Complexes between nascent polypeptides and their molecular chaperones in the cytosol of mammalian cells

Paradoxical imbalance between activated lymphocyte protein synthesis capacity and rapid division rate

Rapid lymphocyte cell division places enormous demands on the protein synthesis machinery. Flow cytometric measurement of puromycylated ribosome-associated nascent chains after treating cells or mice with translation initiation inhibitors reveals that ribosomes in resting lymphocytes in vitro and in vivo elongate at typical rates for mammalian cells. Intriguingly, elongation rates can be increased up to 30% by activation in vivo or fever temperature in vitro. Resting and activated lymphocytes possess abundant monosome populations, most of which actively translate in vivo, while in vitro, nearly all can be stalled prior to activation. Quantitating lymphocyte protein mass and ribosome count reveals a paradoxically high ratio of cellular protein to ribosomes insufficient to support their rapid in vivo division, suggesting that the activated lymphocyte proteome in vivo may be generated in an unusual manner. Our findings demonstrate the importance of a global understanding of protein synthesis in lymphocytes and other rapidly dividing immune cells.

p62 and NBR1 functions are dispensable for aggrephagy in mouse ESCs and ESC-derived neurons

Accumulation of protein aggregates is a hallmark of various neurodegenerative diseases. Selective autophagy mediates the delivery of specific cytoplasmic cargo material into lysosomes for degradation. In aggrephagy, which is the selective autophagy of protein aggregates, the cargo receptors p62 and NBR1 were shown to play important roles in cargo selection. They bind ubiquitinated cargo material via their ubiquitin-associated domains and tether it to autophagic membranes via their LC3-interacting regions. We used mouse embryonic stem cells (ESCs) in combination with genome editing to obtain further insights into the roles of p62 and NBR1 in aggrephagy. Unexpectedly, our data reveal that both ESCs and ESC-derived neurons do not show strong defects in the clearance of protein aggregates upon knockout of p62 or NBR1 and upon mutation of the p62 ubiquitin-associated domain and the LC3-interacting region motif. Taken together, our results show a robust aggregate clearance in ESCs and ESC-derived neurons. Thus, redundancy between the cargo receptors, other factors, and pathways, such as the ubiquitin-proteasome system, may compensate for the loss of function of p62 and NBR1.

Dual Leucine Zipper Kinase Regulates Dscam Expression through a Noncanonical Function of the Cytoplasmic Poly(A)-Binding Protein

Dual leucine zipper kinase (DLK) plays a pivotal role in the development, degeneration, and regeneration of neurons. DLK can regulate gene expression post-transcriptionally, but the underlying mechanism remains poorly understood. The Drosophila DLK, Wallenda (Wnd), regulates the expression of Down syndrome cell adhesion molecule (Dscam) to control presynaptic arbor growth. This regulation is mediated by the 3' untranslated region (3'UTR) of Dscam mRNA, which suggests that RNA binding proteins (RBPs) mediate DLK function. We performed a genome-wide cell-based RNAi screen of RBPs and identified the cytoplasmic poly(A)-binding protein, pAbp, as an RBP that mediates Wnd-induced increase in Dscam expression. Genetic analysis shows that Wnd requires pAbp for promoting presynaptic arbor growth and for enhancing Dscam expression. Our analysis revealed that Dscam mRNAs harbor short poly(A) tails. We identified a region in Dscam 3'UTR that specifically interacts with pAbp. Removing this region significantly reduced Wnd-induced increase in Dscam expression. These suggest that a noncanonical interaction of PABP with the 3'UTR of target transcripts is essential for DLK functions.SIGNIFICANCE STATEMENT The kinase DLK plays key roles in a multitude of neuronal responses, including axon development, neurodegeneration, and nerve injury. Previous studies show that DLK acts via mRNAs to regulate protein synthesis, but how DLK does so is poorly understood. This study demonstrates that DLK regulates the synthesis of Dscam through the poly(A)-binding protein PABP-C. Whereas PABP-C is known as a general translational activator, our study shows that DLK-mediated Dscam expression involves a noncanonical interaction between PABP-C and the Dscam mRNA, which leads to a selective regulation of Dscam translation by PABP-C. Thus, our study provides novel insights into the mechanisms that underlie the function of DLK and regulation of gene expression of PABP-C.

Material properties of phase-separated TFEB condensates regulate the autophagy-lysosome pathway

Very little is known about how the material properties of protein condensates assembled via liquid–liquid phase separation (LLPS) are maintained and affect physiological functions. Here we show that liquid-like condensates of the transcription factor TFEB exhibit low fusion propensity in vitro and in living cells. We directly measured the attraction force between droplets, and we characterized the interfacial tension, viscosity, and elasticity of TFEB condensates. TFEB condensates contain rigid interfacial boundaries that govern their interaction behaviors. Several small molecules, including Ro-3306, modify the material properties of TFEB condensates, increasing their size and fusion propensity. These compounds promote lysosomal biogenesis and function in a TFEB-dependent manner without changing its cytoplasmic-nuclear translocation. Ro-3306 promotes autophagy activity, facilitating degradation of toxic protein aggregates. Our study helps explain how protein condensates are maintained as physically separate entities and reveals that the material properties of TFEB condensates can be harnessed to modulate TFEB activity.

General Structural and Functional Features of Molecular Chaperones

Molecular chaperones are a group of structurally diverse and highly conserved ubiquitous proteins. They play crucial roles in facilitating the correct folding of proteins in vivo by preventing protein aggregation or facilitating the appropriate folding and assembly of proteins. Heat shock proteins form the major class of molecular chaperones that are responsible for protein folding events in the cell. This is achieved by ATP-dependent (folding machines) or ATP-independent mechanisms (holders). Heat shock proteins are induced by a variety of stresses, besides heat shock. The large and varied heat shock protein class is categorised into several subfamilies based on their sizes in kDa namely, small Hsps (HSPB), J domain proteins (Hsp40/DNAJ), Hsp60 (HSPD/E; Chaperonins), Hsp70 (HSPA), Hsp90 (HSPC), and Hsp100. Heat shock proteins are localised to different compartments in the cell to carry out tasks specific to their environment. Most heat shock proteins form large oligomeric structures, and their functions are usually regulated by a variety of cochaperones and cofactors. Heat shock proteins do not function in isolation but are rather part of the chaperone network in the cell. The general structural and functional features of the major heat shock protein families are discussed, including their roles in human disease. Their function is particularly important in disease due to increased stress in the cell. Vector-borne parasites affecting human health encounter stress during transmission between invertebrate vectors and mammalian hosts. Members of the main classes of heat shock proteins are all represented in Plasmodium falciparum, the causative agent of cerebral malaria, and they play specific functions in differentiation, cytoprotection, signal transduction, and virulence.

