Yeast Pth2 is a UBL domain-binding protein that participates in the ubiquitin-proteasome pathway

A Homozygous PTRHD1 Missense Variant (p.Arg122Gln) in an Individual with Intellectual Disability, Generalized Epilepsy, and Juvenile Parkinsonism

Biallelic variants in PTRHD1 have been associated with autosomal recessive intellectual disability, spasticity, and juvenile parkinsonism, with few reported cases. Here, we present the clinical and genetic findings of a female of Austrian origin exhibiting infantile neurodevelopmental abnormalities, intellectual disability, and childhood-onset parkinsonian features, consistent with the established phenotypic spectrum. Notably, she developed genetic generalized epilepsy at age 4, persisting into adulthood. Using diagnostic exome sequencing, we identified a homozygous missense variant (c.365G > A, p.(Arg122Gln)) in PTRHD1 (NM_001013663). In summary, our findings not only support the existing link between biallelic PTRHD1 variants and parkinsonism with neurodevelopmental abnormalities but also suggest a potential extension of the phenotypic spectrum to include generalized epilepsy.

Conserved quality control mechanisms of mitochondrial protein import

Mitochondria carry out essential functions for the cell, including energy production, various biosynthesis pathways, formation of co-factors and cellular signalling in apoptosis and inflammation. The functionality of mitochondria requires the import of about 900-1300 proteins from the cytosol in baker's yeast Saccharomyces cerevisiae and human cells, respectively. The vast majority of these proteins pass the outer membrane in a largely unfolded state through the translocase of the outer mitochondrial membrane (TOM) complex. Subsequently, specific protein translocases sort the precursor proteins into the outer and inner membranes, the intermembrane space and matrix. Premature folding of mitochondrial precursor proteins, defects in the mitochondrial protein translocases or a reduction of the membrane potential across the inner mitochondrial membrane can cause stalling of precursors at the protein import apparatus. Consequently, the translocon is clogged and non-imported precursor proteins accumulate in the cell, which in turn leads to proteotoxic stress and eventually cell death. To prevent such stress situations, quality control mechanisms remove non-imported precursor proteins from the TOM channel. The highly conserved ubiquitin-proteasome system of the cytosol plays a critical role in this process. Thus, the surveillance of protein import via the TOM complex involves the coordinated activity of mitochondria-localized and cytosolic proteins to prevent proteotoxic stress in the cell.

Mitochondrial protein and organelle quality control-Lessons from budding yeast

Mitochondria are essential for normal cellular function and have emerged as key aging determinants. Indeed, defects in mitochondrial function have been linked to cardiovascular, skeletal muscle and neurodegenerative diseases, premature aging, and age-linked diseases. Here, we describe mechanisms for mitochondrial protein and organelle quality control. These surveillance mechanisms mediate repair or degradation of damaged or mistargeted mitochondrial proteins, segregate mitochondria based on their functional state during asymmetric cell division, and modulate cellular fitness, the response to stress, and lifespan control in yeast and other eukaryotes.

The role of the proteasome in mitochondrial protein quality control

The abundance of each cellular protein is dynamically adjusted to the prevailing metabolic and stress conditions by modulation of their synthesis and degradation rates. The proteasome represents the major machinery for the degradation of proteins in eukaryotic cells. How the ubiquitin-proteasome system (UPS) controls protein levels and removes superfluous and damaged proteins from the cytosol and the nucleus is well characterized. However, recent studies showed that the proteasome also plays a crucial role in mitochondrial protein quality control. This mitochondria-associated degradation (MAD) thereby acts on two layers: first, the proteasome removes mature, functionally compromised or mis-localized proteins from the mitochondrial surface; and second, the proteasome cleanses the mitochondrial import pore of import intermediates of nascent proteins that are stalled during translocation. In this review, we provide an overview about the components and their specific functions that facilitate proteasomal degradation of mitochondrial proteins in the yeast Saccharomyces cerevisiae. Thereby we explain how the proteasome, in conjunction with a set of intramitochondrial proteases, maintains mitochondrial protein homeostasis and dynamically adapts the levels of mitochondrial proteins to specific conditions.

