Modulation of the ATPase cycle of BiP by peptides and proteins

Mechanisms of Protein Quality Control in the Endoplasmic Reticulum by a Coordinated Hsp40-Hsp70-Hsp90 System

The Hsp40, Hsp70, and Hsp90 chaperone families are ancient, highly conserved, and critical to cellular protein homeostasis. Hsp40 chaperones can transfer their protein clients to Hsp70, and Hsp70 can transfer clients to Hsp90, but the functional benefits of these transfers are unclear. Recent structural and mechanistic work has opened up the possibility of uncovering how Hsp40, Hsp70, and Hsp90 work together as unified system. In this review, we compile mechanistic data on the ER J-domain protein 3 (ERdj3) (an Hsp40), BiP (an Hsp70), and Grp94 (an Hsp90) chaperones within the endoplasmic reticulum; what is known about how these chaperones work together; and gaps in this understanding. Using calculations, we examine how client transfer could impact the solubilization of aggregates, the folding of soluble proteins, and the triage decisions by which proteins are targeted for degradation. The proposed roles of client transfer among Hsp40-Hsp70-Hsp90 chaperones are new hypotheses, and we discuss potential experimental tests of these ideas.

α-Mangostin Promotes In Vitro and In Vivo Degradation of Androgen Receptor and AR-V7 Splice Variant in Prostate Cancer Cells

A major limitation of current prostate cancer pharmacotherapy approaches is the inability of these compounds to target androgen receptor variants or mutants that develop during prostate cancer progression. The demand for novel therapeutics to prevent, slow, and treat prostate cancer is significant because FDA approved anti-androgens are associated with adverse events and can eventually drive drug-resistant prostate cancer. This study evaluated α-mangostin for its novel ability to degrade the androgen receptor and androgen receptor variants. α-Mangostin is one of more than 70 isoprenylated xanthones isolated from Garcinia mangostana that we have been evaluating for their anticancer potential. Prostate cancer cells treated with α-mangostin exhibited decreased levels of wild-type and mutated androgen receptors. Immunoblot, immunoprecipitation, and transfection experiments demonstrated that the androgen receptor was ubiquitinated and subsequently degraded via the proteasome, which we hypothesize occurs with the assistance of BiP, an ER chaperone protein that we have shown to associate with the androgen receptor. We also evaluated α-mangostin for its antitumor activity and promotion of androgen receptor degradation in vivo. In summary, our study demonstrates that androgen receptor degradation occurs through the novel activation of BiP and suggests a new therapeutic approach for prostate cancer.

The broad range di- and trinucleotide exchanger SLC35B1 displays asymmetrical affinities for ATP transport across the ER membrane

In eukaryotic cells, uptake of cytosolic ATP into the endoplasmic reticulum (ER) lumen is critical for the proper functioning of chaperone proteins. The human transport protein SLC35B1 was recently postulated to mediate ATP/ADP exchange in the ER; however, the underlying molecular mechanisms mediating ATP uptake are not completely understood. Here, we extensively characterized the transport kinetics of human SLC35B1 expressed in yeast that was purified and reconstituted into liposomes. Using [α³²P]ATP uptake assays, we tested the nucleotide concentration dependence of ATP/ADP exchange activity on both sides of the membrane. We found that the apparent affinities of SLC35B1 for ATP/ADP on the internal face were approximately 13 times higher than those on the external side. Because SLC35B1-containing liposomes were preferentially inside-out oriented, these results suggest a low-affinity external site and a high-affinity internal site in the ER. Three different experimental approaches indicated that ATP/ADP exchange by SLC35B1 was not strict, and that other di- and tri-nucleotides could act as suitable counter-substrates for ATP, although mononucleotides and nucleotide sugars were not transported. Finally, bioinformatic analysis and site-directed mutagenesis identified that conserved residues K117 and K120 from transmembrane helix 4 and K277 from transmembrane helix 9 play critical roles in transport. The fact that SLC35B1 can promote ATP transport in exchange for ADP or UDP suggest a more direct coupling between ATP import requirements and the need for eliminating ADP and UDP, which are generated as side products of reactions taking place in the ER-lumen.