Loss of TAX1BP1-Directed Autophagy Results in Protein Aggregate Accumulation in the Brain

Protein aggregates disrupt cellular homeostasis, causing toxicity linked to neurodegeneration. Selective autophagic elimination of aggregates is critical to protein quality control, but how aggregates are selectively targeted for degradation is unclear. We compared the requirements for autophagy receptor proteins: OPTN, NBR1, p62, NDP52, and TAX1BP1 in clearance of proteotoxic aggregates. Endogenous TAX1BP1 is recruited to and required for the clearance of stress-induced aggregates, whereas ectopic expression of TAX1BP1 increases clearance through autophagy, promoting viability of human induced pluripotent stem cell-derived neurons. In contrast, TAX1BP1 depletion sensitizes cells to several forms of aggregate-induced proteotoxicity. Furthermore, TAX1BP1 is more specifically expressed in the brain compared to other autophagy receptor proteins. In vivo, loss of TAX1BP1 results in accumulation of high molecular weight ubiquitin conjugates and premature lipofuscin accumulation in brains of young TAX1BP1 knockout mice. TAX1BP1 mediates clearance of a broad range of cytotoxic proteins indicating therapeutic potential in neurodegenerative diseases.

The science of puromycin: From studies of ribosome function to applications in biotechnology

Puromycin is a naturally occurring aminonucleoside antibiotic that inhibits protein synthesis by ribosome-catalyzed incorporation into the C-terminus of elongating nascent chains, blocking further extension and resulting in premature termination of translation. It is most commonly known as a selection marker for cell lines genetically engineered to express a resistance transgene, but its additional uses as a probe for protein synthesis have proven invaluable across a wide variety of model systems, ranging from purified ribosomes and cell-free translation to intact cultured cells and whole animals. Puromycin is comprised of a nucleoside covalently bound to an amino acid, mimicking the 3′ end of aminoacylated tRNAs that participate in delivery of amino acids to elongating ribosomes. Both moieties can tolerate some chemical substitutions and modifications without significant loss of activity, generating a diverse toolbox of puromycin-based reagents with added functionality, such as biotin for affinity purification or fluorophores for fluorescent microscopy detection. These reagents, as well as anti-puromycin antibodies, have played a pivotal role in advancing our understanding of the regulation and dysregulation of protein synthesis in normal and pathological processes, including immune response and neurological function. This manuscript reviews the current state of puromycin-based research, including structure and mechanism of action, relevant derivatives, use in advanced methodologies and some of the major insights generated using such techniques both in the lab and the clinic.

NoRC Recruitment by H2A.X Deposition at rRNA Gene Promoter Limits Embryonic Stem Cell Proliferation

Embryonic stem cells (ESCs) display an abbreviated cell cycle, resulting in a short doubling time and rapid proliferation. The histone variant H2A.X is critical for proliferation of stem cells, although mechanistic insights have remained obscure. Here, we show that H2A.X defines the rate of mouse ESC proliferation independently of the DNA damage response pathway, and it associates with three major chromatin-modifying complexes. Our functional and biochemical analyses demonstrate that H2A.X-associated factors mediate the H2A.X-dependent effect on ESC proliferation and involve the nucleolar remodeling complex (NoRC). A specific H2A.X deposition at rDNA promoters determines the chromatin recruitment of the NoRC, histone modifications, the rRNA transcription, and the rate of proliferation. Collectively, our results suggest that NoRC assembly by H2A.X deposition at rRNA promoters silences transcription, and this represents an important regulatory component for ESC proliferation.

The MTM1-UBQLN2-HSP complex mediates degradation of misfolded intermediate filaments in skeletal muscle

The ubiquitin proteasome system and autophagy are major protein turnover mechanisms in muscle cells, which ensure stemness and muscle fibre maintenance. Muscle cells contain a high proportion of cytoskeletal proteins, which are prone to misfolding and aggregation; pathological processes that are observed in several neuromuscular diseases called proteinopathies. Despite advances in deciphering the mechanisms underlying misfolding and aggregation, little is known about how muscle cells manage cytoskeletal degradation. Here, we describe a process by which muscle cells degrade the misfolded intermediate filament proteins desmin and vimentin by the proteasome. This relies on the MTM1-UBQLN2 complex to recognize and guide these misfolded proteins to the proteasome and occurs prior to aggregate formation. Thus, our data highlight a safeguarding function of the MTM1-UBQLN2 complex that ensures cytoskeletal integrity to avoid proteotoxic aggregate formation.

The RiboPuromycylation Method (RPM): an Immunofluorescence Technique to Map Translation Sites at the Sub-cellular Level

While isotopic labeling of amino acids remains the reference method in the field for quantifying translation rate, it does not provide any information on spatial localization of translation sites. The rationale behind developing the ribopuromycylation method (RPM) was primarily to map translation sites at the sub-cellular level while avoiding detection of newly synthesized proteins released from ribosomes. RPM visualizes actively translating ribosomes in cells via standard immunofluorescence microscopy in fixed and permeabilized cells using a puromycin-specific monoclonal antibody to detect puromycylated nascent chains trapped on ribosomes treated with a chain elongation inhibitor.

A Plant Biologist's Toolbox to Study Translation

Across a broad range of species and biological questions, more and more studies are incorporating translation data to better assess how gene regulation occurs at the level of protein synthesis. The inclusion of translation data improves upon, and has been shown to be more accurate than, transcriptional studies alone. However, there are many different techniques available to measure translation and it can be difficult, especially for young or aspiring scientists, to determine which methods are best applied in specific situations. We have assembled this review in order to enhance the understanding and promote the utilization of translational methods in plant biology. We cover a broad range of methods to measure changes in global translation (e.g., radiolabeling, polysome profiling, or puromycylation), translation of single genes (e.g., fluorescent reporter constructs, toeprinting, or ribosome density mapping), sequencing-based methods to uncover the entire translatome (e.g., Ribo-seq or translating ribosome affinity purification), and mass spectrometry-based methods to identify changes in the proteome (e.g., stable isotope labeling by amino acids in cell culture or bioorthogonal noncanonical amino acid tagging). The benefits and limitations of each method are discussed with a particular note of how applications from other model systems might be extended for use in plants. In order to make this burgeoning field more accessible to students and newer scientists, our review includes an extensive glossary to define key terms.

