Editorial: The HSP70 Molecular Chaperone Machines

Structural remodeling of AAA+ ATPase p97 by adaptor protein ASPL facilitates posttranslational methylation by METTL21D

p97 is an essential AAA+ ATPase that extracts and unfolds substrate proteins from membranes and protein complexes. Through its mode of action, p97 contributes to various cellular processes, such as membrane fusion, ER-associated protein degradation, DNA repair, and many others. Diverse p97 functions and protein interactions are regulated by a large number of adaptor proteins. Alveolar soft part sarcoma locus (ASPL) is a unique adaptor protein that regulates p97 by disassembling functional p97 hexamers to smaller entities. An alternative mechanism to regulate the activity and interactions of p97 is by posttranslational modifications (PTMs). Although more than 140 PTMs have been identified in p97, only a handful of those have been described in detail. Here we present structural and biochemical data to explain how the p97-remodeling adaptor protein ASPL enables the metastasis promoting methyltransferase METTL21D to bind and trimethylate p97 at a single lysine side chain, which is deeply buried inside functional p97 hexamers. The crystal structure of a heterotrimeric p97:ASPL:METTL21D complex in the presence of cofactors ATP and S-adenosyl homocysteine reveals how structural remodeling by ASPL exposes the crucial lysine residue of p97 to facilitate its trimethylation by METTL21D. The structure also uncovers a role of the second region of homology (SRH) present in the first ATPase domain of p97 in binding of a modifying enzyme to the AAA+ ATPase. Investigation of this interaction in the human, fish, and plant reveals fine details on the mechanism and significance of p97 trimethylation by METTL21D across different organisms.

Real-time single-molecule observation of chaperone-assisted protein folding

The ability of heat shock protein 70 (Hsp70) molecular chaperones to remodel the conformation of their clients is central to their biological function; however, questions remain regarding the precise molecular mechanisms by which Hsp70 machinery interacts with the client and how this contributes toward efficient protein folding. Here, we used total internal reflection fluorescence (TIRF) microscopy and single-molecule fluorescence resonance energy transfer (smFRET) to temporally observe the conformational changes that occur to individual firefly luciferase proteins as they are folded by the bacterial Hsp70 system. We observed multiple cycles of chaperone binding and release to an individual client during refolding and determined that high rates of chaperone cycling improves refolding yield. Furthermore, we demonstrate that DnaJ remodels misfolded proteins via a conformational selection mechanism, whereas DnaK resolves misfolded states via mechanical unfolding. This study illustrates that the temporal observation of chaperone-assisted folding enables the elucidation of key mechanistic details inaccessible using other approaches.

Mitochondrial HSC70-1 Regulates Polar Auxin Transport through ROS Homeostasis in Arabidopsis Roots

Arabidopsis mitochondrial-localized heat shock protein 70-1 (mtHSC70-1) modulates vegetative growth by assisting mitochondrial complex IV assembly and maintaining reactive oxygen species (ROS) homeostasis. In addition, mtHSC70-1 affects embryo development, and this effect is mediated by auxin. However, whether mtHSC70-1 regulates vegetative growth through auxin and knowledge of the link between ROS homeostasis and auxin distribution remain unclear. Here, we found that mtHSC70-1 knockout seedlings (mthsc70-1a) displayed shortened roots, decreased fresh root weight and lateral root number, increased root width and abnormal root morphology. The introduction of the mtHSC70-1 gene into mthsc70-1a restored the growth and development of roots to the level of the wild type. However, sugar and auxin supplementation could not help the mutant roots restore to normal. Moreover, mthsc70-1a seedlings showed a decrease in meristem length and activity, auxin transport carrier (PINs and AUX1) and auxin abundances in root tips. The application of exogenous reducing agents upregulated the levels of PINs in the mutant roots. The introduction of antioxidant enzyme genes (MSD1 or CAT1) into the mthsc70-1a mutant rescued the PIN and local auxin abundances and root growth and development. Taken together, our data suggest that mtHSC70-1 regulates polar auxin transport through ROS homeostasis in Arabidopsis roots.