The ubiquitin-binding protein MdRAD23D1 mediates drought response by regulating degradation of the proline-rich protein MdPRP6 in apple (Malus domestica)

RAD23 (RADIATION SENSITIVE23) proteins are a group of UBL-UBA (ubiquitin-like-ubiquitin-associated) proteins that shuttle ubiquitylated proteins to the 26S proteasome for breakdown. Drought stress is a major environmental constraint that limits plant growth and production, but whether RAD23 proteins are involved in this process is unclear. Here, we demonstrated that a shuttle protein, MdRAD23D1, mediated drought response in apple plants (Malus domestica). MdRAD23D1 levels increased under drought stress, and its suppression resulted in decreased stress tolerance in apple plants. Through in vitro and in vivo assays, we demonstrated that MdRAD23D1 interacted with a proline-rich protein MdPRP6, resulting in the degradation of MdPRP6 by the 26S proteasome. And MdRAD23D1 accelerated the degradation of MdPRP6 under drought stress. Suppression of MdPRP6 resulted in enhanced drought tolerance in apple plants, mainly because the free proline accumulation is changed. And the free proline is also involved in MdRAD23D1-mediated drought response. Taken together, these findings demonstrated that MdRAD23D1 and MdPRP6 oppositely regulated drought response. MdRAD23D1 levels increased under drought, accelerating the degradation of MdPRP6. MdPRP6 negatively regulated drought response, probably by regulating proline accumulation. Thus, "MdRAD23D1-MdPRP6" conferred drought stress tolerance in apple plants.

PTPA variants and impaired PP2A activity in early-onset parkinsonism with intellectual disability

The protein phosphatase 2A complex (PP2A), the major Ser/Thr phosphatase in the brain, is involved in a number of signalling pathways and functions, including the regulation of crucial proteins for neurodegeneration, such as alpha-synuclein, tau and LRRK2. Here, we report the identification of variants in the PTPA/PPP2R4 gene, encoding a major PP2A activator, in two families with early-onset parkinsonism and intellectual disability. We carried out clinical studies and genetic analyses, including genome-wide linkage analysis, whole-exome sequencing, and Sanger sequencing of candidate variants. We next performed functional studies on the disease-associated variants in cultured cells and knock-down of ptpa in Drosophila melanogaster. We first identified a homozygous PTPA variant, c.893T>G (p.Met298Arg), in patients from a South African family with early-onset parkinsonism and intellectual disability. Screening of a large series of additional families yielded a second homozygous variant, c.512C>A (p.Ala171Asp), in a Libyan family with a similar phenotype. Both variants co-segregate with disease in the respective families. The affected subjects display juvenile-onset parkinsonism and intellectual disability. The motor symptoms were responsive to treatment with levodopa and deep brain stimulation of the subthalamic nucleus. In overexpression studies, both the PTPA p.Ala171Asp and p.Met298Arg variants were associated with decreased PTPA RNA stability and decreased PTPA protein levels; the p.Ala171Asp variant additionally displayed decreased PTPA protein stability. Crucially, expression of both variants was associated with decreased PP2A complex levels and impaired PP2A phosphatase activation. PTPA orthologue knock-down in Drosophila neurons induced a significant impairment of locomotion in the climbing test. This defect was age-dependent and fully reversed by L-DOPA treatment. We conclude that bi-allelic missense PTPA variants associated with impaired activation of the PP2A phosphatase cause autosomal recessive early-onset parkinsonism with intellectual disability. Our findings might also provide new insights for understanding the role of the PP2A complex in the pathogenesis of more common forms of neurodegeneration.

The moonlighting of RAD23 in DNA repair and protein degradation

A moonlighting protein is one, which carries out multiple, often wholly unrelated, functions. The RAD23 protein is a fascinating example of this, where the same polypeptide and the embedded domains function independently in both nucleotide excision repair (NER) and protein degradation via the ubiquitin-proteasome system (UPS). Hence, through direct binding to the central NER component XPC, RAD23 stabilizes XPC and contributes to DNA damage recognition. Conversely, RAD23 also interacts directly with the 26S proteasome and ubiquitylated substrates to mediate proteasomal substrate recognition. In this function, RAD23 activates the proteolytic activity of the proteasome and engages specifically in well-characterized degradation pathways through direct interactions with E3 ubiquitin-protein ligases and other UPS components. Here, we summarize the past 40 years of research into the roles of RAD23 in NER and the UPS.