Exploring ribosome composition and newly synthesized proteins through proteomics and potential biomedical applications

INTRODUCTION

Protein synthesis is the outcome of tightly regulated gene expression which is responsive to a variety of conditions. Efforts are ongoing to monitor individual stages of protein synthesis to ensure maximum efficiency and accuracy. Due to post-transcriptional regulation mechanisms, the correlation between translatome and proteome is higher than between transcriptome and proteome. However, the most accurate approach to assess the key modulators and final protein expression is directly by using proteomics. Areas covered: This review covers various proteomic strategies that were used to better understand post-transcriptional regulation, specifically during and early after translation. The methods that identify both regulatory proteins associated with translational components and newly synthesized proteins are discussed. Expert commentary: Emerging proteomic approaches make it possible to monitor protein dynamics in cells, tissues and whole animals. The ability to detect alteration in protein abundance soon after their synthesis enables earlier recognition of disease causing factors and candidates to prevent/rectify disease phenotype.

Titin and Nebulin in Thick and Thin Filament Length Regulation

In this review we discuss the history and the current state of ideas related to the mechanism of size regulation of the thick (myosin) and thin (actin) filaments in vertebrate striated muscles. Various hypotheses have been considered during of more than half century of research, recently mostly involving titin and nebulin acting as templates or 'molecular rulers', terminating exact assembly. These two giant, single-polypeptide, filamentous proteins are bound in situ along the thick and thin filaments, respectively, with an almost perfect match in the respective lengths and structural periodicities. However, evidence still questions the possibility that the proteins function as templates, or scaffolds, on which the thin and thick filaments could be assembled. In addition, the progress in muscle research during the last decades highlighted a number of other factors that could potentially be involved in the mechanism of length regulation: molecular chaperones that may guide folding and assembly of actin and myosin; capping proteins that can influence the rates of assembly-disassembly of the myofilaments; Ca²⁺ transients that can activate or deactivate protein interactions, etc. The entire mechanism of sarcomere assembly appears complex and highly dynamic. This mechanism is also capable of producing filaments of about the correct size without titin and nebulin. What then is the role of these proteins? Evidence points to titin and nebulin stabilizing structures of the respective filaments. This stabilizing effect, based on linear proteins of a fixed size, implies that titin and nebulin are indeed molecular rulers of the filaments. Although the proteins may not function as templates in the assembly of the filaments, they measure and stabilize exactly the same size of the functionally important for the muscles segments in each of the respective filaments.

UBQLN2 Mediates Autophagy-Independent Protein Aggregate Clearance by the Proteasome

Clearance of misfolded and aggregated proteins is central to cell survival. Here, we describe a new pathway for maintaining protein homeostasis mediated by the proteasome shuttle factor UBQLN2. The 26S proteasome degrades polyubiquitylated substrates by recognizing them through stoichiometrically bound ubiquitin receptors, but substrates are also delivered by reversibly bound shuttles. We aimed to determine why these parallel delivery mechanisms exist and found that UBQLN2 acts with the HSP70-HSP110 disaggregase machinery to clear protein aggregates via the 26S proteasome. UBQLN2 recognizes client-bound HSP70 and links it to the proteasome to allow for the degradation of aggregated and misfolded proteins. We further show that this process is active in the cell nucleus, where another system for aggregate clearance, autophagy, does not act. Finally, we found that mutations in UBQLN2, which lead to neurodegeneration in humans, are defective in chaperone binding, impair aggregate clearance, and cause cognitive deficits in mice.

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UBQLN2 clears aggregates independent of autophagy via HSP70 and the proteasome

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A disease mutation in UBQLN2 prevents its binding to HSP70

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Mutant UBQLN2 is defective in clearance of aggregates in vivo

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UBQLN2 knockin mice develop cognitive impairment and brain pathology

Protein Translation Activity: A New Measure of Host Immune Cell Activation

We describe the in vivo ribopuromycylation (RPM) method, which uses a puromycin-specific Ab to fluorescently label ribosome-bound puromycylated nascent chains, enabling measurement of translational activity via immunohistochemistry or flow cytometry. Tissue staining provides a unique view of virus-induced activation of adaptive, innate, and stromal immune cells. RPM flow precisely quantitates virus-induced activation of lymphocytes and innate immune cells, and it provides a unique measure of immune cell deactivation and quiescence. Using RPM we find that high endothelial cells in draining lymph nodes rapidly increase translation in the first day of vaccinia virus infection. We also find a population of constitutively activated splenic T cells in naive mice and further that most bone marrow T cells activate 3 d after vaccinia virus infection. Bone marrow T cell activation is nonspecific, IL-12-dependent, and induces innate memory T cell phenotypic markers. Thus, RPM measures translational activity to uniquely identify cell populations that participate in the immune response to pathogens, other foreign substances, and autoantigens.

Inhibitions of mTORC1 and 4EBP-1 are key events orchestrated by Rottlerin in SK-Mel-28 cell killing

Earlier studies demonstrated that Rottlerin exerts a time- and dose-dependent antiproliferative effect on SK-Mel-28 melanoma cells during 24 h of treatment, but cytotoxicity due to cell death began only after a 48 h exposure. In the current study, in order to identify the type of cell death in this cell line, which is notoriously refractory to most anticancer therapies, and to clarify the underlying mechanisms of this delayed outcome, we searched for apoptotic, necrotic/necroptotic and autophagic traits in Rottlerin-exposed cells. Although SK-Mel-28 cells are both apoptosis and autophagy competent, Western blotting analysis, caspase activity assay, nuclear imaging and the effects of autophagy, apoptosis and necroptosis inhibitors, indicated that Rottlerin cytotoxicity was due to none of the aforementioned death mechanisms. Nevertheless, in growth arrested cells, the death did occur after a prolonged treatment and most likely ensued from the observed blockage of protein synthesis that reached levels expected to be incompatible with cell survival. From a mechanistic point of view, we ascribed this effect to the documented inhibition of mTORC1 activity; mTORC1 inhibition on the one hand led to a not deadly, rather protective autophagic response but, on the other hand caused a near complete arrest of protein synthesis. Interestingly, no cytotoxicity was found towards normal skin fibroblasts, which only resulted mildly growth arrested by the drug.

