Domain structure of the HSC70 cochaperone, HIP

Nucleotide Exchange Factors for Hsp70 Molecular Chaperones: GrpE, Hsp110/Grp170, HspBP1/Sil1, and BAG Domain Proteins

Molecular chaperones of the Hsp70 family are key components of the cellular protein-folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis, and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEFs) facilitate the conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. NEF function can additionally be antagonized by ADP dissociation inhibitors. Beginning with the discovery of the prototypical bacterial NEF, GrpE, a large diversity of nucleotide exchange factors for Hsp70 have been identified, connecting it to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances toward structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1, and BAG domain protein families and discuss how these cochaperones connect protein folding with cellular quality control and degradation pathways.

GrpE, Hsp110/Grp170, HspBP1/Sil1 and BAG domain proteins: nucleotide exchange factors for Hsp70 molecular chaperones

Molecular chaperones of the Hsp70 family are key components of the cellular protein folding machinery. Substrate folding is accomplished by iterative cycles of ATP binding, hydrolysis and release. The ATPase activity of Hsp70 is regulated by two main classes of cochaperones: J-domain proteins stimulate ATPase hydrolysis by Hsp70, while nucleotide exchange factors (NEF) facilitate its conversion from the ADP-bound to the ATP-bound state, thus closing the chaperone folding cycle. Beginning with the discovery of the prototypical bacterial NEF GrpE, a large diversity of Hsp70 nucleotide exchange factors has been identified, connecting Hsp70 to a multitude of cellular processes in the eukaryotic cell. Here we review recent advances towards structure and function of nucleotide exchange factors from the Hsp110/Grp170, HspBP1/Sil1 and BAG domain protein families and discuss how these cochaperones connect protein folding with quality control and degradation pathways.

Effects of HIP in protection of HSP70 for stress-induced cardiomyocytes injury and its glucorticoid receptor pathway

Moderate levels of stress can be beneficial to health, while stress overload can cause injury or contribute to diseases. Despite a number of studies of adaptation or stress damage, the mechanisms of adaptation and stress damage remain far from clear. The effect and mechanisms of adaptation on cardiomyocytes damage caused by stress overload are discussed in this study. Data showed that mild repeated stress mitigated stress overload-induced cardiomyocyte injury both in an animal model of restraint stress and in H9C2 cells with GC (glucocorticoid) treatment. HSP70, HIP expression and interaction between HSP70 and HIP increased during adaptation induced by mild stress both in animals and H9C2 cells. Overexpression or inhibition of HSP70 in H9C2 cells with pCDNA-3.1-Hsp70 or KNK437 (HSP70 inhibitor) showed that HSP70 can protect H9C2 cells from GC-induced cell damage. A luciferase assay showed that Hsp70 plays its protective role through inhibition of GR transcription activity dependent on the interaction with HIP. These results indicated that HSP70 may promote adaptation with its interacting protein HIP, and increased levels of HSP70 and its interacting protein HIP during adaptation may play a protective role on stress-overload-induced cardiomyocyte injury.

Structure and function of Hip, an attenuator of the Hsp70 chaperone cycle

The Hsp70-interacting protein, Hip, cooperates with the chaperone Hsp70 in protein folding and prevention of aggregation. Hsp70 interacts with non-native protein substrates in an ATP-dependent reaction cycle regulated by J-domain proteins and nucleotide exchange factors (NEFs). Hip is thought to delay substrate release by slowing ADP dissociation from Hsp70. Here we present crystal structures of the dimerization domain and the tetratricopeptide repeat (TPR) domain of rat Hip. As shown in a cocrystal structure, the TPR core of Hip interacts with the Hsp70 ATPase domain through an extensive interface, to form a bracket that locks ADP in the binding cleft. Hip and NEF binding to Hsp70 are mutually exclusive, and thus Hip attenuates active cycling of Hsp70-substrate complexes. This mechanism explains how Hip enhances aggregation prevention by Hsp70 and facilitates transfer of specific proteins to downstream chaperones or the proteasome.