Specific Antiproliferative Properties of Proteinaceous Toxin Secretions from the Marine Annelid Eulalia sp. onto Ovarian Cancer Cells

As Yondelis joins the ranks of approved anti-cancer drugs, the benefit from exploring the oceans' biodiversity becomes clear. From marine toxins, relevant bioproducts can be obtained due to their potential to interfere with specific pathways. We explored the cytotoxicity of toxin-bearing secretions of the polychaete Eulalia onto a battery of normal and cancer human cell lines and discovered that the cocktail of proteins is more toxic towards an ovarian cancer cell line (A2780). The secretions' main proteins were identified by proteomics and transcriptomics: 14-3-3 protein, Hsp70, Rab3, Arylsulfatase B and serine protease, the latter two being known toxins. This mixture of toxins induces cell-cycle arrest at G2/M phase after 3h exposure in A2780 cells and extrinsic programmed cell death. These findings indicate that partial re-activation of the G2/M checkpoint, which is inactivated in many cancer cells, can be partly reversed by the toxic mixture. Protein-protein interaction networks partake in two cytotoxic effects: cell-cycle arrest with a link to RAB3C and RAF1; and lytic activity of arylsulfatases. The discovery of both mechanisms indicates that venomous mixtures may affect proliferating cells in a specific manner, highlighting the cocktails' potential in the fine-tuning of anti-cancer therapeutics targeting cell cycle and protein homeostasis.

Arabidopsis mtHSC70-1 physically interacts with the Cox2 subunit of cytochrome c oxidase

The 70-kD heat shock proteins (HSP70s or HSC70s) function as molecular chaperones and are involved in diverse cellular processes. We recently demonstrated the roles of mitochondrial HSC70-1 (mtHSC70-1) in the establishment of cytochrome c oxidase (COX)-dependent respiration and redox homeostasis in Arabidopsis thaliana. Defects in COX assembly were observed in the mtHSC70-1 knockout lines. The levels of Cox2 (COX subunit 2) proteins in COX complex were markedly lower in the mutants than in wild-type plants; however, the levels of total Cox2 proteins in the mutants were not obviously different from those in wild-type plants, suggesting that the stability of COX or the availability of Cox2 was impaired in the mtHSC70-1 mutants. Here, we further detected the interaction between mtHSC70-1 and Cox2 proteins through co-immunoprecipitation, pull-down and firefly luciferase complementation imaging assays. The results showed that mtHSC70-1 could directly combine Cox2 in vivo and in vitro, providing supporting evidence for the role of mtHSC70-1 in COX assembly.

Chaperome Networks - Redundancy and Implications for Cancer Treatment

The chaperome is a large family of proteins composed of chaperones, co-chaperones and a multitude of other factors. Elegant studies in yeast and other organisms have paved the road to how we currently understand the complex organization of this large family into protein networks. The goal of this chapter is to provide an overview of chaperome networks in cancer cells, with a focus on two cellular states defined by chaperome network organization. One state characterized by chaperome networks working in isolation and with little overlap, contains global chaperome networks resembling those of normal, non-transformed, cells. We propose that in this state, redundancy in chaperome networks results in a tumor type unamenable for single-agent chaperome therapy. The second state comprises chaperome networks interconnected in response to cellular stress, such as MYC hyperactivation. This is a state where no redundant pathways can be deployed, and is a state of vulnerability, amenable for chaperome therapy. We conclude by proposing a change in how we discover and implement chaperome inhibitor strategies, and suggest an approach to chaperome therapy where the properties of chaperome networks, rather than genetics or client proteins, are used in chaperome inhibitor implementation.

