Post-translational protein import and folding

Hsp70.1 carbonylation induces lysosomal cell death for lifestyle-related diseases

Alzheimer's disease, type 2 diabetes, and non-alcoholic steatohepatitis (NASH) constitute increasingly prevalent disorders. Individuals with type 2 diabetes are well-known to be susceptible to Alzheimer's disease. Although the pathogenesis of each disorder is multifactorial and the causal relation remains poorly understood, reactive oxygen species (ROS)-induced lipid and protein oxidation conceivably plays a common role. Lipid peroxidation product was recently reported to be a key factor also for non-alcoholic steatohepatitis, because of inducing hepatocyte degeneration/death. Here, we focus on implication of the representative lipid-peroxidation product 'hydroxynonenal' for the cell degeneration/death of brain, pancreas, and liver. Since Hsp70.1 has dual roles as a chaperone and lysosomal membrane stabilizer, hydroxynonenal-mediated oxidative injury (carbonylation) of Hsp70.1 was highlighted. After intake of high-fat diets, oxidation of free fatty acids in mitochondria generates ROS which enhance oxidation of ω-6 polyunsaturated fatty acids (PUFA) involved within biomembranes and generate hydroxynonenal. In addition, hydroxynonenal is generated during cooking deep-fried foods with vegetable oils especially containing linoleic acids. These intrinsic and exogenous hydroxynonenal synergically causes an increase in its serum and organ levels to induce Hsp70.1 oxidation. As it is amphiphilic; being water-soluble but displays strong lipophilic characteristics, hydroxynonenal can diffuse within the cells and react with targets like senile and/or atheromatous plaques outside the cells. Hydroxynonenal can deepen and expand lysosomal injuries by facilitating 'calpain-mediated cleavage of the carbonylated Hsp70.1'. Despite the unique anatomical, physiological, and biochemical characteristics of each organ for its specific disease, there should be a common cascade of the cell degeneration/death which is caused by hydroxynonenal. This review aims to implicate hydroxynonenal-mediated Hsp70.1 carbonylation for lysosomal membrane permeabilization/rupture and the resultant cathepsin leakage for inducing cell degeneration/death. Given the tremendous number of worldwide people suffering various lifestyle-related diseases, it is valuable to consider how ω-6 PUFA-rich vegetable oils is implicated for the organ disorder.

The Dose-Dependent Pleiotropic Effects of the UBB+1 Ubiquitin Mutant

The proteolytic machinery activity diminishes with age, leading to abnormal accumulation of aberrant proteins; furthermore, a decline in protein degradation capacity is associated with multiple age-related proteinopathies. Cellular proteostasis can be maintained via the removal of ubiquitin (Ub)-tagged damaged and redundant proteins by the ubiquitin-proteasome system (UPS). However, during aging, central nervous system (CNS) cells begin to express a frameshift-mutated Ub, UBB⁺¹. Its accumulation is a neuropathological hallmark of tauopathy, including Alzheimer’s disease and polyglutamine diseases. Mechanistically, in cell-free and cell-based systems, an increase in the UBB⁺¹ concentration disrupts proteasome processivity, leading to increased aggregation of toxic proteins. On the other hand, a low level of UBB⁺¹ improves stress resistance and extends lifespan. Here we summarize recent findings regarding the impact of UBB⁺¹ on Ub signaling and neurodegeneration. We also review the molecular basis of how UBB⁺¹ affects UPS components as well as its dose-dependent switch between cytoprotective and cytotoxic roles.