Mitochondrial complexome reveals quality-control pathways of protein import

Mitochondria have crucial roles in cellular energetics, metabolism, signalling and quality control^1-4. They contain around 1,000 different proteins that often assemble into complexes and supercomplexes such as respiratory complexes and preprotein translocases^1,3-7. The composition of the mitochondrial proteome has been characterized^1,3,5,6; however, the organization of mitochondrial proteins into stable and dynamic assemblies is poorly understood for major parts of the proteome^1,4,7. Here we report quantitative mapping of mitochondrial protein assemblies using high-resolution complexome profiling of more than 90% of the yeast mitochondrial proteome, termed MitCOM. An analysis of the MitCOM dataset resolves >5,200 protein peaks with an average of six peaks per protein and demonstrates a notable complexity of mitochondrial protein assemblies with distinct appearance for respiration, metabolism, biogenesis, dynamics, regulation and redox processes. We detect interactors of the mitochondrial receptor for cytosolic ribosomes, of prohibitin scaffolds and of respiratory complexes. The identification of quality-control factors operating at the mitochondrial protein entry gate reveals pathways for preprotein ubiquitylation, deubiquitylation and degradation. Interactions between the peptidyl-tRNA hydrolase Pth2 and the entry gate led to the elucidation of a constitutive pathway for the removal of preproteins. The MitCOM dataset-which is accessible through an interactive profile viewer-is a comprehensive resource for the identification, organization and interaction of mitochondrial machineries and pathways.

Nascent peptide-induced translation discontinuation in eukaryotes impacts biased amino acid usage in proteomes

Robust translation elongation of any given amino acid sequence is required to shape proteomes. Nevertheless, nascent peptides occasionally destabilize ribosomes, since consecutive negatively charged residues in bacterial nascent chains can stochastically induce discontinuation of translation, in a phenomenon termed intrinsic ribosome destabilization (IRD). Here, using budding yeast and a human factor-based reconstituted translation system, we show that IRD also occurs in eukaryotic translation. Nascent chains enriched in aspartic acid (D) or glutamic acid (E) in their N-terminal regions alter canonical ribosome dynamics, stochastically aborting translation. Although eukaryotic ribosomes are more robust to ensure uninterrupted translation, we find many endogenous D/E-rich peptidyl-tRNAs in the N-terminal regions in cells lacking a peptidyl-tRNA hydrolase, indicating that the translation of the N-terminal D/E-rich sequences poses an inherent risk of failure. Indeed, a bioinformatics analysis reveals that the N-terminal regions of ORFs lack D/E enrichment, implying that the translation defect partly restricts the overall amino acid usage in proteomes.

Parkinsonism in Genetic Neurodevelopmental Disorders: A Systematic Review

Background

With advances in clinical genetic testing, associations between genetic neurodevelopmental disorders and parkinsonism are increasingly recognized. In this review, we aimed to provide a comprehensive overview of reports on parkinsonism in genetic neurodevelopmental disorders and summarize findings related to genetic diagnosis, clinical features and proposed disease mechanisms.

Methods

A systematic literature review was conducted in PubMed and Embase on June 15, 2021. Search terms for parkinsonism and genetic neurodevelopmental disorders, using generic terms and the Human Phenotype Ontology, were combined. Study characteristics and descriptive data were extracted from the articles using a modified version of the Cochrane Consumers and Communication Review Group's data extraction template. The protocol was registered in PROSPERO (CRD42020191035).

Results

The literature search yielded 208 reports for data-extraction, describing 69 genetic disorders in 422 patients. The five most reported from most to least frequent were: 22q11.2 deletion syndrome, beta-propeller protein-associated neurodegeneration, Down syndrome, cerebrotendinous xanthomatosis, and Rett syndrome. Notable findings were an almost equal male to female ratio, an early median age of motor onset (26 years old) and rigidity being more common than rest tremor. Results of dopaminergic imaging and response to antiparkinsonian medication often supported the neurodegenerative nature of parkinsonism. Moreover, neuropathology results showed neuronal loss in the majority of cases. Proposed disease mechanisms included aberrant mitochondrial function and disruptions in neurotransmitter metabolism, endosomal trafficking, and the autophagic-lysosomal and ubiquitin-proteasome system.

Conclusion

Parkinsonism has been reported in many GNDs. Findings from this study may provide clues for further research and improve management of patients with GNDs and/or parkinsonism.

