The Hsp110 molecular chaperone stabilizes apolipoprotein B from endoplasmic reticulum-associated degradation (ERAD)

Chaperone-mediated disaggregation of infectious prions releases particles that seed new prion formation in a strain-specific manner

The mammalian prion protein can form infectious, nonnative, and protease resistant aggregates (PrPD), which cause lethal prion diseases like human Creutzfeldt-Jakob disease. PrPD seeds the formation of new infectious prions by interacting with and triggering the refolding of the normally soluble mammalian prion protein, PrPC, into more PrPD. Refolding of misfolded proteins in the cell is carried out by molecular chaperones such as Grp78. We have recently shown that Grp78 sensitizes PrPD to proteases, indicating structural alterations and leading to its degradation. However, the process of chaperone-mediated PrPD disaggregation, the chaperones involved, and the effect of disaggregation on PrPD seeding activity are unclear. We have now monitored the structural modification, disaggregation, and seeding activity of PrPD from two mouse adapted prion strains, 22L and 87V, in the presence of Grp78 and two forms of the Hsp110 disaggregase chaperone family, Hsp105 and Apg-2. We found that both forms of Hsp110 induced similar amounts of disaggregation and structural change in the protease resistant cores of PrPD from both strains. However, 22L PrPD was more susceptible to destabilization and disaggregation by the chaperones than 87V. Surprisingly, despite disaggregation of both strains, only the 22L PrPD aggregates released by the chaperones had seeding activity, with both forms of Hsp110 enhancing the Grp78 mediated release of these aggregates. Our data show that disassembly of PrPD by Grp78 and Hsp110 chaperones can release seeding particles of PrPD in a strain-specific manner, potentially facilitating prion replication and spread.

Characterization of a New Hsp110 Inhibitor as a Potential Antifungal

Fungal infections present a significant global health challenge, prompting ongoing research to discover innovative antifungal agents. The 110 kDa heat shock proteins (Hsp110s) are molecular chaperones essential for maintaining cellular protein homeostasis in eukaryotes. Fungal Hsp110s have emerged as a promising target for innovative antifungal strategies. Notably, 2H stands out as a promising candidate in the endeavor to target Hsp110s and combat fungal infections. Our study reveals that 2H exhibits broad-spectrum antifungal activity, effectively disrupting the in vitro chaperone activity of Hsp110 from Candida auris and inhibiting the growth of Cryptococcus neoformans. Pharmacokinetic analysis indicates that oral administration of 2H may offer enhanced efficacy compared to intravenous delivery, emphasizing the importance of optimizing the AUC/MIC ratio for advancing its clinical therapy.

Sse1, Hsp110 chaperone of yeast, controls the cellular fate during endoplasmic reticulum stress

Sse1 is a cytosolic Hsp110 molecular chaperone of yeast, Saccharomyces cerevisiae. Its multifaceted roles in cellular protein homeostasis as a nucleotide exchange factor (NEF), as a protein-disaggregase and as a chaperone linked to protein synthesis (CLIPS) are well documented. In the current study, we show that SSE1 genetically interacts with IRE1 and HAC1, the endoplasmic reticulum-unfolded protein response (ER-UPR) sensors implicating its role in ER protein homeostasis. Interestingly, the absence of this chaperone imparts unusual resistance to tunicamycin-induced ER stress which depends on the intact Ire1-Hac1 mediated ER-UPR signaling. Furthermore, cells lacking SSE1 show inefficient ER-stress-responsive reorganization of translating ribosomes from polysomes to monosomes that drive uninterrupted protein translation during tunicamycin stress. In consequence, the sse1Δ strain shows prominently faster reversal from ER-UPR activated state indicating quicker restoration of homeostasis, in comparison to the wild-type (WT) cells. Importantly, Sse1 plays a critical role in controlling the ER-stress-mediated cell division arrest, which is escaped in sse1Δ strain during chronic tunicamycin stress. Accordingly, sse1Δ strain shows significantly higher cell viability in comparison to WT yeast imparting the stark fitness following short-term as well as long-term tunicamycin stress. These data, all together, suggest that cytosolic chaperone Sse1 is an important modulator of ER stress response in yeast and it controls stress-induced cell division arrest and cell death during overwhelming ER stress induced by tunicamycin.

VLDL Biogenesis and Secretion: It Takes a Village

The production and secretion of VLDLs (very-low-density lipoproteins) by hepatocytes has a direct impact on liver fat content, as well as the concentrations of cholesterol and triglycerides in the circulation and thus affects both liver and cardiovascular health, respectively. Importantly, insulin resistance, excess caloric intake, and lack of physical activity are associated with overproduction of VLDL, hepatic steatosis, and increased plasma levels of atherogenic lipoproteins. Cholesterol and triglycerides in remnant particles generated by VLDL lipolysis are risk factors for atherosclerotic cardiovascular disease and have garnered increasing attention over the last few decades. Presently, however, increased risk of atherosclerosis is not the only concern when considering today's cardiometabolic patients, as they often also experience hepatic steatosis, a prevalent disorder that can progress to steatohepatitis and cirrhosis. This duality of metabolic risk highlights the importance of understanding the molecular regulation of the biogenesis of VLDL, the lipoprotein that transports triglycerides and cholesterol out of the liver. Fortunately, there has been a resurgence of interest in the intracellular assembly, trafficking, degradation, and secretion of VLDL by hepatocytes, which has led to many exciting new molecular insights that are the topic of this review. Increasing our understanding of the biology of this pathway will aid to the identification of novel therapeutic targets to improve both the cardiovascular and the hepatic health of cardiometabolic patients. This review focuses, for the first time, on this duality.

Multi-omic insights into the cellular response of Phaeodactylum tricornutum (Bacillariophyta) strains under grazing pressure

Background/Aims

Phaeodactylum tricornutum, a model organism of diatoms, plays a crucial role in Earth's primary productivity. Investigating its cellular response to grazing pressure is highly significant for the marine ecological environment. Furthermore, the integration of multi-omics approaches has enhanced the understanding of its response mechanism.

Methods

To assess the molecular and cellular responses of P.tricornutum to grazer presence, we conducted transcriptomic, proteomic, and metabolomic analyses, combined with phenotypic data from previous studies. Sequencing data were obtained by Illumina RNA sequencing, TMT Labeled Quantitative Proteomics and Non-targeted Metabolomics, and WGCNA analysis and statistical analysis were performed.

Results

Among the differentially expressed genes, we observed complex expression patterns of the core genes involved in the phenotypic changes of P.tricornutum under grazing pressure across different strains and multi-omics datasets. These core genes primarily regulate the levels of various proteins and fatty acids, as well as the cellular response to diverse signals.

Conclusion

Our research reveals the association of multi-omics in four strains responses to grazing effects in P.tricornutum. Grazing pressure significantly impacted cell growth, fatty acid composition, stress response, and the core genes involved in phenotype transformation.

A first-in-class inhibitor of Hsp110 molecular chaperones of pathogenic fungi

Proteins of the Hsp110 family are molecular chaperones that play important roles in protein homeostasis in eukaryotes. The pathogenic fungus Candida albicans, which causes infections in humans, has a single Hsp110, termed Msi3. Here, we provide proof-of-principle evidence supporting fungal Hsp110s as targets for the development of new antifungal drugs. We identify a pyrazolo[3,4-b] pyridine derivative, termed HLQ2H (or 2H), that inhibits the biochemical and chaperone activities of Msi3, as well as the growth and viability of C. albicans. Moreover, the fungicidal activity of 2H correlates with its inhibition of in vivo protein folding. We propose 2H and related compounds as promising leads for development of new antifungals and as pharmacological tools for the study of the molecular mechanisms and functions of Hsp110s.