The Ogura sterility-inducing protein forms a large complex without interfering with the oxidative phosphorylation components in rapeseed mitochondria

The Ogura cytoplasmic male sterility causing protein, ORF138, was found to be part of a complex with an apparent size of over 750 kDa in the inner membrane of mitochondria of sterile plants. ORF138 did not colocalize with any of the oxidative phosphorylation complexes, nor did its presence modify their apparent size or amount, compared to samples from fertile isogenic plants. We attempted to detect potential proteins or nucleic acids that could be involved in the large ORF138 complex by 2D PAGE, immunoprecipitation and nuclease treatments of native extracts. All our results suggest that the ORF138 protein is the main, if not only, component of this large complex. The capacities of complexes I, II, IV, and ATP synthase were identical in samples from sterile and fertile plants. Isolated mitochondria from sterile plants showed a higher oxygen consumption than those from fertile plants. In vivo respiration measurements suggest that the difference in O(2) consumption measured at the organelle level is compensated at the cell/tissue level, completely in leaves, but only partially in male reproductive organs.

Duplicate gene evolution toward multiple fates at the Drosophila melanogaster HIP/HIP-Replacement locus

Hsc/Hsp70-interacting protein (HIP) is a rapidly evolving Hsp70 cofactor. Analyses of multiple Drosophila species indicate that the HIP gene is duplicated only in D. melanogaster. The HIP region, in fact, contains seven distinctly evolving duplicated genes. The regional duplication occurred in two steps, fixed rapidly, and illustrates multiple modes of duplicate gene evolution. HIP and its duplicate HIP-R are adaptively evolving in a manner unique to the region: they exhibit elevated divergence from other drosophilids and low polymorphism within D. melanogaster. HIP and HIP-R are virtually identical, share polymorphisms, and are subject to gene conversion. In contrast, two other duplicate genes in the region, CG33221 and GP-CG32779, are pseudogenes, and the chimeric gene Crg1 is subject to balancing selection. HIP and HIP-R are evolving rapidly and adaptively; however, positive selection is not sufficient to explain the molecular evolution of the region as a whole.

Hip Is a Pro-survival Substrate of Granzyme B

The extended substrate specificity of granzyme B (GrB) was used to identify substrates among the chaperone superfamily. This approach identified Hsp90 and Bag1-L as novel GrB substrates, and an additional GrB cleavage site was identified in the Hsc70/Hsp70-Interacting Protein, Hip. Hsp90, Bag1L, and Hip were validated as GrB substrates in vitro, and mutational analysis confirmed the additional cleavage site in Hip. Because the role of Hip in apoptosis is unknown, its proteolysis by GrB was used as a basis to test whether it has anti-apoptotic activity. Previous work on Hip was limited to in vitro characterization; therefore, it was important to demonstrate Hip cleavage in a physiological context and to show its relevance to natural killer (NK) cell-mediated death. Hip is cleaved at both GrB cleavage sites during NK-mediated cell death in a caspase-independent manner, and its cleavage is due solely to GrB and not other granule components. Furthermore, Hip is not cleaved upon stimulation of the Fas receptor in the Jurkat T-cell line, suggesting that Hip is a substrate unique to GrB. RNA interference-mediated reduction of Hip within the K562 cell line rendered the cells more susceptible to NK cell-mediated lysis, indicating that proteolysis by GrB of Hip contributes to death induction. The small effect of RNA interference-mediated Hip deficiency on cytotoxicity is in agreement with the inherent redundancy of NK cell-mediated cell death. The identification of additional members of the chaperone superfamily as GrB substrates and the validation of Hip as an anti-apoptotic protein contribute to understanding the interplay between stress response and apoptosis.