Calcium depletion challenges endoplasmic reticulum proteostasis by destabilising BiP-substrate complexes

The metazoan endoplasmic reticulum (ER) serves both as a hub for maturation of secreted proteins and as an intracellular calcium storage compartment, facilitating calcium-release-dependent cellular processes. ER calcium depletion robustly activates the unfolded protein response (UPR). However, it is unclear how fluctuations in ER calcium impact organellar proteostasis. Here, we report that calcium selectively affects the dynamics of the abundant metazoan ER Hsp70 chaperone BiP, by enhancing its affinity for ADP. In the calcium-replete ER, ADP rebinding to post-ATP hydrolysis BiP-substrate complexes competes with ATP binding during both spontaneous and co-chaperone-assisted nucleotide exchange, favouring substrate retention. Conversely, in the calcium-depleted ER, relative acceleration of ADP-to-ATP exchange favours substrate release. These findings explain the rapid dissociation of certain substrates from BiP observed in the calcium-depleted ER and suggest a mechanism for tuning ER quality control and coupling UPR activity to signals that mobilise ER calcium in secretory cells.

Pan-active imidazolopiperazine antimalarials target the Plasmodium falciparum intracellular secretory pathway

A promising new compound class for treating human malaria is the imidazolopiperazines (IZP) class. IZP compounds KAF156 (Ganaplacide) and GNF179 are effective against Plasmodium symptomatic asexual blood-stage infections, and are able to prevent transmission and block infection in animal models. But despite the identification of resistance mechanisms in P. falciparum, the mode of action of IZPs remains unknown. To investigate, we here combine in vitro evolution and genome analysis in Saccharomyces cerevisiae with molecular, metabolomic, and chemogenomic methods in P. falciparum. Our findings reveal that IZP-resistant S. cerevisiae clones carry mutations in genes involved in Endoplasmic Reticulum (ER)-based lipid homeostasis and autophagy. In Plasmodium, IZPs inhibit protein trafficking, block the establishment of new permeation pathways, and cause ER expansion. Our data highlight a mechanism for blocking parasite development that is distinct from those of standard compounds used to treat malaria, and demonstrate the potential of IZPs for studying ER-dependent protein processing.

Activity of the yeast cytoplasmic Hsp70 nucleotide-exchange factor Fes1 is regulated by reversible methionine oxidation

Cells employ a vast network of regulatory pathways to manage intracellular levels of reactive oxygen species (ROS). An effectual means used by cells to control these regulatory systems are sulfur-based redox switches, which consist of protein cysteine or methionine residues that become transiently oxidized when intracellular ROS levels increase. Here, we describe a methionine-based oxidation event involving the yeast cytoplasmic Hsp70 co-chaperone Fes1. We show that Fes1 undergoes reversible methionine oxidation during excessively-oxidizing cellular conditions, and we map the site of this oxidation to a cluster of three methionine residues in the Fes1 core domain. Making use of recombinant proteins and a variety of in vitro assays, we establish that oxidation inhibits Fes1 activity and, correspondingly, alters Hsp70 activity. Moreover, we demonstrate in vitro and in cells that Fes1 oxidation is reversible and is regulated by the cytoplasmic methionine sulfoxide reductase Mxr1 (MsrA) and a previously unidentified cytoplasmic pool of the reductase Mxr2 (MsrB). We speculate that inactivation of Fes1 activity during excessively-oxidizing conditions may help maintain protein-folding homeostasis in a suboptimal cellular folding environment. The characterization of Fes1 oxidation during cellular stress provides a new perspective as to how the activities of the cytoplasmic Hsp70 chaperones may be attuned by fluctuations in cellular ROS levels and provides further insight into how cells use methionine-based redox switches to sense and respond to oxidative stress.

The endoplasmic reticulum (ER) chaperones BiP and Grp94 selectively associate when BiP is in the ADP conformation

Hsp70 and Hsp90 chaperones are critical for protein quality control in the cytosol, whereas organelle-specific Hsp70/Hsp90 paralogs provide similar protection for mitochondria and the endoplasmic reticulum (ER). Cytosolic Hsp70/Hsp90 can operate sequentially with Hsp90 selectively associating with Hsp70 after Hsp70 is bound to a client protein. This observation has long suggested that Hsp90 could have a preference for interacting with clients at their later stages of folding. However, recent work has shown that cytosolic Hsp70/Hsp90 can directly interact even in the absence of a client, which opens up an alternative possibility that the ordered interactions of Hsp70/Hsp90 with clients could be a consequence of regulated changes in the direct interactions between Hsp70 and Hsp90. However, it is unknown how such regulation could occur mechanistically. Here, we find that the ER Hsp70/Hsp90 (BiP/Grp94) can form a direct complex in the absence of a client. Importantly, the direct interaction between BiP and Grp94 is nucleotide-specific, with BiP and Grp94 having higher affinity under ADP conditions and lower affinity under ATP conditions. We show that this nucleotide-specific association between BiP and Grp94 is largely due to the conformation of BiP. When BiP is in the ATP conformation its substrate-binding domain blocks Grp94; in contrast, Grp94 can readily associate with the ADP conformation of BiP, which represents the client-bound state of BiP. Our observations provide a mechanism for the sequential involvement of BiP and Grp94 in client folding where the conformation of BiP provides the signal for the subsequent recruitment of Grp94.