ER Disposal Pathways in Chronic Liver Disease: Protective, Pathogenic, and Potential Therapeutic Targets

The endoplasmic reticulum is a central player in liver pathophysiology. Chronic injury to the ER through increased lipid content, alcohol metabolism, or accumulation of misfolded proteins causes ER stress, dysregulated hepatocyte function, inflammation, and worsened disease pathogenesis. A key adaptation of the ER to resolve stress is the removal of excess or misfolded proteins. Degradation of intra-luminal or ER membrane proteins occurs through distinct mechanisms that include ER-associated Degradation (ERAD) and ER-to-lysosome-associated degradation (ERLAD), which includes macro-ER-phagy, micro-ER-phagy, and Atg8/LC-3-dependent vesicular delivery. All three of these processes are critical for removing misfolded or unfolded protein aggregates, and re-establishing ER homeostasis following expansion/stress, which is critical for liver function and adaptation to injury. Despite playing a key role in resolving ER stress, the contribution of these degradative processes to liver physiology and pathophysiology is understudied. Analysis of publicly available datasets from diseased livers revealed that numerous genes involved in ER-related degradative pathways are dysregulated; however, their roles and regulation in disease progression are not well defined. Here we discuss the dynamic regulation of ER-related protein disposal pathways in chronic liver disease and cell-type specific roles, as well as potentially targetable mechanisms for treatment of chronic liver disease.

Hsp40s play distinct roles during the initial stages of apolipoprotein B biogenesis

Apolipoprotein B (ApoB) is the primary component of atherogenic lipoproteins, which transport serum fats and cholesterol. Therefore, elevated levels of circulating ApoB are a primary risk factor for cardiovascular disease. During ApoB biosynthesis in the liver and small intestine under nutrient-rich conditions, ApoB cotranslationally translocates into the endoplasmic reticulum (ER) and is lipidated and ultimately secreted. Under lipid-poor conditions, ApoB is targeted for ER Associated Degradation (ERAD). Although prior work identified select chaperones that regulate ApoB biogenesis, the contributions of cytoplasmic Hsp40s are undefined. To this end, we screened ApoB-expressing yeast and determined that a class A ER-associated Hsp40, Ydj1, associates with and facilitates the ERAD of ApoB. Consistent with these results, a homologous Hsp40, DNAJA1, functioned similarly in rat hepatoma cells. DNAJA1 deficient cells also secreted hyperlipidated lipoproteins, in accordance with attenuated ERAD. In contrast to the role of DNAJA1 during ERAD, DNAJB1-a class B Hsp40-helped stabilize ApoB. Depletion of DNAJA1 and DNAJB1 also led to opposing effects on ApoB ubiquitination. These data represent the first example in which different Hsp40s exhibit disparate effects during regulated protein biogenesis in the ER, and highlight distinct roles that chaperones can play on a single ERAD substrate.

Interdomain interactions dictate the function of the Candida albicans Hsp110 protein Msi3

Heat shock proteins of 110 kDa (Hsp110s), a unique class of molecular chaperones, are essential for maintaining protein homeostasis. Hsp110s exhibit a strong chaperone activity preventing protein aggregation (the "holdase" activity) and also function as the major nucleotide-exchange factor (NEF) for Hsp70 chaperones. Hsp110s contain two functional domains: a nucleotide-binding domain (NBD) and substrate-binding domain (SBD). ATP binding is essential for Hsp110 function and results in close contacts between the NBD and SBD. However, the molecular mechanism of this ATP-induced allosteric coupling remains poorly defined. In this study, we carried out biochemical analysis on Msi3, the sole Hsp110 in Candida albicans, to dissect the unique allosteric coupling of Hsp110s using three mutations affecting the domain-domain interface. All the mutations abolished both the in vivo and in vitro functions of Msi3. While the ATP-bound state was disrupted in all mutants, only mutation of the NBD-SBDβ interfaces showed significant ATPase activity, suggesting that the full-length Hsp110s have an ATPase that is mainly suppressed by NBD-SBDβ contacts. Moreover, the high-affinity ATP-binding unexpectedly appears to require these NBD-SBD contacts. Remarkably, the "holdase" activity was largely intact for all mutants tested while NEF activity was mostly compromised, although both activities strictly depended on the ATP-bound state, indicating different requirements for these two activities. Stable peptide substrate binding to Msi3 led to dissociation of the NBD-SBD contacts and compromised interactions with Hsp70. Taken together, our data demonstrate that the exceptionally strong NBD-SBD contacts in Hsp110s dictate the unique allosteric coupling and biochemical activities.

The Targeting of Native Proteins to the Endoplasmic Reticulum-Associated Degradation (ERAD) Pathway: An Expanding Repertoire of Regulated Substrates

All proteins are subject to quality control processes during or soon after their synthesis, and these cellular quality control pathways play critical roles in maintaining homeostasis in the cell and in organism health. Protein quality control is particularly vital for those polypeptides that enter the endoplasmic reticulum (ER). Approximately one-quarter to one-third of all proteins synthesized in eukaryotic cells access the ER because they are destined for transport to the extracellular space, because they represent integral membrane proteins, or because they reside within one of the many compartments of the secretory pathway. However, proteins that mature inefficiently are subject to ER-associated degradation (ERAD), a multi-step pathway involving the chaperone-mediated selection, ubiquitination, and extraction (or “retrotranslocation”) of protein substrates from the ER. Ultimately, these substrates are degraded by the cytosolic proteasome. Interestingly, there is an increasing number of native enzymes and metabolite and solute transporters that are also targeted for ERAD. While some of these proteins may transiently misfold, the ERAD pathway also provides a route to rapidly and quantitatively downregulate the levels and thus the activities of a variety of proteins that mature or reside in the ER.

Purification and biochemical characterization of Msi3, an essential Hsp110 molecular chaperone in Candida albicans

Hsp110s are unique and essential molecular chaperones in the eukaryotic cytosol. They play important roles in maintaining cellular protein homeostasis. Candida albicans is the most prevalent yeast opportunistic pathogen that causes fungal infections in humans. As the only Hsp110 in Candida albicans, Msi3 is essential for the growth and infection of Candida albicans. In this study, we have expressed and purified Msi3 in nucleotide-free state and carried out biochemical analyses. Sse1 is the major Hsp110 in budding yeast S. cerevisiae and the best characterized Hsp110. Msi3 can substitute Sse1 in complementing the temperature-sensitive phenotype of S. cerevisiae carrying a deletion of SSE1 gene although Msi3 shares only 63.4% sequence identity with Sse1. Consistent with this functional similarity, the purified Msi3 protein shares many similar biochemical activities with Sse1 including binding ATP with high affinity, changing conformation upon ATP binding, stimulating the nucleotide-exchange for Hsp70, preventing protein aggregation, and assisting Hsp70 in refolding denatured luciferase. These biochemical characterizations suggested that Msi3 can be used as a model for studying the molecular mechanisms of Hsp110s.