The expanding world of plant J-domain proteins

Plants maintain cellular proteostasis during different phases of growth and development despite a barrage of biotic and abiotic stressors in an ever-changing environment. This requires a collaborative effort of a cadre of molecular chaperones. Hsp70s and their obligate co-chaperones, J-domain proteins (JDPs), are arguably the most ubiquitous and formidable components of the cellular chaperone network, facilitating numerous and diverse cellular processes and allowing survival under a plethora of stressful conditions. JDPs are also among the most versatile chaperones. Compared to Hsp70s, the number of JDP-encoding genes has proliferated, suggesting the emergence of highly complex Hsp70-JDP networks, particularly in plants. Recent studies indicate that besides the increase in the number of JDP encoding genes; regulatory differences, neo- and sub-functionalization, and inter- and intra-class combinatorial interactions, is rapidly expanding the repertoire of Hsp70-JDP systems. This results in highly robust and functionally diverse chaperone networks in plants. Here, we review the current status of plant JDP research and discuss how the paradigm shift in the field can be exploited toward a better understanding of JDP function and evolution.

Arabidopsis mtHSC70-1 plays important roles in the establishment of COX-dependent respiration and redox homeostasis

The 70 kDa heat shock proteins function as molecular chaperones and are involved in diverse cellular processes. However, the functions of the plant mitochondrial HSP70s (mtHSC70s) remain unclear. Severe growth defects were observed in the Arabidopsis thaliana mtHSC70-1 knockout lines, mthsc70-1a and mthsc70-1b. Conversely, the introduction of the mtHSC70-1 gene into the mthsc70-1a background fully reversed the phenotypes, indicating that mtHSC70-1 is essential for plant growth. The loss of mtHSC70-1 functions resulted in abnormal mitochondria and alterations to respiration because of an inhibition of the cytochrome c oxidase (COX) pathway and the activation of the alternative respiratory pathway. Defects in COX assembly were observed in the mtHSC70-1 knockout lines, leading to decreased COX activity. The mtHSC70-1 knockout plants have increased levels of reactive oxygen species (ROS). The introduction of the Mn-superoxide dismutase 1 (MSD1) or the catalase 1 (CAT1) gene into the mthsc70-1a plants decreased ROS levels, reduced the expression of alternative oxidase, and partially rescued growth. Taken together, our data suggest that mtHSC70-1 plays important roles in the establishment of COX-dependent respiration.

HSP70 is required for the proper assembly of pericentriolar material and function of mitotic centrosomes

Background

At the onset of mitosis, the centrosome expands and matures, acquiring enhanced activities for microtubule nucleation and assembly of a functional bipolar mitotic spindle. However, the mechanisms that regulate centrosome expansion and maturation are largely unknown. Previously, we demonstrated in an immortalized human cell line CGL2 and cancer cell line HeLa that the inducible form of heat shock protein 70 (HSP70) accumulates at the mitotic centrosome and is required for centrosome maturation and bipolar spindle assembly.

Results

In this study, we further show that HSP70 accumulated at the spindle pole in a PLK1-dependent manner. HSP70 colocalized with pericentrin (PCNT), CEP215 and γ-tubulin at the spindle pole and was required for the 3D assembly of these three proteins, which supports mitotic centrosome function. Loss of HSP70 disrupted mitotic centrosome structure, reduced pericentriolar material recruitment and induced fragmentation of spindle poles. In addition, HSP70 was necessary for the interaction between PCNT and CEP215 and also facilitated PLK1 accumulation and function at the spindle pole. Furthermore, we found that HSP70 chaperone activity is required for PCNT accumulation at the mitotic centrosome and assembly of mitotic spindles.

Conclusion

Our current results demonstrate that HSP70 is required for the accurate assembly of the pericentriolar material and proper functioning of mitotic centrosomes.

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.