Conserved C-terminal nascent peptide binding domain of HYPK facilitates its chaperone-like activity

Human HYPK (Huntingtin Yeast-two-hybrid Protein K) is an intrinsically unstructured chaperone-like protein with no sequence homology to known chaperones. HYPK is also known to be a part of ribosome-associated protein complex and present in polysomes. The objective of the present study was to investigate the evolutionary influence on HYPK primary structure and its impact on the protein's function. Amino acid sequence analysis revealed 105 orthologs of human HYPK from plants, lower invertebrates to mammals. C-terminal part of HYPK was found to be particularly conserved and to contain nascent polypeptide-associated alpha subunit (NPAA) domain. This region experiences highest selection pressure, signifying its importance in the structural and functional evolution. NPAA domain of human HYPK has unique amino acid composition preferring glutamic acid and happens to be more stable from a conformational point of view having higher content of a-helices than the rest. Cell biology studies indicate that overexpressed C-terminal human HYPK can interact with nascent proteins, co-localizes with huntingtin, increases cell viability and decreases caspase activities in Huntington's disease (HD) cell culture model. This domain is found to be required for the chaperone-like activity of HYPK in vivo. Our study suggested that by virtue of its flexibility and nascent peptide binding activity, HYPK may play an important role in assisting protein (re)folding.

Design of photocaged puromycin for nascent polypeptide release and spatiotemporal monitoring of translation

The antibiotic puromycin, which inhibits protein translation, is used in a broad range of biochemical applications. The synthesis, characterization, and biological applications of NVOC-puromycin, a photocaged derivative that is activated by UV illumination, are presented. The caged compound had no effect either on prokaryotic or eukaryotic translation or on the viability of HEK 293 cells. Furthermore, no significant release of ribosome-bound polypeptide chains was detected in vitro. Upon illumination, cytotoxic activity, in vitro translation inhibition, and polypeptide release triggered by the uncaging of NVOC-puromycin were equivalent to those of the commercial compound. The quantum yield of photolysis was determined to be 1.1±0.2% and the NVOC-puromycin was applied to the detection of newly translated proteins with remarkable spatiotemporal resolution by using two-photon laser excitation, puromycin immunohistochemistry, and imaging in rat hippocampal neurons.

Polysome fractionation and analysis of mammalian translatomes on a genome-wide scale

mRNA translation plays a central role in the regulation of gene expression and represents the most energy consuming process in mammalian cells. Accordingly, dysregulation of mRNA translation is considered to play a major role in a variety of pathological states including cancer. Ribosomes also host chaperones, which facilitate folding of nascent polypeptides, thereby modulating function and stability of newly synthesized polypeptides. In addition, emerging data indicate that ribosomes serve as a platform for a repertoire of signaling molecules, which are implicated in a variety of post-translational modifications of newly synthesized polypeptides as they emerge from the ribosome, and/or components of translational machinery. Herein, a well-established method of ribosome fractionation using sucrose density gradient centrifugation is described. In conjunction with the in-house developed “anota” algorithm this method allows direct determination of differential translation of individual mRNAs on a genome-wide scale. Moreover, this versatile protocol can be used for a variety of biochemical studies aiming to dissect the function of ribosome-associated protein complexes, including those that play a central role in folding and degradation of newly synthesized polypeptides.

Investigating the Chaperone Properties of a Novel Heat Shock Protein, Hsp70.c, from Trypanosoma brucei

The neglected tropical disease, African Trypanosomiasis, is fatal and has a crippling impact on economic development. Heat shock protein 70 (Hsp70) is an important molecular chaperone that is expressed in response to stress and Hsp40 acts as its co-chaperone. These proteins play a wide range of roles in the cell and they are required to assist the parasite as it moves from a cold blooded insect vector to a warm blooded mammalian host. A novel cytosolic Hsp70, from Trypanosoma brucei, TbHsp70.c, contains an acidic substrate binding domain and lacks the C-terminal EEVD motif. The ability of a cytosolic Hsp40 from Trypanosoma brucei J protein 2, Tbj2, to function as a co-chaperone of TbHsp70.c was investigated. The main objective was to functionally characterize TbHsp70.c to further expand our knowledge of parasite biology. TbHsp70.c and Tbj2 were heterologously expressed and purified and both proteins displayed the ability to suppress aggregation of thermolabile MDH and chemically denatured rhodanese. ATPase assays revealed a 2.8-fold stimulation of the ATPase activity of TbHsp70.c by Tbj2. TbHsp70.c and Tbj2 both demonstrated chaperone activity and Tbj2 functions as a co-chaperone of TbHsp70.c. In vivo heat stress experiments indicated upregulation of the expression levels of TbHsp70.c.

Co-translational mechanisms of quality control of newly synthesized polypeptides

During protein synthesis, nascent polypeptides emerge from ribosomes to fold into functional proteins. Misfolding of newly synthesized polypeptides (NSPs) at this stage leads to their aggregation. These misfolded NSPs must be expediently cleared to circumvent the deleterious effects of protein aggregation on cell physiology. To this end, a sizable portion of NSPs are ubiquitinated and rapidly degraded by the proteasome. This suggests the existence of co-translational mechanisms that play a pivotal role in the quality control of NSPs. It is generally thought that ribosomes play a central role in this process. During mRNA translation, ribosomes sense errors that lead to the accumulation of aberrant polypeptides, and serve as a hub for protein complexes that are required for optimal folding and/or proteasome-dependent degradation of misfolded polypeptides. In this review, we discuss recent findings that shed light on the molecular underpinnings of the co-translational quality control of NSPs.

Degradation of newly synthesized polypeptides by ribosome-associated RACK1/c-Jun N-terminal kinase/eukaryotic elongation factor 1A2 complex

Folding of newly synthesized polypeptides (NSPs) into functional proteins is a highly regulated process. Rigorous quality control ensures that NSPs attain their native fold during or shortly after completion of translation. Nonetheless, signaling pathways that govern the degradation of NSPs in mammals remain elusive. We demonstrate that the stress-induced c-Jun N-terminal kinase (JNK) is recruited to ribosomes by the receptor for activated protein C kinase 1 (RACK1). RACK1 is an integral component of the 40S ribosome and an adaptor for protein kinases. Ribosome-associated JNK phosphorylates the eukaryotic translation elongation factor 1A isoform 2 (eEF1A2) on serines 205 and 358 to promote degradation of NSPs by the proteasome. These findings establish a role for a RACK1/JNK/eEF1A2 complex in the quality control of NSPs in response to stress.

Emetine optimally facilitates nascent chain puromycylation and potentiates the ribopuromycylation method (RPM) applied to inert cells

We previously described the ribopuromyclation method (RPM) to visualize and quantitate translating ribosomes in fixed and permeabilized cells by standard immunofluorescence. RPM is based on puromycylation of nascent chains bound to translating ribosomes followed by detection of puromycylated nascent chains with a puromycin-specific mAb. We now demonstrate that emetine optimally enhances nascent chain puromycylation, and describe a modified RPM protocol for identifying ribosome-bound nascent chains in metabolically inert permeabilized cells.

Nuclear translation visualized by ribosome-bound nascent chain puromycylation

A new method for visualizing translation in cells via standard immunofluorescence microscopy provides evidence for translation in the nucleoplasm and nucleolus.