Clearing Traffic Jams During Protein Translocation Across Membranes

Protein translocation across membranes is a critical facet of protein biogenesis in compartmentalized cells as proteins synthesized in the cytoplasm often need to traverse across lipid bilayers via proteinaceous channels to reach their final destinations. It is well established that protein biogenesis is tightly linked to various protein quality control processes, which monitor errors in protein folding, modification, and localization. However, little is known about how cells cope with translocation defective polypeptides that clog translocation channels (translocons) during protein translocation. This review summarizes recent studies, which collectively reveal a set of translocon-associated quality control strategies for eliminating polypeptides stuck in protein-conducting channels in the endoplasmic reticulum and mitochondria.

Chaperoning Endoplasmic Reticulum-Associated Degradation (ERAD) and Protein Conformational Diseases

Misfolded proteins compromise cellular homeostasis. This is especially problematic in the endoplasmic reticulum (ER), which is a high-capacity protein-folding compartment and whose function requires stringent protein quality-control systems. Multiprotein complexes in the ER are able to identify, remove, ubiquitinate, and deliver misfolded proteins to the 26S proteasome for degradation in the cytosol, and these events are collectively termed ER-associated degradation, or ERAD. Several steps in the ERAD pathway are facilitated by molecular chaperone networks, and the importance of ERAD is highlighted by the fact that this pathway is linked to numerous protein conformational diseases. In this review, we discuss the factors that constitute the ERAD machinery and detail how each step in the pathway occurs. We then highlight the underlying pathophysiology of protein conformational diseases associated with ERAD.

Hsp104 facilitates the endoplasmic-reticulum-associated degradation of disease-associated and aggregation-prone substrates

Misfolded proteins in the endoplasmic reticulum (ER) are selected for ER-associated degradation (ERAD). More than 60 disease-associated proteins are substrates for the ERAD pathway due to the presence of missense or nonsense mutations. In yeast, the Hsp104 molecular chaperone disaggregates detergent-insoluble ERAD substrates, but the spectrum of disease-associated ERAD substrates that may be aggregation prone is unknown. To determine if Hsp104 recognizes aggregation-prone ERAD substrates associated with human diseases, we developed yeast expression systems for a hydrophobic lipid-binding protein, apolipoprotein B (ApoB), along with a chimeric protein harboring a nucleotide-binding domain from the cystic fibrosis transmembrane conductance regulator (CFTR) into which disease-causing mutations were introduced. We discovered that Hsp104 facilitates the degradation of ER-associated ApoB as well as a truncated CFTR chimera in which a premature stop codon corresponds to a disease-causing mutation. Chimeras containing a wild-type version of the CFTR domain or a different mutation were stable and thus Hsp104 independent. We also discovered that the detergent solubility of the unstable chimera was lower than the stable chimeras, and Hsp104 helped retrotranslocate the unstable chimera from the ER, consistent with disaggregase activity. To determine why the truncated chimera was unstable, we next performed molecular dynamics simulations and noted significant unraveling of the CFTR nucleotide-binding domain. Because human cells lack Hsp104, these data indicate that an alternate disaggregase or mechanism facilitates the removal of aggregation-prone, disease-causing ERAD substrates in their native environments.

Molecular chaperones: from proteostasis to pathogenesis

Maintaining protein homeostasis (proteostasis) is essential for a functional proteome. A wide range of extrinsic and intrinsic factors perturb proteostasis, causing protein misfolding, misassembly, and aggregation. This compromises cellular integrity and leads to aging and disease, including neurodegeneration and cancer. At the cellular level, protein aggregation is counteracted by powerful mechanisms comprising of a cascade of enzymes and chaperones that operate in a coordinated multistep manner to sense, prevent, and/or dispose of aberrant proteins. Although these processes are well understood for soluble proteins, there is a major gap in our understanding of how cells handle misfolded or aggregated membrane proteins. This article provides an overview of cellular proteostasis with emphasis on membrane protein substrates and suggests host-virus interaction as a tool to clarify outstanding questions in proteostasis.

A novel system to quantify intestinal lipid digestion and transport

The zebrafish larva is a powerful tool for the study of dietary triglyceride (TG) digestion and how fatty acids (FA) derived from dietary lipids are absorbed, metabolized and distributed to the body. While fluorescent FA analogues have enabled visualization of FA metabolism, methods for specifically assaying TG digestion are badly needed. Here we present a novel High Performance Liquid Chromatography (HPLC) method that quantitatively differentiates TG and phospholipid (PL) molecules with one or two fluorescent FA analogues. We show how this tool may be used to discriminate between undigested and digested TG or phosphatidylcholine (PC), and also the products of TG or PC that have been digested, absorbed and re-synthesized into new lipid molecules. Using this approach, we explored the dietary requirement of zebrafish larvae for phospholipids. Here we demonstrate that dietary TG is digested and absorbed in the intestinal epithelium, but without dietary PC, TG accumulates and is not transported out of the enterocytes. Consequently, intestinal ER stress increases and the ingested lipid is not available support the energy and metabolic needs of other tissues. In TG diets with PC, TG is readily transported from the intestine and subsequently metabolized.

Can modulators of apolipoproteinB biogenesis serve as an alternate target for cholesterol-lowering drugs?

Understanding the molecular defects underlying cardiovascular disease is necessary for the development of therapeutics. The most common method to lower circulating lipids, which reduces the incidence of cardiovascular disease, is statins, but other drugs are now entering the clinic, some of which have been approved. Nevertheless, patients cannot tolerate some of these therapeutics, the drugs are costly, and/or the treatments are approved for only rare forms of disease. Efforts to find alternative treatments have focused on other factors, such as apolipoproteinB (apoB), which transports cholesterol in the blood stream. The levels of apoB are regulated by endoplasmic reticulum (ER) associated degradation as well as by a post ER degradation pathway in model systems, and we suggest that these events provide novel therapeutic targets. We discuss first how cardiovascular disease arises and how cholesterol is regulated, and then summarize the mechanisms of action of existing treatments for cardiovascular disease. We then review the apoB biosynthetic pathway, focusing on steps that might be amenable to therapeutic interventions.

Intestinal CD36 and Other Key Proteins of Lipid Utilization: Role in Absorption and Gut Homeostasis

Several proteins have been implicated in fatty acid (FA) transport by enterocytes including the scavenger receptor CD36 (SR-B2), the scavenger receptor B1 (SR-B1) a member of the CD36 family and the FA transport protein 4 (FATP4). Here, we review the regulation of enterocyte FA uptake and its function in lipid absorption including prechylomicron formation, assembly and transport. Emphasis is given to CD36, which is abundantly expressed along the digestive tract of rodents and humans and has been the most studied. We also address the pleiotropic functions of CD36 that go beyond lipid absorption and metabolism to include recent evidence of its impact on intestinal homeostasis and barrier maintenance. Areas of progress involving contribution of membrane phospholipid remodeling and of cytosolic FA-binding proteins, FABP1 and FABP2 to fat absorption will be covered. © 2018 American Physiological Society. Compr Physiol 8:493-507, 2018.

Acetylation of N-terminus and two internal amino acids is dispensable for degradation of a protein that aberrantly engages the endoplasmic reticulum translocon

Conserved homologues of the Hrd1 ubiquitin ligase target for degradation proteins that persistently or aberrantly engage the endoplasmic reticulum translocon, including mammalian apolipoprotein B (apoB; the major protein component of low-density lipoproteins) and the artificial yeast protein Deg1-Sec62. A complete understanding of the molecular mechanism by which translocon-associated proteins are recognized and degraded may inform the development of therapeutic strategies for cholesterol-related pathologies. Both apoB and Deg1-Sec62 are extensively post-translationally modified. Mass spectrometry of a variant of Deg1-Sec62 revealed that the protein is acetylated at the N-terminal methionine and two internal lysine residues. N-terminal and internal acetylation regulates the degradation of a variety of unstable proteins. However, preventing N-terminal and internal acetylation had no detectable consequence for Hrd1-mediated proteolysis of Deg1-Sec62. Our data highlight the importance of empirically validating the role of post-translational modifications and sequence motifs on protein degradation, even when such elements have previously been demonstrated sufficient to destine other proteins for destruction.