Structure, Dynamics and Function of the 26S Proteasome

The 26S proteasome is the most complex ATP-dependent protease machinery, of ~2.5 MDa mass, ubiquitously found in all eukaryotes. It selectively degrades ubiquitin-conjugated proteins and plays fundamentally indispensable roles in regulating almost all major aspects of cellular activities. To serve as the sole terminal "processor" for myriad ubiquitylation pathways, the proteasome evolved exceptional adaptability in dynamically organizing a large network of proteins, including ubiquitin receptors, shuttle factors, deubiquitinases, AAA-ATPase unfoldases, and ubiquitin ligases, to enable substrate selectivity and processing efficiency and to achieve regulation precision of a vast diversity of substrates. The inner working of the 26S proteasome is among the most sophisticated, enigmatic mechanisms of enzyme machinery in eukaryotic cells. Recent breakthroughs in three-dimensional atomic-level visualization of the 26S proteasome dynamics during polyubiquitylated substrate degradation elucidated an extensively detailed picture of its functional mechanisms, owing to progressive methodological advances associated with cryogenic electron microscopy (cryo-EM). Multiple sites of ubiquitin binding in the proteasome revealed a canonical mode of ubiquitin-dependent substrate engagement. The proteasome conformation in the act of substrate deubiquitylation provided insights into how the deubiquitylating activity of RPN11 is enhanced in the holoenzyme and is coupled to substrate translocation. Intriguingly, three principal modes of coordinated ATP hydrolysis in the heterohexameric AAA-ATPase motor  were discovered to regulate intermediate functional steps of the proteasome, including ubiquitin-substrate engagement, deubiquitylation, initiation of substrate translocation and processive substrate degradation. The atomic dissection of the innermost working of the 26S proteasome opens up a new era in our understanding of the ubiquitin-proteasome system and has far-reaching implications in health and disease.

Structural and functional characterization of peptidyl-tRNA hydrolase from Klebsiella pneumoniae

Klebsiella pneumoniae is a member of the ESKAPE panel of pathogens that are top priority to tackle AMR. Bacterial peptidyl tRNA hydrolase (Pth), an essential, ubiquitous enzyme, hydrolyzes the peptidyl-tRNAs that accumulate in the cytoplasm because of premature termination of translation. Pth cleaves the ester bond between 2' or 3' hydroxyl of the ribose in the tRNA and C-terminal carboxylate of the peptide, thereby making free tRNA available for repeated cycles of protein synthesis and preventing cell death by alleviating tRNA starvation. Pth structures have been determined in peptide-bound or peptide-free states. In peptide-bound state, highly conserved residues F67, N69 and N115 adopt a conformation that is conducive to their interaction with peptide moiety of the substrate. While, in peptide-free state, these residues move away from the catalytic center, perhaps, in order to facilitate release of hydrolysed peptide. Here, we present a novel X-ray crystal structure of Pth from Klebsiella pneumoniae (KpPth), at 1.89 Å resolution, in which out of the two molecules in the asymmetric unit, one reflects the peptide-bound while the other reflects peptide-free conformation of the conserved catalytic site residues. Each molecule of the protein has canonical structure with seven stranded β-sheet structure surrounded by six α-helices. MD simulations indicate that both the forms converge over 500 ns simulation to structures with wider opening of the crevice at peptide-binding end. In solution, KpPth is monomeric and its 2D-HSQC spectrum displays a single set of well dispersed peaks. Further, KpPth was demonstrated to be enzymatically active on BODIPY-Lys-tRNA^Lys3.

Lack of PTRHD1 mutation in patients with young-onset and familial Parkinson's disease in a Taiwanese population

Mutations in the peptidyl-tRNA hydrolase domain containing 1 (PTRHD1) gene have been recently identified in consanguineous Iranian and African families with juvenile parkinsonism and intellectual disability. However, the pathogenicity of PTRHD1 mutations in the disease and their role in young-onset Parkinson's disease (PD) remains unclear. We aimed to investigate PTRHD1 mutations in a Taiwanese cohort with young-onset and familial PD. We enrolled 464 participants, including 178 probands from PD pedigrees without known PD-causative gene mutations and 286 patients with young-onset PD (age of onset <50 years). All exons and exon-intron boundary junctions of PTRHD1 were analyzed by Sanger sequencing. We did not find any pathogenic coding variants or previously reported mutations, suggesting that PTRHD1 mutations are rare in young-onset and familial PD in our population.