Functional principles and regulation of molecular chaperones

To be able to perform their biological function, a protein needs to be correctly folded into its three dimensional structure. The protein folding process is spontaneous and does not require the input of energy. However, in the crowded cellular environment where there is high risk of inter-molecular interactions that may lead to protein molecules sticking to each other, hence forming aggregates, protein folding is assisted. Cells have evolved robust machinery called molecular chaperones to deal with the protein folding problem and to maintain proteins in their functional state. Molecular chaperones promote efficient folding of newly synthesized proteins, prevent their aggregation and ensure protein homeostasis in cells. There are different classes of molecular chaperones functioning in a complex interplay. In this review, we discuss the principal characteristics of different classes of molecular chaperones, their structure-function relationships, their mode of regulation and their involvement in human disorders.

Hsp90 and Hsp70 chaperones: Collaborators in protein remodeling

Heat shock proteins 90 (Hsp90) and 70 (Hsp70) are two families of highly conserved ATP-dependent molecular chaperones that fold and remodel proteins. Both are important components of the cellular machinery involved in protein homeostasis and participate in nearly every cellular process. Although Hsp90 and Hsp70 each carry out some chaperone activities independently, they collaborate in other cellular remodeling reactions. In eukaryotes, both Hsp90 and Hsp70 function with numerous Hsp90 and Hsp70 co-chaperones. In contrast, bacterial Hsp90 and Hsp70 are less complex; Hsp90 acts independently of co-chaperones, and Hsp70 uses two co-chaperones. In this review, we focus on recent progress toward understanding the basic mechanisms of Hsp90-mediated protein remodeling and the collaboration between Hsp90 and Hsp70, with an emphasis on bacterial chaperones. We describe the structure and conformational dynamics of these chaperones and their interactions with each other and with client proteins. The physiological roles of Hsp90 in Escherichia coli and other bacteria are also discussed. We anticipate that the information gained from exploring the mechanism of the bacterial chaperone system will provide the groundwork for understanding the more complex eukaryotic Hsp90 system and its modulation by Hsp90 co-chaperones.

Division of Labor: ER-Resident BiP Co-Chaperones Match Substrates to Fates Based on Specific Binding Sequences

In this issue of Molecular Cell, Behnke et al. (2016) describe a novel cell-based peptide-binding assay and use it to analyze the binding specificities of the endoplasmic reticulum Hsp70 chaperone and its co-chaperones and to probe their different roles in protein quality control.

AMPylation targets the rate-limiting step of BiP's ATPase cycle for its functional inactivation

The endoplasmic reticulum (ER)-localized Hsp70 chaperone BiP contributes to protein folding homeostasis by engaging unfolded client proteins in a process that is tightly coupled to ATP binding and hydrolysis. The inverse correlation between BiP AMPylation and the burden of unfolded ER proteins suggests a post-translational mechanism for adjusting BiP's activity to changing levels of ER stress, but the underlying molecular details are unexplored. We present biochemical and crystallographic studies indicating that irrespective of the identity of the bound nucleotide AMPylation biases BiP towards a conformation normally attained by the ATP-bound chaperone. AMPylation does not affect the interaction between BiP and J-protein co-factors but appears to allosterically impair J protein-stimulated ATP-hydrolysis, resulting in the inability of modified BiP to attain high affinity for its substrates. These findings suggest a molecular mechanism by which AMPylation serves as a switch to inactivate BiP, limiting its interactions with substrates whilst conserving ATP.

Biophysical analysis of the effect of chemical modification by 4-oxononenal on the structure, stability, and function of binding immunoglobulin protein (BiP)

Binding immunoglobulin protein (BiP) is a molecular chaperone important for the folding of numerous proteins, which include millions of immunoglobulins in human body. It also plays a key role in the unfolded protein response (UPR) in the endoplasmic reticulum. Free radical generation is a common phenomenon that occurs in cells under healthy as well as under stress conditions such as ageing, inflammation, alcohol consumption, and smoking. These free radicals attack the cell membranes and generate highly reactive lipid peroxidation products such as 4-oxononenal (4-ONE). BiP is a key protein that is modified by 4-ONE. In this study, we probed how such chemical modification affects the biophysical properties of BiP. Upon modification, BiP shows significant tertiary structural changes with no changes in its secondary structure. The protein loses its thermodynamic stability, particularly, that of the nucleotide binding domain (NBD) where ATP binds. In terms of function, the modified BiP completely loses its ATPase activity with decreased ATP binding affinity. However, modified BiP retains its immunoglobulin binding function and its chaperone activity of suppressing non-specific protein aggregation. These results indicate that 4-ONE modification can significantly affect the structure-function of key proteins such as BiP involved in cellular pathways, and provide a molecular basis for how chemical modifications can result in the failure of quality control mechanisms inside the cell.