Membrane Protein Quantity Control at the Endoplasmic Reticulum

The canonical function of the endoplasmic reticulum-associated degradation (ERAD) system is to enforce quality control among membrane-associated proteins by targeting misfolded secreted, intra-organellar, and intramembrane proteins for degradation. However, increasing evidence suggests that ERAD additionally functions in maintaining appropriate levels of a subset of membrane-associated proteins. In this 'quantity control' capacity, ERAD responds to environmental cues to regulate the proteasomal degradation of specific ERAD substrates according to cellular need. In this review, we discuss in detail seven proteins that are targeted by the ERAD quantity control system. Not surprisingly, ERAD-mediated protein degradation is a key regulatory feature of a variety of ER-resident proteins, including HMG-CoA reductase, cytochrome P450 3A4, IP3 receptor, and type II iodothyronine deiodinase. In addition, the ERAD quantity control system plays roles in maintaining the proper stoichiometry of multi-protein complexes by mediating the degradation of components that are produced in excess of the limiting subunit. Perhaps somewhat unexpectedly, recent evidence suggests that the ERAD quantity control system also contributes to the regulation of plasma membrane-localized signaling receptors, including the ErbB3 receptor tyrosine kinase and the GABA neurotransmitter receptors. For these substrates, a proportion of the newly synthesized yet properly folded receptors are diverted for degradation at the ER, and are unable to traffic to the plasma membrane. Given that receptor abundance or concentration within the plasma membrane plays key roles in determining signaling efficiency, these observations may point to a novel mechanism for modulating receptor-mediated cellular signaling.

Endoplasmic reticulum-associated degradation of the renal potassium channel, ROMK, leads to type II Bartter syndrome

Type II Bartter syndrome is caused by mutations in the renal outer medullary potassium (ROMK) channel, but the molecular mechanisms underlying this disease are poorly defined. To rapidly screen for ROMK function, we developed a yeast expression system and discovered that yeast cells lacking endogenous potassium channels could be rescued by WT ROMK but not by ROMK proteins containing any one of four Bartter mutations. We also found that the mutant proteins were significantly less stable than WT ROMK. However, their degradation was slowed in the presence of a proteasome inhibitor or when yeast cells contained mutations in the CDC48 or SSA1 gene, which is required for endoplasmic reticulum (ER)-associated degradation (ERAD). Consistent with these data, sucrose gradient centrifugation and indirect immunofluorescence microscopy indicated that most ROMK protein was ER-localized. To translate these findings to a more relevant cell type, we measured the stabilities of WT ROMK and the ROMK Bartter mutants in HEK293 cells. As in yeast, the Bartter mutant proteins were less stable than the WT protein, and their degradation was slowed in the presence of a proteasome inhibitor. Finally, we discovered that low-temperature incubation increased the steady-state levels of a Bartter mutant, suggesting that the disease-causing mutation traps the protein in a folding-deficient conformation. These findings indicate that the underlying pathology for at least a subset of patients with type II Bartter syndrome is linked to the ERAD pathway and that future therapeutic strategies should focus on correcting deficiencies in ROMK folding.

How Polyomaviruses Exploit the ERAD Machinery to Cause Infection

To infect cells, polyomavirus (PyV) traffics from the cell surface to the endoplasmic reticulum (ER) where it hijacks elements of the ER-associated degradation (ERAD) machinery to penetrate the ER membrane and reach the cytosol. From the cytosol, the virus transports to the nucleus, enabling transcription and replication of the viral genome that leads to lytic infection or cellular transformation. How PyV exploits the ERAD machinery to cross the ER membrane and access the cytosol, a decisive infection step, remains enigmatic. However, recent studies have slowly unraveled many aspects of this process. These emerging insights should advance our efforts to develop more effective therapies against PyV-induced human diseases.

A Non-enveloped Virus Hijacks Host Disaggregation Machinery to Translocate across the Endoplasmic Reticulum Membrane

Mammalian cytosolic Hsp110 family, in concert with the Hsc70:J-protein complex, functions as a disaggregation machinery to rectify protein misfolding problems. Here we uncover a novel role of this machinery in driving membrane translocation during viral entry. The non-enveloped virus SV40 penetrates the endoplasmic reticulum (ER) membrane to reach the cytosol, a critical infection step. Combining biochemical, cell-based, and imaging approaches, we find that the Hsp110 family member Hsp105 associates with the ER membrane J-protein B14. Here Hsp105 cooperates with Hsc70 and extracts the membrane-penetrating SV40 into the cytosol, potentially by disassembling the membrane-embedded virus. Hence the energy provided by the Hsc70-dependent Hsp105 disaggregation machinery can be harnessed to catalyze a membrane translocation event.

How non-enveloped viruses penetrate a host membrane to enter cells and cause disease remains an enigmatic step. To infect cells, the non-enveloped SV40 must transport across the ER membrane to reach the cytosol. In this study, we report that a cellular Hsp105-powered disaggregation machinery pulls SV40 into the cytosol, likely by uncoating the ER membrane-penetrating virus. Because this disaggregation machinery is thought to clarify cellular aggregated proteins, we propose that the force generated by this machinery can also be hijacked by a non-enveloped virus to propel its entry into the host.

Overexpression, Purification and Characterisation of the Plasmodium falciparum Hsp70-z (PfHsp70-z) Protein

Six Hsp70-like genes are represented on the genome of Plasmodium falciparum. Of these two occur in the cytosol: P. falciparum Hsp70-z (PfHsp70-z) and PfHsp70-1. PfHsp70-1 is a well characterised canonical Hsp70 that facilitates protein quality control and is crucial for the development of malaria parasites. There is very little known about PfHsp70-z. However, PfHsp70-z is known to be essential and is implicated in suppressing aggregation of asparagine-rich proteins of P. falciparum. In addition, its expression at the clinical stage of malaria correlates with disease prognosis. Based on structural evidence PfHsp70-z belongs to the Hsp110 family of proteins. Since Hsp110 proteins have been described as nucleotide exchange factors (NEFs) of their canonical Hsp70 counterparts, it has been speculated that PfHsp70-z may serve as a NEF of PfHsp70-1. In the current study, P. falciparum cells cultured in vitro were subjected to heat stress, triggering the enhanced expression of PfHsp70-z. Biochemical assays conducted using recombinant PfHsp70-z protein demonstrated that the protein is heat stable and possesses ATPase activity. Furthermore, we observed that PfHsp70-z is capable of self-association. The structural-functional features of PfHsp70-z provide further evidence for its role as a chaperone and possible nucleotide exchange factor of PfHsp70-1.