Whether protein translation occurs in the nucleus is contentious. To address this question, we developed the ribopuromycylation method (RPM), which visualizes translation in cells via standard immunofluorescence microscopy. The RPM is based on ribosome-catalyzed puromycylation of nascent chains immobilized on ribosomes by antibiotic chain elongation inhibitors followed by detection of puromycylated ribosome-bound nascent chains with a puromycin (PMY)-specific monoclonal antibody in fixed and permeabilized cells. The RPM correlates localized translation with myriad processes in cells and can be applied to any cell whose translation is sensitive to PMY. In this paper, we use the RPM to provide evidence for translation in the nucleoplasm and nucleolus, which is regulated by infectious and chemical stress.

Translation initiation factors and active sites of protein synthesis co-localize at the leading edge of migrating fibroblasts

Cell migration is a highly controlled essential cellular process, often dysregulated in tumour cells, dynamically controlled by the architecture of the cell. Studies involving cellular fractionation and microarray profiling have previously identified functionally distinct mRNA populations specific to cellular organelles and architectural compartments. However, the interaction between the translational machinery itself and cellular structures is relatively unexplored. To help understand the role for the compartmentalization and localized protein synthesis in cell migration, we have used scanning confocal microscopy, immunofluorescence and a novel ribopuromycylation method to visualize translating ribosomes. In the present study we show that eIFs (eukaryotic initiation factors) localize to the leading edge of migrating MRC5 fibroblasts in a process dependent on TGN (trans-Golgi network) to plasma membrane vesicle transport. We show that eIF4E and eIF4GI are associated with the Golgi apparatus and membrane microdomains, and that a proportion of these proteins co-localize to sites of active translation at the leading edge of migrating cells.

Differential regulation and function of 5'-untranslated GR-exon 1 transcripts

Alternative splicing serves to increase biological diversity and adaptation. Many genes, including the glucocorticoid receptor (GR), contain multiple 5'-untranslated exons in their promoter regions that can give rise to various mRNA isoforms encoding the same protein. To date, information on the mouse GR promoter remains sparse. Here, we extensively characterize alternative first exons of the mouse GR to reveal homology to the rat and human. We further find that, although most promoters are broadly expressed in various tissues, transcription of individual promoters can be differentially regulated by growth factor- and depolarization-induced signaling. Moreover, in addition to selective promoter usage, the alternative first exon transcripts differentially control RNA stability and translation efficiency, indicative of their role in GR expression. In conclusion, the composite GR promoter enables multilayered adjustments in gene expression through transcriptional and posttranscriptional mechanisms that may serve varying physiological demands.

Dealing with misfolded proteins: examining the neuroprotective role of molecular chaperones in neurodegeneration

Human neurodegenerative diseases arise from a wide array of genetic and environmental factors. Despite the diversity in etiology, many of these diseases are considered "conformational" in nature, characterized by the accumulation of pathological, misfolded proteins. These misfolded proteins can induce cellular stress by overloading the proteolytic machinery, ultimately resulting in the accumulation and deposition of aggregated protein species that are cytotoxic. Misfolded proteins may also form aberrant, non-physiological protein-protein interactions leading to the sequestration of other normal proteins essential for cellular functions. The progression of such disease may therefore be viewed as a failure of normal protein homeostasis, a process that involves a network of molecules regulating the synthesis, folding, translocation and clearance of proteins. Molecular chaperones are highly conserved proteins involved in the folding of nascent proteins, and the repair of proteins that have lost their typical conformations. These functions have therefore made molecular chaperones an active area of investigation within the field of conformational diseases. This review will discuss the role of molecular chaperones in neurodegenerative diseases, highlighting their functional classification, regulation, and therapeutic potential for such diseases.

Indirect participation of Hsp90 in the regulation of the cyclin E turnover

Cyclin E is the Cdk2-regulatory subunit required for the initiation of DNA replication at the G1/S transition. It accumulates in late G1 phase and gets rapidly degraded by the ubiquitin/proteasome pathway during S phase. The degradation of cyclin E is a consequence of its phosphorylation and subsequent isomerization by the peptidyl-prolyl isomerase Pin1. We show that in the colon cancer cells HT-29 the inhibition of the chaperone function of Hsp90 by geldanamycin (GA) enhances the ubiquitinylation of cyclin E and triggers active degradation via the proteasome pathway. As Hsp90 forms multiprotein complexes with and regulates the function and cell contents of numerous signaling proteins, this observation suggests a direct interaction between Hsp90 and cyclin E. However, experiments using cell lysate fractionation did not reveal the presence of complexes containing both Hsp90 and cyclin E. Coupled transcription/translation experiments also failed to detect the formation of complexes between newly synthesized cyclin E and Hsp90. We conclude that Hsp90 can regulate the degradation of cellular proteins without binding to them, by an indirect mechanism. This conclusion postulates a new category of proteins that are affected by the inactivation of Hsp90. Our observations do not support the possible involvement of a PPIase in this indirect mechanism. Besides, we did not observe active geldanamycin-dependent degradation of cyclin E in the prostate cancer-derived cell line DU-145, indicating that the Hsp90-dependent stabilization of cyclin E requires specific regulatory mechanism which may be lost in certain types of cancer cells.

Fine-tuning of translation termination efficiency in Saccharomyces cerevisiae involves two factors in close proximity to the exit tunnel of the ribosome

In eukaryotes, release factors 1 and 3 (eRF1 and eRF3) are recruited to promote translation termination when a stop codon on the mRNA enters at the ribosomal A-site. However, their overexpression increases termination efficiency only moderately, suggesting that other factors might be involved in the termination process. To determine such unknown components, we performed a genetic screen in Saccharomyces cerevisiae that identified genes increasing termination efficiency when overexpressed. For this purpose, we constructed a dedicated reporter strain in which a leaky stop codon is inserted into the chromosomal copy of the ade2 gene. Twenty-five antisuppressor candidates were identified and characterized for their impact on readthrough. Among them, SSB1 and snR18, two factors close to the exit tunnel of the ribosome, directed the strongest antisuppression effects when overexpressed, showing that they may be involved in fine-tuning of the translation termination level.

