in clinical diseases and therapeutic potential

Heat shock cognate protein 70 (HSC70) is a constitutively expressed molecular chaperone which belongs to the heat shock protein 70 (HSP70) family. HSC70 shares some of the structural and functional similarity with HSP70. HSC70 also has different properties compared with HSP70 and other heat shock family members. HSC70 performs its full functions by the cooperation of co-chaperones. It interacts with many other molecules as well and regulates various cellular functions. It is also involved in various diseases and may become a biomarker for diagnosis and potential therapeutic targets for design, discovery, and development of novel drugs to treat various diseases. In this article, we provide a comprehensive review on HSC70 from the literatures including the basic general information such as classification, structure and cellular location, genetics and function, as well as its protein association and interaction with other proteins. In addition, we also discussed the relationship of HSC70 and related clinical diseases such as cancer, cardiovascular, neurological, hepatic and many other diseases and possible therapeutic potential and highlight the progress and prospects of research in this field. Understanding the functions of HSC70 and its interaction with other molecules will help us to reveal other novel properties of this protein. Scientists may be able to utilize this protein as a biomarker and therapeutic target to make significant advancement in scientific research and clinical setting in the future.

Conserved inserts in the Hsp60 (GroEL) and Hsp70 (DnaK) proteins are essential for cellular growth

The Hsp60 and Hsp70 chaperones contain a number of conserved inserts that are restricted to particular phyla of bacteria. A one aa insert in the E. coli GroEL and a 21-23 insert in the DnaK proteins are specific for most Gram-negative bacteria. Two other inserts in DnaK are limited to certain groups of proteobacteria. The requirement of these inserts for cellular growth was examined by carrying out complementation studies with temperature-sensitive (T(s)) mutants of E. coli groEL or dnaK. Our results demonstrate that deletion or most changes in these inserts completely abolished the complementation ability of the mutant proteins. Studies with GroEL and DnaK from some other species that either lacked or contained these inserts also indicated that these inserts are essential for growth of E. coli. The DnaK from some bacteria contains a two aa insert that is not found in E. coli. Introduction of this insert into the E. coli DnaK also led to its inactivation, indicating that these inserts are specific for different groups. We postulate that these conserved inserts that are localized in loop regions on protein surfaces, are involved in some ancillary functions that are essential for the groups of bacteria where they are found.

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.

Plasmodium falciparum heat shock protein 70 is able to suppress the thermosensitivity of an Escherichia coli DnaK mutant strain

Heat shock protein 70 (Hsp 70) and heat shock protein 40 (Hsp 40) are molecular chaperones that ensure that the proteins of the cell are properly folded and functional under both normal and stressful conditions. The malaria parasite Plasmodium falciparum is known to overproduce a heat shock protein 70 (PfHsp 70) in response to thermal stress; however, the in vivo function of this protein still needs to be explored. Using in vivo complementation assays, we found that PfHsp 70 was able to suppress the thermosensitivity of an Escherichia coli dnaK 756 strain, but not that of the corresponding deletion strain (DeltadnaK 52) or dnaK 103 strain, which produces a truncated DnaK. Constructs were generated that encoded the ATPase domain of PfHsp 70 fused to the substrate-binding domain (SBD) of E. coli DnaK (referred to as PfK), and the ATPase domain of E. coli DnaK coupled to the SBD of PfHsp 70 (KPf). PfK was unable to suppress the thermosensitivity of any of the E. coli strains. In contrast, KPf was able to suppress the thermosensitivity in the E. coli dnaK 756 strain. We also identified two key amino acid residues (V 401 and Q 402) in the linker region between the ATPase domain and SBD that are essential for the in vivo function of PfHsp 70. This is the first example of an Hsp70 from a eukaryotic parasite that can suppress thermosensitivity in a prokaryotic system. In addition, our results also suggest that interdomain communication is critical for the function of the PfHsp 70 and PfHsp 70-DnaK chimeras. We discuss the implications of these data for the mechanism of action of the Hsp70-Hsp 40 chaperone machinery.

Conformational properties of bacterial DnaK and yeast mitochondrial Hsp70

Among the eukaryotic members of the Hsp70 family, mitochondrial Hsp70 shows the highest degree of sequence identity with bacterial DnaK. Although they share a functional mechanism and homologous co-chaperones, they are highly specific and cannot be exchanged between Escherichia coli and yeast mitochondria. To provide a structural basis for this finding, we characterized both proteins, as well as two DnaK/mtHsp70 chimeras constructed by domain swapping, using biochemical and biophysical methods. Here, we show that DnaK and mtHsp70 display different conformational and biochemical properties. Replacing different regions of the DnaK peptide-binding domain with those of mtHsp70 results in chimeric proteins that: (a) are not able to support growth of an E. coli DnaK deletion strain at stress temperatures (e.g. 42 degrees C); (b) show increased accessibility and decreased thermal stability of the peptide-binding pocket; and (c) have reduced activation by bacterial, but not mitochondrial co-chaperones, as compared with DnaK. Importantly, swapping the C-terminal alpha-helical subdomain promotes a conformational change in the chimeras to an mtHsp70-like conformation. Thus, interaction with bacterial co-chaperones correlates well with the conformation that natural and chimeric Hsp70s adopt in solution. Our results support the hypothesis that a specific protein structure might regulate the interaction of Hsp70s with particular components of the cellular machinery, such as Tim44, so that they perform specific functions.