The endoplasmic reticulum: A hub of protein quality control in health and disease

One third of the eukaryotic proteome is synthesized at the endoplasmic reticulum (ER), whose unique properties provide a folding environment substantially different from the cytosol. A healthy, balanced proteome in the ER is maintained by a network of factors referred to as the ER quality control (ERQC) machinery. This network consists of various protein folding chaperones and modifying enzymes, and is regulated by stress response pathways that prevent the build-up as well as the secretion of potentially toxic and aggregation-prone misfolded protein species. Here, we describe the components of the ERQC machinery, investigate their response to different forms of stress, and discuss the consequences of ERQC break-down.

Crowding Activates Heat Shock Protein 90

Hsp90 is a dimeric ATP-dependent chaperone involved in the folding, maturation, and activation of diverse target proteins. Extensive in vitro structural analysis has led to a working model of Hsp90's ATP-driven conformational cycle. An implicit assumption is that dilute experimental conditions do not significantly perturb Hsp90 structure and function. However, Hsp90 undergoes a dramatic open/closed conformational change, which raises the possibility that this assumption may not be valid for this chaperone. Indeed, here we show that the ATPase activity of Hsp90 is highly sensitive to molecular crowding, whereas the ATPase activities of Hsp60 and Hsp70 chaperones are insensitive to crowding conditions. Polymer crowders activate Hsp90 in a non-saturable manner, with increasing efficacy at increasing concentration. Crowders exhibit a non-linear relationship between their radius of gyration and the extent to which they activate Hsp90. This experimental relationship can be qualitatively recapitulated with simple structure-based volume calculations comparing open/closed configurations of Hsp90. Thermodynamic analysis indicates that crowding activation of Hsp90 is entropically driven, which is consistent with a model in which excluded volume provides a driving force that favors the closed active state of Hsp90. Multiple Hsp90 homologs are activated by crowders, with the endoplasmic reticulum-specific Hsp90, Grp94, exhibiting the highest sensitivity. Finally, we find that crowding activation works by a different mechanism than co-chaperone activation and that these mechanisms are independent. We hypothesize that Hsp90 has a higher intrinsic activity in the cell than in vitro.

Noncanonical binding of BiP ATPase domain to Ire1 and Perk is dissociated by unfolded protein CH1 to initiate ER stress signaling

The unfolded protein response (UPR) is an essential cell signaling system that detects the accumulation of misfolded proteins within the endoplasmic reticulum (ER) and initiates a cellular response in order to maintain homeostasis. How cells detect the accumulation of misfolded proteins remains unclear. In this study, we identify a noncanonical interaction between the ATPase domain of the ER chaperone BiP and the luminal domains of the UPR sensors Ire1 and Perk that dissociates when authentic ER unfolded protein C_H1 binds to the canonical substrate binding domain of BiP. Unlike the interaction between chaperone and substrates, we found that the interaction between BiP and UPR sensors was unaffected by nucleotides. Thus, we discover that BiP is dual functional UPR sensor, sensing unfolded proteins by canonical binding to substrates and transducing this event to noncanonical, signaling interaction to Ire1 and Perk. Our observations implicate BiP as the key component for detecting ER stress and suggest an allosteric mechanism for UPR induction.

DOI:http://dx.doi.org/10.7554/eLife.03522.001

Proteins perform many essential tasks in cells, but to be able to work they first have to correctly fold into a specific three-dimensional shape. Within the cell, many proteins are folded with the help of ‘chaperone’ proteins. If any proteins fold incorrectly, the normal workings of the cell can be disturbed, which may damage the cell. This is more likely to happen if a cell suddenly requires a large number of proteins to be made, which can overwhelm the chaperone proteins.

In humans and other eukaryotic organisms, many proteins are folded in a compartment within the cell called the endoplasmic reticulum. Inside this compartment there is a system called the unfolded protein response that detects misfolded proteins and boosts the cell's capacity to re-fold them. As part of this system, two sensor proteins detect when misfolded proteins are present, but it is not clear how they do so. It has been suggested that a chaperone protein called BiP may be able to activate these sensor proteins in order to turn on the unfolded protein response.