Rkr1/Ltn1 Ubiquitin Ligase-mediated Degradation of Translationally Stalled Endoplasmic Reticulum Proteins

Aberrant nonstop proteins arise from translation of mRNA molecules beyond the coding sequence into the 3'-untranslated region. If a stop codon is not encountered, translation continues into the poly(A) tail, resulting in C-terminal appendage of a polylysine tract and a terminally stalled ribosome. In Saccharomyces cerevisiae, the ubiquitin ligase Rkr1/Ltn1 has been implicated in the proteasomal degradation of soluble cytosolic nonstop and translationally stalled proteins. Rkr1 is essential for cellular fitness under conditions associated with increased prevalence of nonstop proteins. Mutation of the mammalian homolog causes significant neurological pathology, suggesting broad physiological significance of ribosome-associated quality control. It is not known whether and how soluble or transmembrane nonstop and translationally stalled proteins targeted to the endoplasmic reticulum (ER) are detected and degraded. We generated and characterized model soluble and transmembrane ER-targeted nonstop and translationally stalled proteins. We found that these proteins are indeed subject to proteasomal degradation. We tested three candidate ubiquitin ligases (Rkr1 and ER-associated Doa10 and Hrd1) for roles in regulating abundance of these proteins. Our results indicate that Rkr1 plays the primary role in targeting the tested model ER-targeted nonstop and translationally stalled proteins for degradation. These data expand the catalog of Rkr1 substrates and highlight a previously unappreciated role for this ubiquitin ligase at the ER membrane.

Safety assessment of potential food ingredients in canine hepatocytes

This research aimed to develop in vitro methods to assess hazard of canine food ingredients. Canine hepatocytes were harvested and cell viability of clove-leaf oil (CLO), eugenol (EUG), lemongrass oil (LGO), guanosine monophosphate (GMP), inosine monophosphate (IMP), sorbose, ginger-root extract (GRE), cinnamon-bark oil (CBO), cinnamaldehyde (CINA), thymol oil (TO), thymol (THYM), and citric acid were assessed with positive controls: acetaminophen (APAP), aflatoxin B1 and xylitol. Molecular Toxicology PathwayFinder array (MTPF) analyzed toxicity mechanisms for LGO. LC50 for APAP was similar among human (3.45), rat (2.35), dog (4.26 mg/ml). Aflatoxin B1 had an LC50 of 4.43 (human), 5.78 (rat) and 6.05 (dog) µg/ml; xylitol did not decrease viability. LC50 of CLO (0.185 ± 0.075(SD)), EUG (0.165 ± 0.112), LGO (0.220 ± 0.012), GRE (1.54 ± 0.31) mg/ml; GMP (166.03 ± 41.83), GMP + IMP (208.67 ± 15.27) mM; CBO (0.08 ± 0.03), CINA (0.11 ± 0.01), TO (0.21 ± 0.03), THYM (0.05 ± 0.01), citric acid (1.58 ± 0.08) mg/ml, while sorbose was non-toxic. LGO induced upregulation of 16 and down-regulation of 24 genes, which CYP and heat shock most affected. These results suggest that in vitro assays such as this may be useful for hazard assessment of food ingredients for altered hepatic function.

Structural mechanisms of chaperone mediated protein disaggregation

The ClpB/Hsp104 and Hsp70 classes of molecular chaperones use ATP hydrolysis to dissociate protein aggregates and complexes, and to move proteins through membranes. ClpB/Hsp104 are members of the AAA+ family of proteins which form ring-shaped hexamers. Loops lining the pore in the ring engage substrate proteins as extended polypeptides. Interdomain rotations and conformational changes in these loops coupled to ATP hydrolysis unfold and pull proteins through the pore. This provides a mechanism that progressively disrupts local secondary and tertiary structure in substrates, allowing these chaperones to dissociate stable aggregates such as β-sheet rich prions or coiled coil SNARE complexes. While the ClpB/Hsp104 mechanism appears to embody a true power-stroke in which an ATP powered conformational change in one protein is directly coupled to movement or structural change in another, the mechanism of force generation by Hsp70s is distinct and less well understood. Both active power-stroke and purely passive mechanisms in which Hsp70 captures spontaneous fluctuations in a substrate have been proposed, while a third proposed mechanism-entropic pulling-may be able to generate forces larger than seen in ATP-driven molecular motors without the conformational coupling required for a power-stroke. The disaggregase activity of these chaperones is required for thermotolerance, but unrestrained protein complex/aggregate dissociation is potentially detrimental. Disaggregating chaperones are strongly auto-repressed, and are regulated by co-chaperones which recruit them to protein substrates and activate the disaggregases via mechanisms involving either sequential transfer of substrate from one chaperone to another and/or simultaneous interaction of substrate with multiple chaperones. By effectively subjecting substrates to multiple levels of selection by multiple chaperones, this may insure that these potent disaggregases are only activated in the appropriate context.

ERAD and how viruses exploit it

Endoplasmic reticulum (ER)-associated degradation (ERAD) is a universally important process among eukaryotic cells. ERAD is necessary to preserve cell integrity since the accumulation of defective proteins results in diseases associated with neurological dysfunction, cancer, and infections. This process involves recognition of misfolded or misassembled proteins that have been translated in association with ER membranes. Recognition of ERAD substrates leads to their extraction through the ER membrane (retrotranslocation or dislocation), ubiquitination, and destruction by cytosolic proteasomes. This review focuses on ERAD and its components as well as how viruses use this process to promote their replication and to avoid the immune response.

The metazoan protein disaggregase and amyloid depolymerase system: Hsp110, Hsp70, Hsp40, and small heat shock proteins

A baffling aspect of metazoan proteostasis is the lack of an Hsp104 ortholog that rapidly disaggregates and reactivates misfolded polypeptides trapped in stress induced disordered aggregates, preamyloid oligomers, or amyloid fibrils. By contrast, in bacteria, protozoa, chromista, fungi, and plants, Hsp104 orthologs are highly conserved and confer huge selective advantages in stress tolerance. Moreover, in fungi, the amyloid remodeling activity of Hsp104 has enabled deployment of prions for various beneficial modalities. Thus, a longstanding conundrum has remained unanswered: how do metazoan cells renature aggregated proteins or resolve amyloid fibrils without Hsp104? Here, we highlight recent advances that unveil the metazoan protein-disaggregase machinery, comprising Hsp110, Hsp70, and Hsp40, which synergize to dissolve disordered aggregates, but are unable to rapidly solubilize stable amyloid fibrils. However, Hsp110, Hsp70, and Hsp40 exploit the slow monomer exchange dynamics of amyloid, and can slowly depolymerize amyloid fibrils from their ends in a manner that is stimulated by small heat shock proteins. Upregulation of this system could have key therapeutic applications in various protein-misfolding disorders. Intriguingly, yeast Hsp104 can interface with metazoan Hsp110, Hsp70, and Hsp40 to rapidly eliminate disease associated amyloid. Thus, metazoan proteostasis is receptive to augmentation with exogenous disaggregases, which opens a number of therapeutic opportunities.

Suppression of polyglutamine protein toxicity by co-expression of a heat-shock protein 40 and a heat-shock protein 110

A network of heat-shock proteins mediates cellular protein homeostasis, and has a fundamental role in preventing aggregation-associated neurodegenerative diseases. In a Drosophila model of polyglutamine (polyQ) disease, the HSP40 family protein, DNAJ-1, is a superior suppressor of toxicity caused by the aggregation of polyQ containing proteins. Here, we demonstrate that one specific HSP110 protein, 70 kDa heat-shock cognate protein cb (HSC70cb), interacts physically and genetically with DNAJ-1 in vivo, and that HSC70cb is necessary for DNAJ-1 to suppress polyglutamine-induced cell death in Drosophila. Expression of HSC70cb together with DNAJ-1 significantly enhanced the suppressive effects of DNAJ-1 on polyQ-induced neurodegeneration, whereas expression of HSC70cb alone did not suppress neurodegeneration in Drosophila models of either general polyQ disease or Huntington's disease. Furthermore, expression of a human HSP40, DNAJB1, together with a human HSP110, APG-1, protected cells from polyQ-induced neural degeneration in flies, whereas expression of either component alone had little effect. Our data provide a functional link between HSP40 and HSP110 in suppressing the cytotoxicity of aggregation-prone proteins, and suggest that HSP40 and HSP110 function together in protein homeostasis control.