The Agrobacterium tumefaciens DnaK: ATPase cycle, oligomeric state and chaperone properties

DnaK is a molecular chaperone that promotes cell survival during stress by preventing protein misfolding. The chaperone activity is regulated by nucleotide binding and hydrolysis events in the N-terminal ATPase domain, which in turn mediate substrate binding and release in the C-terminal substrate binding domain. In this study we determined that ATP hydrolysis was the rate limiting step in the ATPase cycle of Agrobacterium tumefaciens DnaK (Agt DnaK); however the data suggested that Agt DnaK had a significantly lower affinity for ATP than Escherichia coli DnaK. We show for the first time that Agt DnaK was very effective at preventing thermal aggregation of malate dehydrogenase (MDH) in a concentration dependent manner. This is in contrast to E. coli DnaK which was ineffective at preventing thermal aggregation of MDH. A mutant Agt DnaK-V431F, with a blocked hydrophobic pocket in the substrate binding domain, was unable to suppress the thermosensitivty of an E. coli dnaK103 deletion strain. However the mutation did not inhibit Agt DnaK-V431F from preventing the thermal aggregation of MDH. The oligomeric state of Agt DnaK was studied using size exclusion chromatography. We demonstrated that dilution of the Agt DnaK protein, the addition of ATP and the removal of the 10kDa C-terminal alpha-helical subdomain reduced higher order associations but did not abrogate dimerisation. Our research implies that the C-terminal alpha-helical subdomain is involved in higher order associations, while the substrate binding domain is possibly involved in dimerisation.

Molecular cloning and expression analysis of a heat shock protein (Hsp90) gene from black tiger shrimp (Penaeus monodon)

The techniques of homology cloning and anchored PCR were used to clone the Hsp90 gene from black tiger shrimp. The full length cDNA of black tiger shrimp Hsp90 (btsHsp90) contained a 5' untranslated region (UTR) of 72 bp, an ORF (open reading frame) of 2160 bp encoding a polypeptide of 720 amino acids with an estimated molecular mass of 83-kDa and a 3' UTR of 288 bp. The sequence of the coding region showed 90 and 84% homology with that of the Chiromantes haematocheir and Homo sapiens, respectively. Conserved signature sequences of Hsp90 gene family were found in the btsHsp90 deduced amino acid sequence. The temporal expressions of Hsp90 gene were constitutively in the black tiger shrimp tissues including liver, ovary, muscle, brain stomach, and heart, and their levels were markedly enhanced after 30-min heat treatment at 37 degrees C. In ovarian maturation stages, the expression of btsHsp90 was strongest in the second stage, weaker in the fourth and first stage.

Irreversible aggregation of protein synthesis machinery after focal brain ischemia

Focal brain ischemia leads to a slow type of neuronal death in the penumbra that starts several hours after ischemia and continues to mature for days. During this maturation period, blood flow, cellular ATP and ionic homeostasis are gradually recovered in the penumbral region. In striking contrast, protein synthesis is irreversibly inhibited. This study used a rat focal brain ischemia model to investigate whether or not irreversible translational inhibition is due to abnormal aggregation of translational complex components, i.e. the ribosomes and their associated nascent polypeptides, protein synthesis initiation factors and co-translational chaperones. Under electron microscopy, most rosette-shaped polyribosomes were relatively evenly distributed in the cytoplasm of sham-operated control neurons, but clumped into large abnormal aggregates in penumbral neurons subjected to 2 h of focal ischemia followed by 4 h of reperfusion. The abnormal ribosomal protein aggregation lasted until the onset of delayed neuronal death at 24-48 h of reperfusion after ischemia. Biochemical study further suggested that translational complex components, including small ribosomal subunit protein 6 (S6), large subunit protein 28 (L28), eukaryotic initiation factors 2alpha, 4E and 3eta, and co-translational chaperone heat-shock cognate protein 70 (HSC70) and co-chaperone Hdj1, were all irreversibly clumped into large abnormal protein aggregates after ischemia. Translational complex components were also highly ubiquitinated. This study clearly demonstrates that focal ischemia leads to irreversible aggregation of protein synthesis machinery that contributes to neuronal death after focal brain ischemia.

Three immunoproteasome-associated subunits cooperatively generate a cytotoxic T-lymphocyte epitope of Epstein-Barr virus LMP2A by overcoming specific structures resistant to epitope liberation

The precise roles of gamma interferon-inducible immunoproteasome-associated molecules in generation of cytotoxic T-lymphocyte (CTL) epitopes have yet to be fully elucidated. We describe here a unique epitope derived from the Epstein-Barr virus (EBV) latent membrane protein 2A (LMP2A) presented by HLA-A*2402 molecules. Generation of the epitope, designated LMP2A(222-230), from the full-length protein requires the immunoproteasome subunit low-molecular-weight protein 7 (ip-LMP7) and the proteasome activator 28-alpha subunit and is accelerated by ip-LMP2, as revealed by gene expression experiments using an LMP2A(222-230)-specific CTL clone as a responder in enzyme-linked immunospot assays. The unequivocal involvement of all three components was confirmed by RNA interference gene silencing. Interestingly, the LMP2A(222-230) epitope could be efficiently generated from incomplete EBV-LMP2A fragments that were produced by puromycin treatment or gene-engineered shortened EBV-LMP2A lacking some of its hydrophobic domains. In addition, epitope generation was increased by a single amino acid substitution from leucine to alanine immediately flanking the C terminus, this being predicted by a web-accessible program to increase the cleavage strength. Taken together, the data indicate that the generation of LMP2A(222-230) is influenced not only by extrinsic factors such as immunoproteasomes but also by intrinsic factors such as the length of the EBV-LMP2A protein and proteasomal cleavage strength at specific positions in the source antigen.

Pathways of chaperone-mediated protein folding in the cytosol

Cells are faced with the task of folding thousands of different polypeptides into a wide range of conformations. For many proteins, the folding process requires the action of molecular chaperones. In the cytosol of prokaryotic and eukaryotic cells, molecular chaperones of different structural classes form a network of pathways that can handle substrate polypeptides from the point of initial synthesis on ribosomes to the final stages of folding.

Molecular cloning of the 82-kDa heat shock protein (HSP90) of Toxoplasma gondii associated with the entry into and growth in host cells

Among the monoclonal antibodies (mAb) against Toxoplasma gondii, mAb Tg485 specifically reacted with an 82-kDa cytoplasmic protein of tachyzoites. The protein was secreted from extracellular tachyzoites, but was not released into the parasitophorous vacuole after invasion. The cDNA fragment encoding the protein was obtained by screening a T. gondii cDNA expression library with Tg485. The full-length cDNA was amplified by the 5(')-RACE method and sequenced. The deduced amino acid sequence of the 82 kDa protein reacting with Tg485 revealed a polypeptide of 708 amino acids showing significant homology to the heat shock protein 90 (HSP90) family of other organisms, especially to those of apicomplexan species. Treatment with geldanamycin, a drug known to interfere with HSP90 function, did not affect the secretion of TgHSP90 from extracellular tachyzoites, but the entry of the tachyzoites into host cells and the intracellular growth of the parasite were significantly disturbed.