In this study, Carrara et al. studied the sensor proteins and BiP using an artificial set-up in the laboratory. The experiments show that both of the sensor proteins can bind to a section of the BiP chaperone called the ATPase domain. However, in the presence of an unfolded protein, BiP stopped interacting with the sensor proteins, which could allow the sensor proteins to activate the unfolded protein response. The experiments also show that BiP must bind to the unfolded protein to activate the unfolded protein response.

Carrara et al.'s findings suggest that BiP has a dual role in cells: to sense unfolded proteins by binding to them, and then to activate the sensor proteins that trigger the unfolded protein response.

Together, these results suggest a new model for how cells detect and respond to misfolded proteins within the endoplasmic reticulum, and may provide new targets for therapies to treat diseases caused by defects in protein folding.

DOI:http://dx.doi.org/10.7554/eLife.03522.002

BiP and its nucleotide exchange factors Grp170 and Sil1: mechanisms of action and biological functions

BiP (immunoglobulin heavy-chain binding protein) is the endoplasmic reticulum (ER) orthologue of the Hsp70 family of molecular chaperones and is intricately involved in most functions of this organelle through its interactions with a variety of substrates and regulatory proteins. Like all Hsp70 family members, the ability of BiP to bind and release unfolded proteins is tightly regulated by a cycle of ATP binding, hydrolysis, and nucleotide exchange. As a characteristic of the Hsp70 family, multiple DnaJ-like co-factors can target substrates to BiP and stimulate its ATPase activity to stabilize the binding of BiP to substrates. However, only in the past decade have nucleotide exchange factors for BiP been identified, which has shed light not only on the mechanism of BiP-assisted folding in the ER but also on Hsp70 family members that reside throughout the cell. We will review the current understanding of the ATPase cycle of BiP in the unique environment of the ER and how it is regulated by the nucleotide exchange factors, Grp170 (glucose-regulated protein of 170kDa) and Sil1, both of which perform unanticipated roles in various biological functions and disease states.

How hsp70 molecular machines interact with their substrates to mediate diverse physiological functions

Hsp70 molecular chaperones are implicated in a wide variety of cellular processes, including protein biogenesis, protection of the proteome from stress, recovery of proteins from aggregates, facilitation of protein translocation across membranes, and more specialized roles such as disassembly of particular protein complexes. It is a fascinating question to ask how the mechanism of these deceptively simple molecular machines is matched to their roles in these wide-ranging processes. The key is a combination of the nature of the recognition and binding of Hsp70 substrates and the impact of Hsp70 action on their substrates. In many cases, the binding, which relies on interaction with an extended, accessible short hydrophobic sequence, favors more unfolded states of client proteins. The ATP-mediated dissociation of the substrate thus releases it in a relatively less folded state for downstream folding, membrane translocation, or hand-off to another chaperone. There are cases, such as regulation of the heat shock response or disassembly of clathrin coats, however, where binding of a short hydrophobic sequence selects conformational states of clients to favor their productive participation in a subsequent step. This Perspective discusses current understanding of how Hsp70 molecular chaperones recognize and act on their substrates and the relationships between these fundamental processes and the functional roles played by these molecular machines.

BiP clustering facilitates protein folding in the endoplasmic reticulum

The chaperone BiP participates in several regulatory processes within the endoplasmic reticulum (ER): translocation, protein folding, and ER-associated degradation. To facilitate protein folding, a cooperative mechanism known as entropic pulling has been proposed to demonstrate the molecular-level understanding of how multiple BiP molecules bind to nascent and unfolded proteins. Recently, experimental evidence revealed the spatial heterogeneity of BiP within the nuclear and peripheral ER of S. cerevisiae (commonly referred to as 'clusters'). Here, we developed a model to evaluate the potential advantages of accounting for multiple BiP molecules binding to peptides, while proposing that BiP's spatial heterogeneity may enhance protein folding and maturation. Scenarios were simulated to gauge the effectiveness of binding multiple chaperone molecules to peptides. Using two metrics: folding efficiency and chaperone cost, we determined that the single binding site model achieves a higher efficiency than models characterized by multiple binding sites, in the absence of cooperativity. Due to entropic pulling, however, multiple chaperones perform in concert to facilitate the resolubilization and ultimate yield of folded proteins. As a result of cooperativity, multiple binding site models used fewer BiP molecules and maintained a higher folding efficiency than the single binding site model. These insilico investigations reveal that clusters of BiP molecules bound to unfolded proteins may enhance folding efficiency through cooperative action via entropic pulling.