Hsp70 targets a cytoplasmic quality control substrate to the San1p ubiquitin ligase

Accumulation of misfolded proteins in cellular compartments can result in stress-induced cell death. In the endoplasmic reticulum (ER), ER-associated degradation clears aberrant proteins from the secretory pathway. In the cytoplasm and nucleus, this job is left to the cytoplasmic quality control (CytoQC) machinery. Both processes utilize chaperones and the ubiquitin-proteasome system to aid in protein elimination. Previous studies in yeast have drawn comparisons between these processes using data from structurally and topologically different substrates. We sought to draw a direct comparison between ERAD and CytoQC by studying the elimination of a single misfolded domain that, depending on its residence, is disposed by either of these pathways. The truncated, second nucleotide binding domain (NBD2*) from a yeast ERAD substrate, Ste6p*, resides at the cytoplasmic face of the ER. We show that a soluble form of NBD2* is cytoplasmic and unlike wild-type NBD2 is targeted for proteasome-mediated degradation. In contrast to Ste6p*, which employs the ER-localized Doa10p ubiquitin ligase, NBD2* is ubiquitinated by a nuclear E3 ligase San1p, a factor that is also required for its degradation. Although the yeast cytoplasmic Hsp70 chaperone, Ssa1p, has been thought to facilitate the nuclear import or to maintain the solubility of most CytoQC substrates, we discovered that Ssa1p facilitates the interaction between San1p and NBD2*, demonstrating that chaperones can aid in substrate recognition and San1p-dependent protein degradation. These results emphasize the diverse action of molecular chaperones during CytoQC.

The Lhs1/GRP170 chaperones facilitate the endoplasmic reticulum-associated degradation of the epithelial sodium channel

The epithelial sodium channel, ENaC, plays a critical role in maintaining salt and water homeostasis, and not surprisingly defects in ENaC function are associated with disease. Like many other membrane-spanning proteins, this trimeric protein complex folds and assembles inefficiently in the endoplasmic reticulum (ER), which results in a substantial percentage of the channel being targeted for ER-associated degradation (ERAD). Because the spectrum of factors that facilitates the degradation of ENaC is incomplete, we developed yeast expression systems for each ENaC subunit. We discovered that a conserved Hsp70-like chaperone, Lhs1, is required for maximal turnover of the ENaC α subunit. By expressing Lhs1 ATP binding mutants, we also found that the nucleotide exchange properties of this chaperone are dispensable for ENaC degradation. Consistent with the precipitation of an Lhs1-αENaC complex, Lhs1 holdase activity was instead most likely required to support the ERAD of αENaC. Moreover, a complex containing the mammalian Lhs1 homolog GRP170 and αENaC co-precipitated, and GRP170 also facilitated ENaC degradation in human, HEK293 cells, and in a Xenopus oocyte expression system. In both yeast and higher cell types, the effect of Lhs1 on the ERAD of αENaC was selective for the unglycosylated form of the protein. These data establish the first evidence that Lhs1/Grp170 chaperones can act as mediators of ERAD substrate selection.

The endoplasmic reticulum-associated degradation pathways of budding yeast

Protein misfolding is a common cellular event that can produce intrinsically harmful products. To reduce the risk, quality control mechanisms are deployed to detect and eliminate misfolded, aggregated, and unassembled proteins. In the secretory pathway, it is mainly the endoplasmic reticulum-associated degradation (ERAD) pathways that perform this role. Here, specialized factors are organized to monitor and process the folded states of nascent polypeptides. Despite the complex structures, topologies, and posttranslational modifications of client molecules, the ER mechanisms are the best understood among all protein quality-control systems. This is the result of convergent and sometimes serendipitous discoveries by researchers from diverse fields. Although major advances in ER quality control and ERAD came from all model organisms, this review will focus on the discoveries culminating from the simple budding yeast.

Aberrant substrate engagement of the ER translocon triggers degradation by the Hrd1 ubiquitin ligase

The Hrd1 ubiquitin ligase plays a role in quality control of two substrates associated with the Sec61 translocon.

Little is known about quality control of proteins that aberrantly or persistently engage the endoplasmic reticulum (ER)-localized translocon en route to membrane localization or the secretory pathway. Hrd1 and Doa10, the primary ubiquitin ligases that function in ER-associated degradation (ERAD) in yeast, target distinct subsets of misfolded or otherwise abnormal proteins based primarily on degradation signal (degron) location. We report the surprising observation that fusing Deg1, a cytoplasmic degron normally recognized by Doa10, to the Sec62 membrane protein rendered the protein a Hrd1 substrate. Hrd1-dependent degradation occurred when Deg1-Sec62 aberrantly engaged the Sec61 translocon channel and underwent topological rearrangement. Mutations that prevent translocon engagement caused a reversion to Doa10-dependent degradation. Similarly, a variant of apolipoprotein B, a protein known to be cotranslocationally targeted for proteasomal degradation, was also a Hrd1 substrate. Hrd1 therefore likely plays a general role in targeting proteins that persistently associate with and potentially obstruct the translocon.

Finding the will and the way of ERAD substrate retrotranslocation

ER-associated degradation (ERAD) is a mechanism by which numerous ER-localized proteins are targeted for cytosolic degradation by the ubiquitin-proteasome system. A surprising and still-cryptic requirement of this process is the energy dependent retrotranslocation of both lumenal and membrane-embedded ER proteins into the cytosol for ongoing ubiquitination and proteasomal destruction. The current understanding, results, and open questions are discussed below for this intriguing and critical process of ERAD.

Intracellular trafficking and secretion of VLDL

Steady increase in the incidence of atherosclerosis is becoming a major concern not only in the United States but also in other countries. One of the major risk factors for the development of atherosclerosis is high concentrations of plasma low-density lipoprotein, which are metabolic products of very low-density lipoprotein (VLDL). VLDLs are synthesized and secreted by the liver. In this review, we discuss various stages through which VLDL particles go from their biogenesis to secretion in the circulatory system. Once VLDLs are synthesized in the lumen of the endoplasmic reticulum, they are transported to the Golgi. The transport of nascent VLDLs from the endoplasmic reticulum to Golgi is a complex multistep process, which is mediated by a specialized transport vesicle, the VLDL transport vesicle. The VLDL transport vesicle delivers VLDLs to the cis-Golgi lumen where nascent VLDLs undergo a number of essential modifications. The mature VLDL particles are then transported to the plasma membrane and secreted in the circulatory system. Understanding of molecular mechanisms and identification of factors regulating the complex intracellular VLDL trafficking will provide insight into the pathophysiology of various metabolic disorders associated with abnormal VLDL secretion and identify potential new therapeutic targets.