Signal recognition particle-dependent protein targeting, universal to all kingdoms of life

The signal recognition particle (SRP) and its membrane-bound receptor represent a ubiquitous protein-targeting device utilized by organisms as different as bacteria and humans, archaea and plants. The unifying concept of SRP-dependent protein targeting is that SRP binds to signal sequences of newly synthesized proteins as they emerge from the ribosome. In eukaryotes this interaction arrests or retards translation elongation until SRP targets the ribosome-nascent chain complexes via the SRP receptor to the translocation channel. Such channels are present in the endoplasmic reticulum of eukaryotic cells, the thylakoids of chloroplasts, or the plasma membrane of prokaryotes. The minimal functional unit of SRP consists of a signal sequence-recognizing protein and a small RNA. The as yet most complex version is the mammalian SRP whose RNA, together with six proteinaceous subunits, undergo an intricate assembly process. The preferential substrates of SRP possess especially hydrophobic signal sequences. Interactions between SRP and its receptor, the ribosome, the signal sequence, and the target membrane are regulated by GTP hydrolysis. SRP-dependent protein targeting in bacteria and chloroplasts slightly deviate from the canonical mechanism found in eukaryotes. Pro- and eukaryotic cells harbour regulatory mechanisms to prevent a malfunction of the SRP pathway.

Heat shock protein 90 function is essential for Plasmodium falciparum growth in human erythrocytes

Hsp90 is important for normal growth and development in eukaryotes. Together with Hsp70 and other accessory proteins, Hsp90 not only helps newly synthesized proteins to fold but also regulates activities of transcription factors and protein kinases. Although the gene coding for heat shock protein 90 from Plasmodium falciparum (PfHsp90) has been characterized previously, there is very little known regarding its function in the parasite. We have analyzed PfHsp90 complexes and addressed its role in parasite life cycle using Geldanamycin (GA), a drug known to interfere with Hsp90 function. Sedimentation analysis and size exclusion chromatography showed PfHsp90 to be in 11 s(20,(w)) complexes of approximately 300 kDa in size. Similar to the hetero-oligomeric complexes of Hsp90 in mammals, PfHsp70 was found to be present in PfHsp90 complexes. Homology modeling revealed a putative GA-binding pocket at the amino terminus of PfHsp90. The addition of GA inhibited parasite growth with LD(50) of 0.2 microm. GA inhibited parasite growth by arresting transition from Ring to trophozoite. Transition from trophozoite to schizonts and reinvasion of new erythrocytes were less significantly affected. While inducing the synthesis of PfHsp70 and PfHsp90, GA did not significantly alter the pattern of newly synthesized proteins. Pre-exposure to heat shock attenuated GA-mediated growth inhibition, suggesting the involvement of heat shock proteins. Specificity of GA action on PfHsp90 was evident from selective inhibition of PfHsp90 phosphorylation in GA-treated cultures. In addition to suggesting an essential role for PfHsp90 during parasite growth, our results highlight PfHsp90 as a potential drug target to control malaria.

Redox signaling of cardiac HSF1 DNA binding

Experiments involving chemical induction of the heat shock response in simple biological systems have generated the hypothesis that protein denaturation and consequential binding of heat shock transcription factor 1 (HSF1) to proximal heat shock elements (HSEs) on heat shock protein (hsp) genes are the result of oxidation and/or depletion of intracellular thiols. The purpose of the present investigation was to determine the role of redox signaling of HSF1 in the intact animal in response to physiological and pharmacological perturbations. Heat shock and exercise induced HSF1-HSE DNA binding in the rat myocardium (P < 0.001) in the absence of changes in reduced glutathione (GSH), the major nonprotein thiol in the cell. Ischemia-reperfusion, which decreased GSH content (P < 0.05), resulted in nonsignificant HSF1-HSE formation. This dissociation between physiological induction of HSF1 and changes in GSH was not gender dependent. Pharmacological ablation of GSH with L-buthionine-[S,R]-sulfoximine (BSO) treatment increased myocardial HSF1-HSE DNA binding in estrogen-naive animals (P = 0.007). Thus, although physiological induction of HSF1-HSE DNA binding is likely regulated by mediators of protein denaturation other than cellular redox status, the proposed signaling pathway may predominate with pharmacological oxidation and may represent a plausible and accessible strategy in the development of HSP-based therapies.

Hsp70 and thermal pretreatment mitigate developmental damage caused by mitotic poisons in Drosophila

To assess the ability of the heat-inducible molecular chaperone heat-shock protein 70 (Hsp70) to mitigate a specific developmental lesion, we administered the antimicrotubule drugs vinblastine (VB) and colchicine (COL) to larvae of Drosophila engineered to express differing levels of Hsp70 after heat pretreatment (HP). VB and COL decreased survival during metamorphosis, disrupted development of the adult eye and other structures as well as their precursor imaginal disks, and induced chromosome nondisjunction in the wing imaginal disk as indicated by the somatic mutation and recombination test (SMART) assay. Hsp70-inducing HP reduced many of these effects. For the traits viability, adult eye morphology, eye imaginal disk morphology, cell death in the eye imaginal disks, and single and total mutant clone formation in the SMART assay, HP reduced the impact of VB to a greater extent in Drosophila with 6 hsp70 transgenes than in a sister strain from which the transgenes had been excised. Because the extra-copy strain has higher levels of Hsp70 than does the excision strain but is otherwise almost identical in genetic background to the excision strain, these outcomes are attributable to Hsp70. The hsp70 copy number had a variable interaction with HP and COL administration.

The action of molecular chaperones in the early secretory pathway

The endoplasmic reticulum (ER) serves as a way-station during the biogenesis of nearly all secreted proteins, and associated with or housed within the ER are factors required to catalyze their import into the ER and facilitate their folding. To ensure that only properly folded proteins are secreted and to temper the effects of cellular stress, the ER can target aberrant proteins for degradation and/or adapt to the accumulation of misfolded proteins. Molecular chaperones play critical roles in each of these phenomena.