The dual function gene RAD23 contributes to Cryptococcus neoformans virulence independently of its role in nucleotide excision DNA repair

Genes involved in the repair of DNA damage are emerging as playing important roles during the disease processes caused by pathogenic fungi. However, there are potentially hundreds of genes involved in DNA repair in a fungus and some of those genes can play additional roles within the cell. One such gene is RAD23, required for virulence of the human pathogenic fungus Cryptococcus neoformans, that encodes a protein involved in the nucleotide excision repair (NER) pathway. However, Rad23 is a dual function protein, with a role in either repair of damaged DNA or protein turn over by directing proteins to the proteasome. Here, these two functions of Rad23 were tested by the creation of a series of domain deletion alleles of RAD23 and the assessment of the strains for DNA repair, proteasome functions, and virulence properties. Deletion of the different domains was able to uncouple the two functions of Rad23, and the phenotypes of strains carrying such forms indicated that the role of RAD23 in virulence is due to its function in proteasomal-mediated protein degradation rather than NER.

Homozygous mutation in PTRH2 gene causes progressive sensorineural deafness and peripheral neuropathy

PTRH2 is an evolutionarily highly conserved mitochondrial protein that belongs to a family of peptidyl-tRNA hydrolases. Recently, patients from two consanguineous families with mutations in the PTRH2 gene were reported. Global developmental delay associated with microcephaly, growth retardation, progressive ataxia, distal muscle weakness with ankle contractures, demyelinating sensorimotor neuropathy, and sensorineural hearing loss were present in all patients, while facial dysmorphism with widely spaced eyes, exotropia, thin upper lip, proximally placed thumbs, and deformities of the fingers and toes were present in some individuals. Here, we report a new family with three siblings affected by sensorineural hearing loss and peripheral neuropathy. Autozygosity mapping followed by exome sequencing identified a previously reported homozygous missense mutation in PTRH2 (c.254A>C; p.(Gln85Pro)). Sanger sequencing confirmed that the variant segregated with the phenotype. In contrast to the previously reported patient, the affected siblings had normal intelligence, milder microcephaly, delayed puberty, myopia, and moderate insensitivity to pain. Our findings expand the clinical phenotype and further demonstrate the clinical heterogeneity related to PTRH2 variants.

The deubiquitylating enzyme Ubp12 regulates Rad23-dependent proteasomal degradation

The consecutive actions of the ubiquitin-selective segregase Cdc48 and the ubiquitin shuttle factor Rad23 result in the delivery of ubiquitylated proteins at the proteasome. Here, we show that the deubiquitylating enzyme Ubp12 interacts with Cdc48 and regulates proteasomal degradation of Rad23-dependent substrates in Saccharomyces cerevisiae. Overexpression of Ubp12 results in stabilization of Rad23-dependent substrates. We show that Ubp12 removes short ubiquitin chains from the N-terminal ubiquitin-like domain (UbL) of Rad23. Preventing ubiquitylation of Rad23 by mutation of lysine residues within the UbL domain, Rad23^UbLK0, does not affect the non-proteolytic role of Rad23 in DNA repair but causes an increase in ubiquitylated cargo bound to the UBA2 domain of Rad23, recapitulating the stabilization of Rad23-dependent substrates observed upon overexpression of Ubp12. Expression of Rad23^UbLK0 or overexpression of Ubp12 impairs the ability of yeast to cope with proteotoxic stress, consistent with inefficient clearance of misfolded proteins by the ubiquitin-proteasome system. Our data suggest that ubiquitylation of Rad23 plays a stimulatory role in the degradation of ubiquitylated substrates by the proteasome.

OsFTIP1-Mediated Regulation of Florigen Transport in Rice Is Negatively Regulated by the Ubiquitin-Like Domain Kinase OsUbDKγ4