Human heat shock protein 105/110 kDa (Hsp105/110) regulates biogenesis and quality control of misfolded cystic fibrosis transmembrane conductance regulator at multiple levels

Heat shock protein 105/110-kDa (Hsp105/110), a member of the Hsp70 super family of molecular chaperones, serves as a nucleotide exchange factor for Hsc70, independently prevents the aggregation of misfolded proteins, and functionally relates to Hsp90. We investigated the roles of human Hsp105α, the constitutively expressed isoform, in the biogenesis and quality control of the cystic fibrosis transmembrane conductance regulator (CFTR). In the endoplasmic reticulum (ER), Hsp105 facilitates CFTR quality control at an early stage in its biosynthesis but promotes CFTR post-translational folding. Deletion of Phe-508 (ΔF508), the most prevalent mutation causing cystic fibrosis, interferes with de novo folding of CFTR, impairing its export from the ER and accelerating its clearance in the ER and post-Golgi compartments. We show that Hsp105 preferentially associates with and stabilizes ΔF508 CFTR at both levels. Introduction of the Hsp105 substrate binding domain potently increases the steady state level of ΔF508 CFTR by reducing its early-stage degradation. This in turn dramatically enhances ΔF508 CFTR cell surface functional expression in cystic fibrosis airway epithelial cells. Although other Hsc70 nucleotide exchange factors such as HspBP1 and BAG-2 inhibit CFTR post-translational degradation in the ER through cochaperone CHIP, Hsp105 has a primary role promoting CFTR quality control at an earlier stage. The Hsp105-mediated multilevel regulation of ΔF508 CFTR folding and quality control provides new opportunities to understand how chaperone machinery regulates the homeostasis and functional expression of misfolded proteins in the cell. Future studies in this direction will inform therapeutics development for cystic fibrosis and other protein misfolding diseases.

The degradation of apolipoprotein B100: multiple opportunities to regulate VLDL triglyceride production by different proteolytic pathways

Very low density lipoproteins (VLDL) are a major secretory product of the liver. They serve to transport endogenously synthesized lipids, mainly triglycerides (but also some cholesterol and cholesteryl esters) to peripheral tissues. VLDL is also the precursor of LDL. ApoB100 is absolutely required for VLDL assembly and secretion. The amount of VLDL triglycerides secreted by the liver depends on the amount loaded onto each lipoprotein particle, as well as the number of particles. Each VLDL has one apoB100 molecule, making apoB100 availability a key determinant of the number of VLDL particles, and hence, triglycerides, that can be secreted by hepatic cells. Surprisingly, the pool of apoB100 in the liver is typically regulated not by its level of synthesis, which is relatively constant, but by its level of degradation. It is now recognized that there are multiple opportunities for the hepatic cell to intercept apoB100 molecules and to direct them to distinct degradative processes. This mini-review will summarize progress in understanding these processes, with an emphasis on autophagy, the most recently described pathway of apoB100 degradation, and the one with possibly the most physiologic relevance to common metabolic perturbations affecting VLDL production. This article is part of a Special Issue entitled Triglyceride Metabolism and Disease.

Protein disulfide isomerases contribute differentially to the endoplasmic reticulum-associated degradation of apolipoprotein B and other substrates

ER-associated degradation (ERAD) rids the early secretory pathway of misfolded or misprocessed proteins. Some members of the protein disulfide isomerase (PDI) family appear to facilitate ERAD substrate selection and retrotranslocation, but a thorough characterization of PDIs during the degradation of diverse substrates has not been undertaken, in part because there are 20 PDI family members in mammals. PDIs can also exhibit disulfide redox, isomerization, and/or chaperone activity, but which of these activities is required for the ERAD of different substrate classes is unknown. We therefore examined the fates of unique substrates in yeast, which expresses five PDIs. Through the use of a yeast expression system for apolipoprotein B (ApoB), which is disulfide rich, we discovered that Pdi1 interacts with ApoB and facilitates degradation through its chaperone activity. In contrast, Pdi1's redox activity was required for the ERAD of CPY* (a misfolded version of carboxypeptidase Y that has five disulfide bonds). The ERAD of another substrate, the alpha subunit of the epithelial sodium channel, was Pdi1 independent. Distinct effects of mammalian PDI homologues on ApoB degradation were then observed in hepatic cells. These data indicate that PDIs contribute to the ERAD of proteins through different mechanisms and that PDI diversity is critical to recognize the spectrum of potential ERAD substrates.

Unique peptide substrate binding properties of 110-kDa heat-shock protein (Hsp110) determine its distinct chaperone activity

The molecular chaperone 70-kDa heat-shock proteins (Hsp70s) play essential roles in maintaining protein homeostasis. Hsp110, an Hsp70 homolog, is highly efficient in preventing protein aggregation but lacks the hallmark folding activity seen in Hsp70s. To understand the mechanistic differences between these two chaperones, we first characterized the distinct peptide substrate binding properties of Hsp110s. In contrast to Hsp70s, Hsp110s prefer aromatic residues in their substrates, and the substrate binding and release exhibit remarkably fast kinetics. Sequence and structure comparison revealed significant differences in the two peptide-binding loops: the length and properties are switched. When we swapped these two loops in an Hsp70, the peptide binding properties of this mutant Hsp70 were converted to Hsp110-like, and more impressively, it functionally behaved like an Hsp110. Thus, the peptide substrate binding properties implemented in the peptide-binding loops may determine the chaperone activity differences between Hsp70s and Hsp110s.

Molecular chaperones and substrate ubiquitination control the efficiency of endoplasmic reticulum-associated degradation

The endoplasmic reticulum (ER) must contend with a large protein flux, which is especially notable in cells dedicated to secreting hormone-regulated gene products. Because of the complexity of the protein folding pathway and the potential for genetic or stochastic errors, a significant percentage of these nascent secreted proteins fail to acquire their native conformations. If these species cannot be cleared from the ER, they may aggregate, which leads to cell death. To lessen the effects of potentially toxic polypeptides, aberrant ER proteins are destroyed via a process known as ER-associated degradation (ERAD). ERAD substrates are selected by molecular chaperones and chaperone-like proteins, and prior to degradation most substrates are ubiquitin-modified. Together with the unfolded protein response, the ERAD pathway is a critical component of the protein quality control machinery in the ER. Although emerging data continue to link ERAD with human diseases, most of our knowledge of this pathway arose from studies using a model eukaryote, the yeast Saccharomyces cerevisiae. In this review, we will summarize the discoveries that led to our current understanding of this pathway, focusing primarily on experiments in yeast. We will also indicate links between ERAD and disease and emphasize future research avenues.

Hsp70- and Hsp90-mediated proteasomal degradation underlies TPI sugarkill pathogenesis in Drosophila

Triosephosphate isomerase (TPI) deficiency is a severe glycolytic enzymopathy that causes progressive locomotor impairment and neurodegeneration, susceptibility to infection, and premature death. The recessive missense TPI(sugarkill) mutation in Drosophila melanogaster exhibits phenotypes analogous to human TPI deficiency such as progressive locomotor impairment, neurodegeneration, and reduced life span. We have shown that the TPI(sugarkill) protein is an active stable dimer; however, the mutant protein is turned over by the proteasome reducing cellular levels of this glycolytic enzyme. As proteasome function is often coupled with molecular chaperone activity, we hypothesized that TPI(sugarkill) is recognized by molecular chaperones that mediate the proteasomal degradation of the mutant protein. Coimmunoprecipitation data and analyses of TPI(sugarkill) turnover in animals with reduced or enhanced molecular chaperone activity indicate that both Hsp90 and Hsp70 are important for targeting TPI(sugarkill) for degradation. Furthermore, molecular chaperone and proteasome activity modified by pharmacological or genetic manipulations resulted in improved TPI(sugarkill) protein levels and rescue some but not all of the disease phenotypes suggesting that TPI deficiency pathology is complex. Overall, these data demonstrate a surprising role for Hsp70 and Hsp90 in the progression of neural dysfunction associated with TPI deficiency.