Heat shock protein 70 interacts with aquaporin-2 and regulates its trafficking

The trafficking of aquaporin-2 (AQP2) involves multiple complex pathways, including regulated, cAMP-, and cGMP-mediated pathways, as well as a constitutive recycling pathway. Although several accessory proteins have been indirectly implicated in AQP2 recycling, the direct protein-protein interactions that regulate this process remain largely unknown. Using yeast two-hybrid screening of a human kidney cDNA library, we have identified the 70-kDa heat shock proteins as AQP2-interacting proteins. Interaction was confirmed by mass spectrometry of proteins pulled down from rat kidney papilla extract using a GST-AQP2 C-terminal fusion protein (GST-A2C) as a bait, by co-immunoprecipitation (IP) assays, and by direct binding assays using purified hsc70 and the GST-A2C. The direct interaction of AQP2 with hsc70 is partially inhibited by ATP, and the Ser-256 residue in the AQP2 C terminus is important for this direct interaction. Vasopressin stimulation in cells enhances the interaction of hsc70 with AQP2 in IP assays, and vasopressin stimulation in vivo induces an increased co-localization of hsc70 and AQP2 on the apical membrane of principal cells in rat kidney collecting ducts. Functional knockdown of hsc70 activity in AQP2 expressing cells results in membrane accumulation of AQP2 and reduced endocytosis of rhodamine-transferrin. Our data also show that AQP2 interacts with hsp70 in multiple in vitro binding assays. Finally, in addition to hsc70 and hsp70, AQP2 interacts with several other key components of the endocytotic machinery in co-IP assays, including clathrin, dynamin, and AP2. To summarize, we have identified the 70-kDa heat shock proteins as a AQP2 interactors and have shown for hsc70 that this interaction is involved in AQP2 trafficking.

Functional similarity between the chloroplast translocon component, Tic40, and the human co-chaperone, Hsp70-interacting protein (Hip)

Tic40 is a component of the protein import apparatus of the inner envelope of chloroplasts, but its role in the import mechanism has not been clearly defined. The C terminus of Tic40 shares weak similarity with the C-terminal Sti1 domains of the mammalian Hsp70-interacting protein (Hip) and Hsp70/Hsp90-organizing protein (Hop) co-chaperones. Additionally, Tic40 may possess a tetratricopeptide repeat (TPR) protein-protein interaction domain, another characteristic feature of Hip/Hop co-chaperones. To investigate the functional importance of different parts of the Tic40 protein and to determine whether the homology between Tic40 and co-chaperones is functionally significant, different Tic40 deletion and Tic40:Hip fusion constructs were generated and assessed for complementation activity in the Arabidopsis Tic40 knock-out mutant, tic40. Interestingly, all Tic40 deletion constructs failed to complement tic40, indicating that each part removed is essential for Tic40 function; these included a construct lacking the Sti1-like domain (DeltaSti1), a second lacking a central region, including the putative TPR domain (DeltaTPR), and a third lacking the predicted transmembrane anchor region. Moreover, the DeltaSti1 and DeltaTPR constructs caused strong dominant-negative, albino phenotypes in tic40 transformants, indicating that the truncated Tic40 proteins interfere with the residual chloroplast protein import that occurs in tic40 plants. Remarkably, the Tic40:Hip fusion constructs showed that the Sti1 domain of human Hip is functionally equivalent to the Sti1-like region of Tic40, strongly suggesting a co-chaperone role for the Tic40 protein. Supporting this notion, yeast two-hybrid and bimolecular fluorescence complementation assays demonstrated the in vivo interaction of Tic40 with Tic110, a protein believed to recruit stromal chaperones to protein import sites.

What we know about ST13, a co-factor of heat shock protein, or a tumor suppressor?