The Heat Shock Protein HSP70 Promotes Th17 Genes' Expression via Specific Regulation of microRNA

T helper cells type 17 (Th17) are orchestrators of autoimmune conditions, including multiple sclerosis (MS), but mechanisms of Th17 pathogenicity remain unknown. MicroRNAs (miRNA) are known to control T cells. To understand the function of miRNA in Th17, we have established a T cell line, EL4-TCR⁺, that resembles the expression pattern of the Th17 cells. Subsequently, we have evaluated the crosstalk between miRNA and Th17 genes' expression using a combination of gene expression profiling, gene expression manipulation, RNA and protein immunoprecipitation, as well as confocal microscopy. We have found that Th17-related miRNA were strongly expressed in EL4-TCR⁺ cells following the binding of the cluster of differentiation 3 (CD3) component of the T cell receptor (TCR). Furthermore, a specific inhibition of these miRNA resulted in downregulation of the critical Th17 genes' expression. Surprisingly, this mechanism relied on the function of the stress signal regulator heat shock protein 70 (HSP70). Upon activation, HSP70 co-localized intracellularly with miRNA processing proteins. Precipitation of HSP70 resulted in enrichment of the Th17-associated miRNA. Finally, HSP70 inhibition led to downregulation of the Th17 genes' expression and ameliorated development of autoimmune demyelination. Our study demonstrated that HSP70 facilitates specific miRNA function leading to Th17 genes' expression, a mechanism linking stress and autoimmunity.

Backbone and methyl resonance assignments of the 42 kDa human Hsc70 nucleotide binding domain in the ADP state

Hsc70 is the constitutively expressed mammalian heat shock 70 kDa (Hsp70) cytosolic chaperone. It plays a central role in cellular proteostasis and protein trafficking. Here, we present the backbone and methyl group assignments for the 386-residue nucleotide binding domain of the human protein. This domain controls the chaperone's allostery, binds multiple co-chaperones and is the target of several classes of known chemical Hsp70 inhibitors. The NMR assignments are based on common triple resonance experiments with triple labeled protein, and on several ¹⁵N and ¹³C-resolved 3D NOE experiments with methyl-reprotonated samples. A combination of computer and manual data interpretation was used.

The remarkable multivalency of the Hsp70 chaperones

Hsp70 proteins are key to maintaining intracellular protein homeostasis. To carry out this task, they employ a large number of cochaperones and adapter proteins. Here, we review what is known about the interaction between the chaperones and partners, with a strong slant toward structural biology. Hsp70s in general, and Hsc70 (HSPA8) in particular, display an amazing array of interfaces with their protein cofactors. We also review the known interactions between Hsp70s with lipids and with active compounds that may become leads toward Hsp70 modulation for treatment of a variety of diseases.

Cross Talk of Proteostasis and Mitostasis in Cellular Homeodynamics, Ageing, and Disease

Mitochondria are highly dynamic organelles that provide essential metabolic functions and represent the major bioenergetic hub of eukaryotic cell. Therefore, maintenance of mitochondria activity is necessary for the proper cellular function and survival. To this end, several mechanisms that act at different levels and time points have been developed to ensure mitochondria quality control. An interconnected highly integrated system of mitochondrial and cytosolic chaperones and proteases along with the fission/fusion machinery represents the surveillance scaffold of mitostasis. Moreover, nonreversible mitochondrial damage targets the organelle to a specific autophagic removal, namely, mitophagy. Beyond the organelle dynamics, the constant interaction with the ubiquitin-proteasome-system (UPS) has become an emerging aspect of healthy mitochondria. Dysfunction of mitochondria and UPS increases with age and correlates with many age-related diseases including cancer and neurodegeneration. In this review, we discuss the functional cross talk of proteostasis and mitostasis in cellular homeodynamics and the impairment of mitochondrial quality control during ageing, cancer, and neurodegeneration.