The Hsp70-Hsp90 Chaperone Cascade in Protein Folding

Conserved families of molecular chaperones assist protein folding in the cell. Here we review the conceptual advances on three major folding routes: (i) spontaneous, chaperone-independent folding; (ii) folding assisted by repetitive Hsp70 cycles; and (iii) folding by the Hsp70-Hsp90 cascades. These chaperones prepare their protein clients for folding on their own, without altering their folding path. A particularly interesting role is reserved for Hsp90. The function of Hsp90 in folding is its ancient function downstream of Hsp70, free of cochaperone regulation and present in all kingdoms of life. Eukaryotic signalling networks, however, embrace Hsp90 by a plethora of cochaperones, transforming the profolding machinery to a folding-on-demand factor. We discuss implications for biology and molecular medicine.

Mechanisms of protein homeostasis (proteostasis) maintain stem cell identity in mammalian pluripotent stem cells

Protein homeostasis, or proteostasis, is essential for cell function, development, and organismal viability. The composition of the proteome is adjusted to the specific requirements of a particular cell type and status. Moreover, multiple metabolic and environmental conditions challenge the integrity of the proteome. To maintain the quality of the proteome, the proteostasis network monitors proteins from their synthesis through their degradation. Whereas somatic stem cells lose their ability to maintain proteostasis with age, immortal pluripotent stem cells exhibit a stringent proteostasis network associated with their biological function and intrinsic characteristics. Moreover, growing evidence indicates that enhanced proteostasis mechanisms play a central role in immortality and cell fate decisions of pluripotent stem cells. Here, we will review new insights into the melding fields of proteostasis and pluripotency and their implications for the understanding of organismal development and survival.

Peptide fragments of Hsp70 modulate its chaperone activity and sensitize tumor cells to anti-cancer drugs

Most Hsp70 chaperone inhibitors exert anti-cancer effects; however, their high cytotoxicity proposed the use of peptide fragments of the chaperone as safer modulators of its activity and as complements to customary drugs. One such peptide, ICit-2, was found to inhibit substrate-binding and refolding activities of the chaperone. Using various approaches, we established that ICit-2 binds Hsp70, which may explain its inhibitory action. ICit-2 penetrates A-431 cancer cells and, in combination with doxorubicin (Dox), enhances the cytotoxicity and growth inhibitory effect of the drug. Similarly, using the B16 mouse melanoma model, we found that ICit-2 inhibits the rate of tumor growth by 48% compared to Dox alone, confirming that the peptide can be employed to sensitize resistant tumors to cytostatic medicines.

Differential Targeting of Hsp70 Heat Shock Proteins HSPA6 and HSPA1A with Components of a Protein Disaggregation/Refolding Machine in Differentiated Human Neuronal Cells following Thermal Stress

Heat shock proteins (Hsps) co-operate in multi-protein machines that counter protein misfolding and aggregation and involve DNAJ (Hsp40), HSPA (Hsp70), and HSPH (Hsp105α). The HSPA family is a multigene family composed of inducible and constitutively expressed members. Inducible HSPA6 (Hsp70B') is found in the human genome but not in the genomes of mouse and rat. To advance knowledge of this little studied HSPA member, the targeting of HSPA6 to stress-sensitive neuronal sites with components of a disaggregation/refolding machine was investigated following thermal stress. HSPA6 targeted the periphery of nuclear speckles (perispeckles) that have been characterized as sites of transcription. However, HSPA6 did not co-localize at perispeckles with DNAJB1 (Hsp40-1) or HSPH1 (Hsp105α). At 3 h after heat shock, HSPA6 co-localized with these members of the disaggregation/refolding machine at the granular component (GC) of the nucleolus. Inducible HSPA1A (Hsp70-1) and constitutively expressed HSPA8 (Hsc70) co-localized at nuclear speckles with components of the machine immediately after heat shock, and at the GC layer of the nucleolus at 1 h with DNAJA1 and BAG-1. These results suggest that HSPA6 exhibits targeting features that are not apparent for HSPA1A and HSPA8.