Loss of Hsp110 leads to age-dependent tau hyperphosphorylation and early accumulation of insoluble amyloid beta

Accumulation of tau into neurofibrillary tangles is a pathological consequence of Alzheimer's disease and other tauopathies. Failures of the quality control mechanisms by the heat shock proteins (Hsps) positively correlate with the appearance of such neurodegenerative diseases. However, in vivo genetic evidence for the roles of Hsps in neurodegeneration remains elusive. Hsp110 is a nucleotide exchange factor for Hsp70, and direct substrate binding to Hsp110 may facilitate substrate folding. Hsp70 complexes have been implicated in tau phosphorylation state and amyloid precursor protein (APP) processing. To provide evidence for a role for Hsp110 in central nervous system homeostasis, we have generated hsp110(-)(/)(-) mice. Our results show that hsp110(-)(/)(-) mice exhibit accumulation of hyperphosphorylated-tau (p-tau) and neurodegeneration. We also demonstrate that Hsp110 is in complexes with tau, other molecular chaperones, and protein phosphatase 2A (PP2A). Surprisingly, high levels of PP2A remain bound to tau but with significantly reduced activity in brain extracts from aged hsp110(-)(/)(-) mice compared to brain extracts from wild-type mice. Mice deficient in the Hsp110 partner (Hsp70) also exhibit a phenotype comparable to that of hsp110(-)(/)(-) mice, confirming a critical role for Hsp110-Hsp70 in maintaining tau in its unphosphorylated form during aging. In addition, crossing hsp110(-)(/)(-) mice with mice overexpressing mutant APP (APPβsw) leads to selective appearance of insoluble amyloid β42 (Aβ42), suggesting an essential role for Hsp110 in APP processing and Aβ generation. Thus, our findings provide in vivo evidence that Hsp110 plays a critical function in tau phosphorylation state through maintenance of efficient PP2A activity, confirming its role in pathogenesis of Alzheimer's disease and other tauopathies.

Sti1 regulation of Hsp70 and Hsp90 is critical for curing of Saccharomyces cerevisiae [PSI+] prions by Hsp104

Although propagation of Saccharomyces cerevisiae prions requires Hsp104 protein disaggregating activity, overproducing Hsp104 "cures" cells of [PSI(+)] prions. Earlier evidence suggests that the Hsp70 mutant Ssa1-21 impairs [PSI(+)] by a related mechanism. Here, we confirm this link by finding that deletion of STI1 both suppresses Ssa1-21 impairment of [PSI(+)] and blocks Hsp104 curing of [PSI(+)]. Hsp104's tetratricopeptide repeat (TPR) interaction motif was dispensable for curing; however, cells expressing Sti1 defective in Hsp70 or Hsp90 interaction cured less efficiently, and the Hsp90 inhibitor radicicol abolished curing, implying that Sti1 acts in curing through Hsp70 and Hsp90 interactions. Accordingly, strains lacking constitutive or inducible Hsp90 isoforms cured at reduced rates. We confirm an earlier finding that elevating free ubiquitin levels enhances curing, but it did not overcome inhibition of curing caused by Hsp90 defects, suggesting that Hsp90 machinery is important for the contribution of ubiquitin to curing. We also find curing associated with cell division. Our findings point to crucial roles of Hsp70, Sti1, and Hsp90 for efficient curing by overexpressed Hsp104 and provide evidence supporting the earlier suggestion that destruction of prions by protein disaggregation does not adequately explain the curing.

The biogenesis of chylomicrons

The absorption of dietary fat is of increasing concern given the rise of obesity not only in the United States but throughout the developed world. This review explores what happens to dietary fat within the enterocyte. Absorbed fatty acids and monoacylglycerols are required to be bound to intracellular proteins and/or to be rapidly converted to triacylglycerols to prevent cellular membrane disruption. The triacylglycerol produced at the level of the endoplasmic reticulum (ER) is either incorporated into prechylomicrons within the ER lumen or shunted to triacylglycerol storage pools. The prechylomicrons exit the ER in a specialized transport vesicle in the rate-limiting step in the intracellular transit of triacylglycerol across the enterocyte. The prechylomicrons are further processed in the Golgi and are transported to the basolateral membrane via a separate vesicular system for exocytosis into the intestinal lamina propria. Fatty acids and monoacylglycerols entering the enterocyte via the basolateral membrane are also incorporated into triacylglycerol, but the basolaterally entering lipid is much more likely to enter the triacylglycerol storage pool than the lipid entering via the apical membrane.

Hsp110 chaperones control client fate determination in the hsp70-Hsp90 chaperone system

The Hsp110 family of protein chaperones was known to promote maturation of Hsp90 client proteins. The yeast Hsp110 ortholog Sse1 is now shown to influence the decision to fold or degrade substrates of the Hsp70–Hsp90 chaperone system when maturation is compromised.

Heat shock protein 70 (Hsp70) plays a central role in protein homeostasis and quality control in conjunction with other chaperone machines, including Hsp90. The Hsp110 chaperone Sse1 promotes Hsp90 activity in yeast, and functions as a nucleotide exchange factor (NEF) for cytosolic Hsp70, but the precise roles Sse1 plays in client maturation through the Hsp70–Hsp90 chaperone system are not fully understood. We find that upon pharmacological inhibition of Hsp90, a model protein kinase, Ste11ΔN, is rapidly degraded, whereas heterologously expressed glucocorticoid receptor (GR) remains stable. Hsp70 binding and nucleotide exchange by Sse1 was required for GR maturation and signaling through endogenous Ste11, as well as to promote Ste11ΔN degradation. Overexpression of another functional NEF partially compensated for loss of Sse1, whereas the paralog Sse2 fully restored GR maturation and Ste11ΔN degradation. Sse1 was required for ubiquitinylation of Ste11ΔN upon Hsp90 inhibition, providing a mechanistic explanation for its role in substrate degradation. Sse1/2 copurified with Hsp70 and other proteins comprising the “early-stage” Hsp90 complex, and was absent from “late-stage” Hsp90 complexes characterized by the presence of Sba1/p23. These findings support a model in which Hsp110 chaperones contribute significantly to the decision made by Hsp70 to fold or degrade a client protein.

The ribosome-bound Hsp70 homolog Ssb of Saccharomyces cerevisiae

The Hsp70 homolog Ssb directly binds to the ribosome and contacts a variety of newly synthesized polypeptide chains as soon as they emerge from the ribosomal exit tunnel. For this reason a general role of Ssb in the de novo folding of newly synthesized proteins is highly suggestive. However, for more than a decade client proteins which require Ssb for proper folding have remained elusive. It was therefore speculated that Ssb, despite its ability to interact with a large variety of nascent polypeptides, may assist the folding of only a small and specific subset. Alternatively, it has been suggested that Ssb's function may be limited to the protection of nascent polypeptides from aggregation until downstream chaperones take over and actively fold their substrates. There is also evidence that Ssb, in parallel to a classical chaperone function, is involved in the regulation of cellular signaling processes. Here we aim to summarize what is currently known about Ssb's multiple functions and what remains to be ascertained by future research.