The mitochondrial protein import motor

Mitochondrial proteins are synthesized as precursor proteins in the cytosol and are posttranslationally imported into the organelle. A complex system of translocation machineries recognizes and transports the precursor polypeptide across the mitochondrial membranes. Energy for the translocation process is mainly supplied by the mitochondrial membrane potential (deltapsi) and the hydrolysis of ATP. Mitochondrial Hsp70 (mtHsp70) has been identified as the major ATPase driving the membrane transport of the precursor polypeptides into the mitochondrial matrix. Together with the partner proteins Tim44 and Mge1, mtHsp70 forms an import motor complex interacting with the incoming preproteins at the inner face of the inner membrane. This import motor complex drives the movement of the polypeptides in the translocation channel and the unfolding of carboxy-terminal parts of the preproteins on the outside of the outer membrane. Two models of the molecular mechanism of mtHsp70 during polypeptide translocation are discussed. In the 'trapping' model, precursor movement is generated by Brownian movement of the polypeptide chain in the translocation pore. This random movement is made vectorial by the interaction with mtHsp70 in the matrix. The detailed characterization of conditional mutants of the import motor complex provides the basis for an extended model. In this 'pulling' model, the attachment of mtHsp70 at the inner membrane via Tim44 and a conformational change induced by ATP results in the generation of an inward-directed force on the bound precursor polypeptide. This active role of the import motor complex is necessary for the translocation of proteins containing tightly folded domains. We suggest that both mechanisms complement each other to reach a high efficiency of preprotein import.

Size-dependent disaggregation of stable protein aggregates by the DnaK chaperone machinery

Classic in vitro studies show that the Hsp70 chaperone system from Escherichia coli (DnaK-DnaJ-GrpE, the DnaK system) can bind to proteins, prevent aggregation, and promote the correct refolding of chaperone-bound polypeptides into native proteins. However, little is known about how the DnaK system handles proteins that have already aggregated. In this study, glucose-6-phosphate dehydrogenase was used as a model system to generate stable populations of protein aggregates comprising controlled ranges of particle sizes. The DnaK system recognized the glucose-6-phosphate dehydrogenase aggregates as authentic substrates and specifically solubilized and refolded the protein into a native enzyme. The efficiency of disaggregation by the DnaK system was high with small aggregates, but the efficiency decreased as the size of the aggregates increased. High folding efficiency was restored by either excess DnaK or substoichiometric amounts of the chaperone ClpB. We suggest a mechanism whereby the DnaK system can readily solubilize small aggregates and refold them into active proteins. With large aggregates, however, the binding sites for the DnaK system had to be dynamically exposed with excess DnaK or the catalytic action of ClpB and ATP. Disaggregation by the DnaK machinery in the cell can solubilize early aggregates that formed accidentally during chaperone-assisted protein folding or that escaped the protection of "holding" chaperones during stress.

Directionality of polypeptide transfer in the mitochondrial pathway of chaperone-mediated protein folding

Protein folding in mitochondria depends on the functional cooperation of the Hsp70 and Hsp60 chaperone systems, at least for a subset of mitochondrial polypeptides. As suggested previously, Hsp70 and Hsp60 act sequentially. However, recent proposals that the chaperonin Hsp60 functions by releasing substrate protein in an unfolded state would predict a lateral partitioning of folding intermediates between chaperone systems. Firefly luciferase, carrying a mitochondrial targeting signal, was used as a model protein to analyze the degree of coupling and the directionality of substrate transfer between the Hsp70 and Hsp60 chaperones. In vitro, Hsp60 binds unfolded luciferase with high affinity but is unable to promote its folding, whereas the Hsp70 system assists the folding of luciferase efficiently. Upon import into yeast mitochondria, luciferase interacted first with Hsp70. Surprisingly, most of the protein subsequently accumulated in a complex with Hsp60 and never reached the native state. Import into mitochondria that lack a functional Hsp60 did not result in increased folding, but in the aggregation of luciferase. Thus, in intact organelles the two chaperone systems do not function independently in de novo folding of aggregation-sensitive proteins but rather act in an ordered pathway with substrate transfer predominantly in the direction from Hsp70 to Hsp60.