This article is to summarize the molecular and functional analysis of the gene "suppression of tumorigenicity 13" (ST13). ST13 is in fact the gene encoding Hsp70 interacting protein (Hip), a co-factor (co-chaperone) of the 70-kDa heat shock proteins (Hsc/Hsp70). By collaborating with other positive co-factors such as Hsp40 and the Hsp70-Hsp90 organizing protein (Hop), or competing with negative co-factors such as Bcl2-associated athanogen 1 (Bag1), Hip facilitates may facilitate the chaperone function of Hsc/Hsp70 in protein folding and repair, and in controlling the activity of regulatory proteins such as steroid receptors and regulators of proliferation or apoptosis. Although the nomenclature of ST13 implies a role in the suppression of tumorigenicity (ST), to date available experimental data are not sufficient to support its role in cancer development, except for the possible down-regulation of ST13 in gastric and colorectal cancers. Further investigation of this gene at the physiological level would benefit our understanding of diseases such as endocrinological disorders, cancer, and neurodegeneration commonly associated with protein misfolding.

Hsp70 Chaperone as a Survival Factor in Cell Pathology

Heat shock protein Hsp70 is implicated in the mechanism of cell reaction to a variety of cytotoxic factors. The protective function of Hsp70 is related to its ability to promote folding of nascent polypeptides and to remove denatured proteins. Many types of cancer cells contain high amounts of Hsp70, whose protective capacity may pose a problem for therapy in oncology. Hsp70 was shown to be expressed on the surface of cancer cells and to participate in the presentation of tumor antigens to immune cells. Therefore, the chaperone activity of Hsp70 is an important factor that should be taken into consideration in cancer therapy. The protective role of Hsp70 is also evident in neuropathology. Many neurodegenerative processes are associated with the accumulation of insoluble aggregates of misfolded proteins in neural cells. These aggregates hamper intracellular transport, inhibit metabolism, and activate apoptosis through diverse pathways. The increase of Hsp70 content results in the reduction of aggregate size and number and ultimately enhances cell viability.

Genome-scale expression of proteins from Bacillus subtilis

We have applied high throughput methods for cloning and expression of more than 850 genes from the Bacillus subtilis genome. The process uses 96-well plates and is automated from the level of primer design to the detection of soluble protein by a tag detection screen. This process was applied to a set of cytoplasmic targets from Bacillus subtilis to produce clones expressing soluble protein for incorporation into the structure determination pipeline of the Midwest Center for Structural Genomics. We also evaluated the feasibility of these plate-based methods for domain-based cloning and expression of secretory proteins and putative soluble domains of membrane proteins. This approach shows promise for implementation in a high throughput format and could provide additional target resources for structure determination. The continued development of new technologies that can be implemented in an automated format will be essential for continued success in the structural genomic programs.

A brain-specific isoform of small glutamine-rich tetratricopeptide repeat-containing protein binds to Hsc70 and the cysteine string protein

Small glutamine-rich tetratricopeptide repeat-containing protein (SGT) is a ubiquitously expressed cochaperone of heat shock cognate protein of 70 kDa (Hsc70). SGT binds to the C terminus of Hsc70, a site used by several tetratricopeptide repeat-containing binding partners to recruit Hsc70 into complexes of diverse function. We describe here an isoform of SGT with 60% amino acid sequence identity that we name betaSGT. In contrast to the previously published alphaSGT, betaSGT is almost exclusively expressed in brain. Both isoforms of SGT possess similar binding properties toward Hsc70 and cysteine string protein, a synaptic vesicle-associated J-domain-containing protein. In addition, SGTs oligomerize without preferences among isoforms. The distribution of protein binding motifs on SGTs reveals a modular structure. The N-terminal domains mediate oligomerization. Binding to Hsc70 is impaired by mutations of basic residues within the central tetratricopeptide repeat domain of betaSGT, indicating a two-carboxylate clamp as the binding mode. The tetratricopeptide repeats are also necessary for binding to the cysteine string protein. However, this binding mode is distinct from the two-carboxylate clamp that is involved in Hsc70 binding. The C-terminal regions of SGTs might constitute independent protein interaction domains. We conclude that betaSGT is likely to cooperate with alphaSGT as co-chaperone of Hsc70 in the brain. The modular structure of SGTs allows them to recruit client proteins to Hsc70 and to direct the resulting complex toward downstream proteins that take over the respective client proteins.