Conformational selection in substrate recognition by Hsp70 chaperones

Hsp70s are molecular chaperones involved in the folding and assembly of proteins. They recognize hydrophobic amino acid stretches in their substrate binding groove. However, a detailed understanding of substrate specificity is still missing. Here, we use the endoplasmic reticulum-resident Hsp70 BiP to identify binding sites in a natural client protein. Two sites are mutually recognized and form stable Hsp70-substrate complexes. In silico and in vitro analyses revealed an extended substrate conformation as a crucial factor for interaction and show an unexpected plasticity of the substrate binding groove. The basic binding mechanism is conserved among different Hsp70s.

Spatial localisation of chaperone distribution in the endoplasmic reticulum of yeast

In eukaryotes, the endoplasmic reticulum (ER) serves as the first membrane-enclosed organelle in the secretory pathway, with functions including protein folding, maturation and transport. Molecular chaperones, of the Hsp70 family of proteins, participate in assisting these processes and are essential to cellular function and survival. BiP is a resident Hsp70 chaperone in the ER of Saccharomyces cerevisiae. In this study the authors have created a partial differential equation model to examine how BiP interacts with the membrane-bound co-chaperone Sec63 in translocation, a process in which BiP assists in guiding a nascent protein into the ER lumen. It has been found that when Sec63 participates in translocation through localisation at the membrane, the spatial distribution of BiP is inhomogeneous, with more BiP at the surface. When translocation is inhibited through a disabling of Sec63's membrane tether, the concentration of BiP throughout the ER becomes homogeneous. The computational simulations suggest that Sec63's localisation and the resulting binding to BiP near the membrane surface of the ER enable a heterogeneous distribution of BiP within the ER, and may facilitate BiP's role in translocation. [Includes supplementary material].

ADP ribosylation adapts an ER chaperone response to short-term fluctuations in unfolded protein load

Gene expression programs that regulate the abundance of the chaperone BiP adapt the endoplasmic reticulum (ER) to unfolded protein load. However, such programs are slow compared with physiological fluctuations in secreted protein synthesis. While searching for mechanisms that fill this temporal gap in coping with ER stress, we found elevated levels of adenosine diphosphate (ADP)-ribosylated BiP in the inactive pancreas of fasted mice and a rapid decline in this modification in the active fed state. ADP ribosylation mapped to Arg470 and Arg492 in the substrate-binding domain of hamster BiP. Mutations that mimic the negative charge of ADP-ribose destabilized substrate binding and interfered with interdomain allosteric coupling, marking ADP ribosylation as a rapid posttranslational mechanism for reversible inactivation of BiP. A kinetic model showed that buffering fluctuations in unfolded protein load with a recruitable pool of inactive chaperone is an efficient strategy to minimize both aggregation and costly degradation of unfolded proteins.

Localization of BiP to translating ribosomes increases soluble accumulation of secreted eukaryotic proteins in an Escherichia coli cell-free system

The endoplasmic reticulum (ER) resident Hsp70 chaperone, BiP, docks to the Sec translocon and interacts co-translationally with polypeptides entering the ER to encourage proper folding. In order to recreate this interaction in Escherichia coli cell-free protein synthesis (CFPS) reactions, a fusion protein was formed between the ribosome-binding portion of the E. coli protein trigger factor (TF) and BiP. The biophysical affinity to ribosomes as well as the characteristic Hsp70 ATPase activity were both verified for the fusion protein. When added to E. coli-based CFPS reactions, the TF-BiP fusion chaperone increased soluble yields of several protein fragments that are normally secreted through the ER and have poor solubility in typical CFPS reactions. For comparison, a fusion between TF and the native E. coli Hsp70, DnaK, was also constructed. This fusion was also biologically active and increased soluble yields of certain protein targets in CFPS. The TF-BiP fusion described in this study can be seen as a first step in reconstituting and better understanding ER folding pathways in the prokaryotic environment of E. coli CFPS.