Flowering time is a critical agronomic trait that determines successful seed production and adaptation of crop plants. Photoperiodic control of this process in flowering plants is mediated by the long-distance mobile signal called florigen partly encoded by FLOWERING LOCUS T (FT) in Arabidopsis thaliana and its orthologs in other plant species. Despite the progress in understanding FT transport in the dicot model Arabidopsis, the mechanisms of florigen transport in monocots, which provide most of the biomass in agriculture, are unknown. Here, we show that rice FT-INTERACTING PROTEIN1 (OsFTIP1), a member of the family of multiple C2 domain and transmembrane region proteins (MCTPs) and the closest ortholog of Arabidopsis FTIP1, is required for export of RICE FLOWERING LOCUS T 1 (RFT1) from companion cells to sieve elements. This affects RFT1 movement to the shoot apical meristem and its regulation of rice flowering time under long days. We further reveal that a ubiquitin-like domain kinase γ4, OsUbDKγ4, interacts with OsFTIP1 and modulates its degradation in leaves through the 26S proteasome, which in turn affects RFT1 transport to the shoot apical meristem. Thus, dynamic modulation of OsFTIP1 abundance in leaves by a negative regulator OsUbDKγ4 is integral to the role of OsFTIP1 in mediating RFT1 transport in rice and provides key evidence for a conserved role of FTIP1-like MCTPs in mediating florigen transport in flowering plants.

PTRHD1 (C2orf79) mutations lead to autosomal-recessive intellectual disability and parkinsonism

INTRODUCTION

Atypical parkinsonism is a neurodegenerative disease that includes diverse neurological and psychiatric manifestations.

OBJECTIVES

We aimed to identify the disease-cauisng mutations in a consanguineous family featuring intellectual disability and parkinsonism.

METHODS

Full phenotypic characterization, followed by genome-wide single-nucleotide polymorphism genotyping and whole-genome sequencing, was carried out in all available family members.

RESULTS

The chromosome, 2p23.3, was identified as the disease-associated locus, and a homozygous PTRHD1 mutation (c.157C>T) was then established as the disease-causing mutation. The pathogenicity of this PTRHD1 mutation was supported by its segregation with the disease status, its location in a functional domain of the encoding protein, as well as its absence in public databases and ethnicity-matched control chromosomes.

CONCLUSION

Given the role of 2p23 locus in patients with intellectual disability and the previously reported PTRHD1 mutation (c.155G>A) in patients with parkinsonism and cognitive dysfunction, we concluded that the PTRHD1 mutation identified in this study is likely to be responsible for the phenotypic features of the family under consideration. © 2016 International Parkinson and Movement Disorder Society.

The life cycle of the 26S proteasome: from birth, through regulation and function, and onto its death

The 26S proteasome is a large, ∼2.5 MDa, multi-catalytic ATP-dependent protease complex that serves as the degrading arm of the ubiquitin system, which is the major pathway for regulated degradation of cytosolic, nuclear and membrane proteins in all eukaryotic organisms.

Yeast Irc22 Is a Novel Dsk2-Interacting Protein that Is Involved in Salt Tolerance

The yeast ubiquitin-like and ubiquitin-associated protein Dsk2 is one of the ubiquitin receptors that function in the ubiquitin-proteasome pathway. We screened the Dsk2-interacting proteins in Saccharomyces cerevisiae by a two-hybrid assay and identified a novel Dsk2-interacting protein, Irc22, the gene locus of which has previously been described as YEL001C, but the function of which is unknown. IRC22/YEL001C encodes 225 amino acid residues with a calculated molecular weight of 25 kDa. The Irc22 protein was detected in yeast cells. IRC22 was a nonessential gene for yeast growth, and its homologs were found among ascomycetous yeasts. Irc22 interacted with Dsk2 in yeast cells, but not with Rad23 and Ddi1. Ubiquitin-dependent degradation was impaired mildly by over-expression or disruption of IRC22. Compared with the wild-type strain, dsk2Δ exhibited salt sensitivity while irc22Δ exhibited salt tolerance at high temperatures. The salt-tolerant phenotype that was observed in irc22Δ disappeared in the dsk2Δirc22Δ double disruptant, indicating that DSK2 is positively and IRC22 is negatively involved in salt stress tolerance. IRC22 disruption did not affect any responses to DNA damage and oxidative stress when comparing the irc22Δ and wild-type strains. Collectively, these results suggest that Dsk2 and Irc22 are involved in salt stress tolerance in yeast.