The degradation of apolipoprotein B100 is mediated by the ubiquitin-proteasome pathway and involves heat shock protein 70

Apolipoprotein B (apoB) is the major protein component of atherogenic lipoproteins of hepatic origin. In HepG2 cells, the standard cell culture model of human hepatic lipoprotein metabolism, there is a limited availability of core lipids in the endoplasmic reticulum for association with nascent apoB. Under these conditions, apoB is partially translocated, interacts with cytosolic Hsp70, and undergoes rapid degradation. We show that increasing the expression of Hsp70 in HepG2 cells promotes apoB degradation. In addition, apoB is polyubiquitinated and its degradation both normally and after Hsp70 induction is blocked by inhibitors of the proteasome. The apoB that accumulates after proteasome inhibition is endoplasmic reticulum-associated and can be assembled into lipoproteins and secreted if new lipid synthesis is stimulated. Thus, apoB is the first example of a wild-type mammalian protein whose secretion is regulated by degradation in the cytosol via the ubiquitin-proteasome pathway. Furthermore, targeting of this secretory protein to the proteasome is regulated by the molecular chaperone Hsp70 and the availability of apoB's lipid-ligands.

A role for HDJ-2/HSDJ in correcting subnuclear trafficking, transactivation, and transrepression defects of a glucocorticoid receptor zinc finger mutant

All steroid receptors possess a bipartite nuclear localization signal sequence (NLS) that localizes within the second zinc finger of their DNA-binding domain. Fine-structure mapping of the rat glucocorticoid receptor (rGS) NLS identified a composite signal composed of three distinct proto-NLSs that function effectively when present in unique pairs. At least one of the rGR proto-NLSs appears to influence receptor trafficking within the nucleus, as revealed by a unique nuclear staining pattern of receptors possessing a point mutation (i.e., arginine at position 496; R496), at proto-NLS, pNLS-2. Specifically, carboxyl-terminal-truncated rGRs possessing various point mutations at R496 localized within a limited number of large foci in nuclei of transiently transfected COS-1 cells. R496 mutations did not affect subnuclear targeting when present in full-length rGR, reflecting a protective effect of the receptor's ligand-binding domain that can be exerted in cis and in trans. The effects of rGR R496 mutations on subnuclear targeting were not autonomous because we also observed a coincident localization of hsp70, the 70-kDa heat shock protein, within nuclear foci that include r496 mutant receptors. The elimination of R496 mistargeting by overexpression of an hsp70 partner (i.e., the DnaJ homologue, HDJ-2/HSDJ) suggests that the hsp70/DnaJ chaperone system is mobilized to specific sites within the nucleus in response to inappropriate targeting or folding of specific mutant receptors. HDJ-2/HSDJ overexpression also corrects defective transactivation and transrepression activity of R496 mutant GRs. Thus, molecular chaperones, such as members of the hsp70 and DnaJ families, may survey the nucleus for misfolded proteins and actively participate in their refolding into biologically active conformational states.

Subcellular distribution and targeting of the intracellular chloride channel p64

p64 is an intracellular chloride channel originally identified in bovine kidney microsomes. Using a combination of immunofluorescent and electron microscopic technique, we demonstrate that p64 resides in the limiting membranes of perinuclear dense core vesicles which appear to be regulated secretory vesicles. Heterologous expression of p64 in PancI cells, a cell type which does not normally express p64, results in targeting to a similar compartment. Mutagenesis experiments demonstrate that both the N- and C-terminal domains of the protein independently contribute to subcellular distribution of the protein. The C-terminal domain functions to prevent expression of p64 on the plasma membrane and the N-terminal domain is necessary to deliver p64 to the appropriate membrane compartment.

FtsY, the prokaryotic signal recognition particle receptor homologue, is essential for biogenesis of membrane proteins

In mammalian cells, many secretory proteins are targeted to the endoplasmic reticulum co-translationally, by the signal recognition particle (SRP) and its receptor. In Escherichia coli, the targeting of secretory proteins to the inner membrane can be accomplished post-translationally. Unexpectedly, despite this variance, E. coli contains essential genes encoding Ffh and FtsY with a significant similarity to proteins of the eukaryotic SRP machinery. In this study, we investigated the possibility that the prokaryotic SRP-like machinery is involved in biogenesis of membrane proteins in E. coli. The data presented here demonstrate that the SRP-receptor homologue, FtsY, is indeed essential for expression of integral membrane proteins in E. coli, indicating that, in the case of this group of proteins, FtsY and the mammalian SRP receptor have similar functions.