Human delta opioid receptor biogenesis is regulated via interactions with SERCA2b and calnexin

Sarco(endo)plasmic reticulum calcium ATPase (SERCA)2b maintains the cellular Ca(2+) homeostasis by transferring Ca(2+) from the cytosol to the lumen of the endoplasmic reticulum (ER). Recently, SERCA2b has also been shown to be involved in the biosynthesis of secreted and membrane proteins via direct protein-protein interactions, involving components of the ER folding and quality-control machinery, as well as newly synthesized G protein-coupled receptors. Here we demonstrate that the human delta opioid receptor (hdeltaOR) exists in a ternary complex with SERCA2b and the ER molecular chaperone calnexin. The interaction between SERCA2b and hdeltaOR in vivo did not require calnexin as it was independent of the C-terminal calnexin-interacting domain of SERCA2b. However, the receptor was able to mediate co-immunoprecipitation of calnexin with the C-terminally truncated SERCA2b. The association of SERCA2b with hdeltaOR was regulated in vitro by Ca(2+) and ATP in a manner that was opposite to the calnexin-hdeltaOR interaction. Importantly, co-expression of the catalytically inactive SERCA2b(D351A) or calnexin binding-compromised SERCA2bDeltaC mutants with the receptor decreased the expression of mature receptors in a manner that did not directly relate to changes in the ER Ca(2+) concentration. We conclude that dynamic interactions among SERCA2b, calnexin and the hdeltaOR precursor orchestrate receptor biogenesis and are regulated by Ca(2+) and ATP. We further hypothesize that the primary role of SERCA2b in this process is to act as a Ca(2+) sensor in the vicinity of active translocons, integrating protein folding with local fluctuations of ER Ca(2+) levels.

Overexpression of the endoplasmic reticulum chaperone BiP3 regulates XA21-mediated innate immunity in rice

Recognition of pathogen-associated molecular patterns by pattern recognition receptors (PRRs) activates the innate immune response. Although PRR-mediated signaling events are critical to the survival of plants and animals, secretion and localization of PRRs have not yet been clearly elucidated. Here we report the in vivo interaction of the endoplasmic reticulum (ER) chaperone BiP3 with the rice XA21 PRR, which confers resistance to the Gram negative bacterium, Xanthomonas oryzae pv. oryzae (Xoo). We show that XA21 is glycosylated and is primarily localized to the ER and also to the plasma membrane (PM). In BiP3-overexpressing rice plants, XA21-mediated immunity is compromised, XA21 stability is significantly decreased, and XA21 proteolytic cleavage is inhibited. BiP3 overexpression does not affect the general rice defense response, cell death or brassinolide-induced responses. These results indicate that BiP3 regulates XA21 protein stability and processing and that this regulation is critical for resistance to Xoo.

Folding of conotoxins: formation of the native disulfide bridges during chemical synthesis and biosynthesis of Conus peptides

Conopeptides from >700 species of predatory marine Conus snails provide an impressive molecular diversity of cysteine-rich peptides. Most of the estimated 50,000-100,000 distinct conopeptides range in size from 10 to 50 amino acid residues, often with multiple posttranslational modifications. The great majority contain from two to four disulfide bridges. As the biosynthetic and chemical production of this impressive repertoire of disulfide-rich peptides has been investigated, particularly the formation of native disulfide bridges, differences between in vivo and in vitro oxidative folding have become increasingly evident. In this article, we provide an overview of the molecular diversity of conotoxins with an emphasis on the cysteine patterns and disulfide frameworks. The conotoxin folding studies reviewed include regioselective and direct oxidation strategies, recombinant expression, optimization of folding methods, mechanisms of in vitro folding, and preliminary data on the biosynthesis of conotoxins in venom ducts. Despite these studies, how the cone snails efficiently produce properly folded conotoxins remains unanswered. As chemists continue to master oxidative folding techniques, insights gleaned from how conotoxins are folded in vivo will likely lead to the development of the new folding methods, as well as shed some light on fundamental mechanisms relevant to the protein folding problem.

The heat shock protein 70 molecular chaperone network in the pancreatic endoplasmic reticulum - a quantitative approach

Traditionally, the canine pancreatic endoplasmic reticulum (ER) has been the workhorse for cell-free studies on protein transport into the mammalian ER. These studies have revealed multiple roles for the major ER-luminal heat shock protein (Hsp) 70, IgG heavy chain-binding protein (BiP), at least one of which also involves the second ER-luminal Hsp70, glucose-regulated protein (Grp) 170. In addition, at least one of these BiP activities depends on Hsp40. Up to now, five Hsp40s and two nucleotide exchange factors, Sil1 and Grp170, have been identified in the ER of different mammalian cell types. Here we quantified the various proteins of this chaperone network in canine pancreatic rough microsomes. We also characterized the various purified proteins with respect to their affinities for BiP and their effect on the ATPase activity of BiP. The results identify Grp170 as the major nucleotide exchange factor for BiP, and the resident ER-membrane proteins ER-resident J-domain protein 1 plus ER-resident J-domain protein 2/Sec63 as prime candidates for cochaperones of BiP in protein transport in the pancreatic ER. Thus, these data represent a comprehensive analysis of the BiP chaperone network that was recently linked to two human inherited diseases, polycystic liver disease and Marinesco-Sjögren syndrome.