Tic40, a membrane-anchored co-chaperone homolog in the chloroplast protein translocon

The function of Tic40 during chloroplast protein import was investigated. Tic40 is an inner envelope membrane protein with a large hydrophilic domain located in the stroma. Arabidopsis null mutants of the atTic40 gene were very pale green and grew slowly but were not seedling lethal. Isolated mutant chloroplasts imported precursor proteins at a lower rate than wild-type chloroplasts. Mutant chloroplasts were normal in allowing binding of precursor proteins. However, during subsequent translocation across the inner membrane, fewer precursors were translocated and more precursors were released from the mutant chloroplasts. Cross-linking experiments demonstrated that Tic40 was part of the translocon complex and functioned at the same stage of import as Tic110 and Hsp93, a member of the Hsp100 family of molecular chaperones. Tertiary structure prediction and immunological studies indicated that the C-terminal portion of Tic40 contains a TPR domain followed by a domain with sequence similarity to co-chaperones Sti1p/Hop and Hip. We propose that Tic40 functions as a co-chaperone in the stromal chaperone complex that facilitates protein translocation across the inner membrane.

Comparison of the carboxy-terminal DP-repeat region in the co-chaperones Hop and Hip

Functional steroid receptor complexes are assembled and maintained by an ordered pathway of interactions involving multiple components of the cellular chaperone machinery. Two of these components, Hop and Hip, serve as co-chaperones to the major heat shock proteins (Hsps), Hsp70 and Hsp90, and participate in intermediate stages of receptor assembly. In an effort to better understand the functions of Hop and Hip in the assembly process, we focused on a region of similarity located near the C-terminus of each co-chaperone. Contained within this region is a repeated sequence motif we have termed the DP repeat. Earlier mutagenesis studies implicated the DP repeat of either Hop or Hip in Hsp70 binding and in normal assembly of the co-chaperones with progesterone receptor (PR) complexes. We report here that the DP repeat lies within a protease-resistant domain that extends to or is near the C-terminus of both co-chaperones. Point mutations in the DP repeats render the C-terminal regions hypersensitive to proteolysis. In addition, a Hop DP mutant displays altered proteolytic digestion patterns, which suggest that the DP-repeat region influences the folding of other Hop domains. Although the respective DP regions of Hop and Hip share sequence and structural similarities, they are not functionally interchangeable. Moreover, a double-point mutation within the second DP-repeat unit of Hop that converts this to the sequence found in Hip disrupts Hop function; however, the corresponding mutation in Hip does not alter its function. We conclude that the DP repeats are important structural elements within a C-terminal domain, which is important for Hop and Hip function.

Checkpoint kinase 2 (Chk2) monomers or dimers phosphorylate Cdc25C after DNA damage regardless of threonine 68 phosphorylation

We have purified and characterized human Chk2 both from baculovirus-infected insect cells and from either untreated or DNA damage-stressed human HCT116 cells. Chk2 from unstressed human cells is largely monomeric and inactive in phosphorylating its substrate, Cdc25C. It is also unphosphorylated at Thr-68, a site that is the target of the ataxia telangiectasia-mutated protein kinase. After treatment of HCT116 cells with a radiomimetic compound neocarzinostatin, active Chk2 exists as stable Thr-68-phosphorylated dimers as well as interconvertable Thr-68-unphosphorylated monomers and dimers. Interestingly, Chk2 from insect cells behaves by all criteria tested like active Chk2 from neocarzinostatin-treated HCT116 cells. Based on Stokes radius and sedimentation coefficient values, Chk2 monomers and dimers have asymmetric rather than globular shapes. Both Thr-68-phosphorylated and Thr-68-unphosphorylated forms of active Chk2 are capable of phosphorylating Cdc25C. Thus, although phosphorylation of Thr-68 may be required for initial oligomerization and activation of Chk2, it is not needed for maintenance of dimerization or kinase activity.