Protein import into mitochondria

Mitochondria import many hundreds of different proteins that are encoded by nuclear genes. These proteins are targeted to the mitochondria, translocated through the mitochondrial membranes, and sorted to the different mitochondrial subcompartments. Separate translocases in the mitochondrial outer membrane (TOM complex) and in the inner membrane (TIM complex) facilitate recognition of preproteins and transport across the two membranes. Factors in the cytosol assist in targeting of preproteins. Protein components in the matrix partake in energetically driving translocation in a reaction that depends on the membrane potential and matrix-ATP. Molecular chaperones in the matrix exert multiple functions in translocation, sorting, folding, and assembly of newly imported proteins.

A novel DnaJ-like protein in Escherichia coli inserts into the cytoplasmic membrane with a type III topology

We describe a novel Escherichia coli protein, DjlA, containing a highly conserved J-region motif, which is present in the DnaJ protein chaperone family and required for interaction with DnaK. Remarkably, DjlA is shown to be a membrane protein, localized to the inner membrane with the unusual Type III topology (N-out, C-in). Thus, DjlA appears to present an extremely short N-terminus to the periplasm and has a single transmembrane domain (TMD) and a large cytoplasmic domain containing the C-terminal J-region. Analysis of the TMD of DjlA and recently identified homologues in Coxiella burnetti and Haemophilus influenzae revealed a striking pattern of conserved glycines (or rarely alanine), with a four-residue spacing. This motif, predicted to form a spiral groove in the TMD, is more marked than a repeating glycine motif, implicated in the dimerization of TMDs of some eukaryotic proteins. This feature of DjlA could represent a promiscuous docking mechanism for interaction with a variety of membrane proteins. DjlA null mutants can be isolated but these appear rapidly to accumulate suppressors to correct envelope and growth defects. Moderate (10-fold) overproduction of DjlA suppresses a mutation in FtsZ but markedly perturbs cell division and cell-envelope growth in minimal medium. We propose that DjlA plays a role in the correct assembly, activity and/or maintenance of a number of membrane proteins, including two-component signal-transduction systems.

Roles for hsp70 in protein translocation across membranes of organelles

The family of hsp70 molecular chaperones plays an essential and diverse role in cellular physiology. Hsp70 proteins appear to elicit their effects by interaction with polypeptides that present domains which exhibit non-native conformations at distinct stages during their life in the cell. Work pertaining to the functions of hsp70 proteins in driving protein translocation across membranes is reviewed herein. Hsp70 proteins function to deliver polypeptides to protein translocation channels, unfold polypeptides during transit across membranes and drive the translocation process. All these reactions are facilitated in an ATP-dependent reaction cycle with the assistance of different partner proteins that modulate the function of hsp70.

Hip, a novel cochaperone involved in the eukaryotic Hsc70/Hsp40 reaction cycle

The Hsc70-interacting protein Hip, a tetratricopeptide repeat protein, participates in the regulation of the eukaryotic 70 kDa heat shock cognate Hsc70. One Hip oligomer binds the ATPase domains of at least two Hsc70 molecules dependent on activation of the Hsc70 ATPase by Hsp40. While hydrolysis remains the rate-limiting step in the ATPase cycle, Hip stabilizes the ADP state of Hsc70 that has a high affinity for substrate protein. Through its own chaperone activity, Hip may contribute to the interaction of Hsc70 with various target proteins. We propose a mechanism for the regulation of eukaryotic Hsc70 that is distinct from that of bacterial Hsp70. The Hsc70/Hsp40/Hip system is apparently independent of a GrpE-like nucleotide exchange factor.