Conserved Structural and Functional Properties of D-domain Containing Redox-active and -inactive Protein Disulfide Isomerase-related Protein Chaperones

The structure and mode of binding of the endoplasmic reticulum protein disulfide isomerase-related proteins to their substrates is currently a focus of intensive research. We have recently determined the crystal structure of the Drosophila melanogaster protein disulfide isomerase-related protein Wind and have described two essential substrate binding sites within the protein, one within the thioredoxin b-domain and another within the C-terminal D-domain. Although a mammalian ortholog of Wind (ERp29/28) is known, conflicting interpretations of its structure and putative function have been postulated. Here, we have provided evidence indicating that ERp29 is indeed similar in both structure and function to its Drosophila ortholog. Using a site-directed mutagenesis approach, we have demonstrated that homodimerization of the b-domains is significantly reduced in vitro upon replacement of key residues at the predicted dimerization interface. Investigation of Wind-ERp29 fusion constructs showed that mutants of the D-domain of ERp29 prevent transport of a substrate protein (Pipe) in a manner consistent with the presence of a discrete, conserved peptide binding site in the D-domain. Finally, we have highlighted the general applicability of these findings by showing that the D-domain of a redox-active disulfide isomerase, from the slime mold Dictyostelium discoideum, can also functionally replace the Wind D-domain in vivo.

The binding of FKBP23 to BiP modulates BiP’s ATPase activity with its PPIase activity

Peptidyl-prolyl cis-trans-isomerases (PPIases) are enzymes that can cis-trans-isomerize a Xaa-Pro peptide bond. Three families of PPIases are known: cyclophilins, FKBPs, and parvulins. The physiological functions of the PPIases are only poorly understood. In previous work, we reported that the mouse FK506-binding protein 23 (mFKBP23), which comprises an N-terminal PPIase domain and a C-terminal domain with Ca(2+)-binding sites, binds to mBiP in the endoplasmic reticulum (ER) and this binding is affected by the Ca(2+) concentration. In this study, we demonstrate the ability of mFKBP23 to modulate the ATPase activity of BiP, and that the bound mFKBP23, but not the free mFKBP23, can suppress the ATPase activity of mBiP through its PPIase activity.

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.

The molecular mechanisms underlying BiP-mediated gating of the Sec61 translocon of the endoplasmic reticulum

The Sec61 translocon of the endoplasmic reticulum membrane forms an aqueous pore that is gated by the lumenal Hsp70 chaperone BiP. We have explored the molecular mechanisms governing BiP-mediated gating activity, including the coupling between gating and the BiP ATPase cycle, and the involvement of the substrate-binding and J domain–binding regions of BiP. Translocon gating was assayed by measuring the collisional quenching of fluorescent probes incorporated into nascent chains of translocation intermediates engaged with microsomes containing various BiP mutants and BiP substrate. Our results indicate that BiP must assume the ADP-bound conformation to seal the translocon, and that the reopening of the pore requires an ATP binding–induced conformational change. Further, pore closure requires functional interactions between both the substrate-binding region and the J domain–binding region of BiP and membrane proteins. The mechanism by which BiP mediates translocon pore closure and opening is therefore similar to that in which Hsp70 chaperones associate with and dissociate from substrates.

Hsp70 and Hsp90--a relay team for protein folding

Molecular chaperones are a functionally defined set of proteins which assist the structure formation of proteins in vivo. Without certain protective mechanisms, such as binding nascent polypeptide chains by molecular chaperones, cellular protein concentrations would lead to misfolding and aggregation. In the mammalian system, the molecular chaperones Hsp70 and Hsp90 are involved in the folding and maturation of key regulatory proteins, like steroid hormone receptors, transcription factors, and kinases, some of which are involved in cancer progression. Hsp70 and Hsp90 form a multichaperone complex, in which both are connected by a third protein called Hop. The connection of and the interplay between the two chaperone machineries is of crucial importance for cell viability. This review provides a detailed view of the Hsp70 and Hsp90 machineries, their cofactors and their mode of regulation. It summarizes the current knowledge in the field, including the ATP-dependent regulation of the Hsp70/Hsp90 multichaperone cycle and elucidates the complex interplay and their synergistic interaction.