Divergent functional properties of the ribosome-associated molecular chaperone Ssb compared with other Hsp70s

Ssbs of Saccharomyces cerevisiae are ribosome-associated molecular chaperones, which can be cross-linked to nascent polypeptide chains. Because Ssbs are members of a divergent subclass of Hsp70s found thus far only in fungi, we asked if the structural requirements for in vivo function were similar to those of "classic" Hsp70s. An intact peptide-binding domain is essential and an alteration of a conserved residue in the peptide-binding cleft (V442) affects function. However, Ssb tolerates a number of alterations in the peptide-binding cleft, revealing a high degree of flexibility in its functional requirements. Because binding of Ssb to peptide substrates in vitro was undetectable, we assessed the importance of substrate binding using the chimera BAB, in which the peptide binding domain of Ssb is exchanged for the analogous domain of the more "classical" Hsp70, Ssa. BAB, which binds peptide substrates in vitro, can substitute for Ssb in vivo. Alteration of a residue in the peptide-binding cleft of BAB creates a protein with a reduced affinity for peptide and altered ribosome binding that is unable to substitute for Ssb in vivo. These results indicate that Ssb's ability to bind unfolded polypeptides is likely critical for its function. This binding accounts, in part, for its stable interaction with translating ribosomes, even although it has a low affinity for peptides that detectably bind to other Hsp70s in vitro. These unusual properties may allow Ssb to function efficiently as a chaperone for ribosome-bound nascent chains.

Not such a dismal science: the economics of protein synthesis, folding, degradation and antigen processing

There is a pronounced tendency among cell biologists to focus on qualitative aspects of cell physiology. The remarkable accomplishments of evolution in creating cells can only be fully appreciated, however, by combining this qualitative analysis with a quantitative assessment of cellular constituents and processes. Here, I consider the overall protein economy of cells as it relates to recent advances in understanding protein folding, ubiquitin-targeted proteasome-mediated degradation of proteins and the generation of peptide ligands for major histocompatibility complex (MHC) class I molecules.

Folding of a nascent peptide on the ribosome

Even though very significant progress has been made recently in elucidating the structure of the bacterial ribosome and topological assignments of its functional parts, the molecular mechanism of how a peptide is formed and how the nascent peptides is folded on the ribosomes remains uncertain. Here, the current progress and remaining problems are considered from the standpoint of the authors. Topics considered include formation of peptide bonds and models that represent this process, the vicinity of RNA to the nascent peptide, the cotranslational folding hypothesis, evidence that some but not all nascent peptides pass through a region within the 50S ribosomal subunit, presumably the tunnel, in which they are folded and sheltered, pause-site peptides, and the involvement of chaperones in folding of nascent proteins on ribosomes. The chaperone-like activity of the large ribosomal subunit in renaturation of denatured proteins is reviewed. It is concluded that cotranslational folding of some but not all nascent peptides occurs in the large ribosomal subunit. It is suggested that this folding is facilitated by changes in the conformation of the ribosome that are related to the reaction cycle of peptide elongation.

Maturation of steroid receptors: an example of functional cooperation among molecular chaperones and their associated proteins

The selective modulation of transcription exerted by steroids depends upon recognition of signalling molecules by properly folded cytoplasmic receptors and their subsequent translocation into the nucleus. These events require a sequential and dynamic series of protein-protein interactions in order to fashion receptors that bind stably to steroids. Central to receptor maturation, therefore, are several molecular chaperones and their accessory proteins; Hsp70, Hsp40, and hip modulate the 3-dimensional conformation of steroid receptors, permitting reaction via hop with Hsp90, arguably the central protein in the process. Binding to Hsp90 leads to dissociation of some proteins from the receptor complex while others are recruited. Notably, p23 stabilizes receptors in a steroid binding state, and the immunophilins, principally CyP40 and Hsp56, arrive late in receptor complex assembly. In this review, the functions of molecular chaperones during steroid receptor maturation are explored, leading to a general mechanistic model indicative of chaperone cooperation in protein folding.

Induced hsp70 is in small, cytoplasmic complexes in a cell culture model of renal ischemia: a comparative study with heat shock

A number of clinical conditions are known to result in the induction of heat shock proteins, but detailed studies on stress response have focused mostly on heat shock as a model. We have analyzed the induction and intracellular distribution of heat shock proteins in a reversible adenosine triphosphate (ATP) depletion model of renal ischemia. Two Hsp70 homologues, Hsp70 in the cytoplasm and BiP in the endoplasmic reticulum (ER) lumen, were found significantly induced during the recovery phase of ATP depletion. Other members of the heat shock protein family, such as Hsp90, constitutive Hsc70, and a related protein Hop60, were not induced. The induction of stress proteins on ATP depletion differed from that after heat shock in the kinds of proteins elaborated, their induction kinetics, and their intracellular distributions. Biochemical fractionation and indirect immunofluorescence experiments indicated that Hsp70 was predominantly cytoplasmic in the recovery phase of ischemia-like stress. Velocity sedimentation on sucrose gradients showed that induced Hsp70 sedimented as small, soluble complexes, ranging in size from 4S20,w to 8S20,w. The results suggest a role for induced Hsp70 that may be different from one of protecting aggregated proteins as under heat shock and emphasize the need for their characterization in other clinical conditions that result in stress response.

Co-translational interactions of apoprotein B with the ribosome and translocon during lipoprotein assembly or targeting to the proteasome

Hepatic lipoprotein assembly and secretion can be regulated by proteasomal degradation of newly synthesized apoB, especially if lipid synthesis or lipid transfer is low. Our previous studies in HepG2 cells showed that, under these conditions, newly synthesized apoB remains stably associated with the endoplasmic reticulum (ER) membrane (Mitchell, D. M., Zhou, M., Pariyarath, R., Wang, H., Aitchison, J. D., Ginsberg, H. N., and Fisher, E. A. (1998) Proc. Natl. Acad. Sci. U. S. A. 95, 14733-14738). We now show that independent of lipid synthesis, apoB chains that appear full-length are, in fact, incompletely translated polypeptides still engaged by the ribosome and associated with the ER translocon. In the presence of active lipid synthesis and transfer, translation and lipoprotein assembly are completed, and the complexes exit the ER. Upon omitting fatty acids from, or adding a microsomal triglyceride transfer protein inhibitor to, culture media to reduce lipid synthesis or transfer, respectively, apoB was degraded while it remained associated with the ER and complexed with cytosolic hsp70 and proteasomes. Thus, unlike other ER substrates of the proteasome, such as major histocompatibility complex class I molecules, apoB does not fully retrotranslocate to the cytosol before entering the ubiquitin-proteasome pathway. Although, upon immunofluorescence, apoB in proteasome-inhibited cells accumulated in punctate structures similar in appearance to aggresomes (cytosolic structures containing molecules irreversibly lost from the secretory pathway), these apoB molecules could be secreted when lipid synthesis was stimulated. The results suggest a model in which 1) apoB translation does not complete until lipoprotein assembly terminates, and 2) assembly with lipids or entry into the ubiquitin-proteasome pathway occurs while apoB polypeptides remain associated with the translocon and attached to the ribosome.