Premature translational termination products are rapidly degraded substrates for MHC class I presentation

Nearly thirty percent of all newly synthesized polypeptides are targeted for rapid proteasome-mediated degradation. These rapidly degraded polypeptides (RDPs) are a source of antigenic substrates for the MHC class I presentation pathway, allowing for immunosurveillance of newly synthesized proteins by cytotoxic T lymphocytes. Despite the recognized role of RDPs in MHC I presentation, it remains unclear what molecular characteristics distinguish RDPs from their more stable counterparts. It has been proposed that premature translational termination products may constitute a form of RDP; indeed, in prokaryotes translational drop-off products are normal by-products of protein synthesis and are subsequently rapidly degraded. To study the cellular fate of premature termination products, we used the antibiotic puromycin as a means to experimentally manipulate prematurely terminated polypeptide production in human cells. At low concentrations, puromycin enhanced flux into rapidly degraded polypeptide pools, with small polypeptides being markedly more labile then high molecular weight puromycin adducts. Immunoprecipitation experiments using anti-puromycin antisera demonstrated that the majority of peptidyl-puromycins are rapidly degraded in a proteasome-dependent manner. Low concentrations of puromycin increased the recovery of cell surface MHC I-peptide complexes, indicating that prematurely terminated polypeptides can be processed for presentation via the MHC I pathway. In the continued presence of puromycin, however, MHC I export to the cell surface was inhibited, coincident with the accumulation of polyubiquitinated proteins. The time- and dose-dependent effects of puromycin suggest that the pool of peptidyl-puromycin adducts differ in their targeting to various proteolytic pathways that, in turn, differ in the efficiency with which they access the MHC I presentation machinery. These studies highlight the diversity of cellular proteolytic pathways necessary for the metabolism and immunosurveillance of prematurely terminated polypeptides that are, by their nature, highly heterogeneous.

The ubiquitin proteasome system in neurodegenerative diseases: culprit, accomplice or victim?

A shared hallmark for many neurodegenerative disorders is the accumulation of toxic protein species which is assumed to be the cause for these diseases. Since the ubiquitin proteasome system (UPS) is the most important pathway for selective protein degradation it is likely that it is involved in the aetiology neurodegenerative disorders. Indeed, impairment of the UPS has been reported to occur during neurodegeneration. Although accumulation of toxic protein species (amyloid β) are in turn known to impair the UPS the relationship is not necessarily causal. We provide an overview of the most recent insights in the roles the UPS plays in protein degradation and other processes. Additionally, we discuss the role of the UPS in clearance of the toxic proteins known to accumulate in the hallmarks of neurodegenerative diseases. The present paper will focus on critically reviewing the involvement of the UPS in specific neurodegenerative diseases and will discuss if UPS impairment is a cause, a consequence or both of the disease.

The defective proteasome but not substrate recognition function is responsible for the null phenotypes of the Arabidopsis proteasome subunit RPN10

Ubiquitylated substrate recognition during ubiquitin/proteasome-mediated proteolysis (UPP) is mediated directly by the proteasome subunits RPN10 and RPN13 and indirectly by ubiquitin-like (UBL) and ubiquitin-associated (UBA) domain-containing factors. To dissect the complexity and functional roles of UPP substrate recognition in Arabidopsis thaliana, potential UPP substrate receptors were characterized. RPN10 and members of the UBL-UBA-containing RAD23 and DSK2 families displayed strong affinities for Lys-48-linked ubiquitin chains (the major UPP signals), indicating that they are involved in ubiquitylated substrate recognition. Additionally, RPN10 uses distinct interfaces as primary proteasomal docking sites for RAD23s and DSK2s. Analyses of T-DNA insertion knockout or RNA interference knockdown mutants of potential UPP ubiquitin receptors, including RPN10, RPN13, RAD23a-d, DSK2a-b, DDI1, and NUB1, demonstrated that only the RPN10 mutant gave clear phenotypes. The null rpn10-2 showed decreased double-capped proteasomes, increased 20S core complexes, and pleiotropic vegetative and reproductive growth phenotypes. Surprisingly, the observed rpn10-2 phenotypes were rescued by a RPN10 variant defective in substrate recognition, indicating that the defectiveness of RPN10 in proteasome but not substrate recognition function is responsible for the null phenotypes. Our results suggest that redundant recognition pathways likely are used in Arabidopsis to target ubiquitylated substrates for proteasomal degradation and that their specific roles in vivo require further examination.

Proteasomal recognition of ubiquitylated substrates

Ubiquitin/26S proteasome-mediated proteolysis controls the half-life of numerous critical regulatory proteins and is an intimate regulatory component for nearly all aspects of cellular processes. In addition to ubiquitin conjugation, an additional level of substrate specificity is regulated at the step of proteasomal recognition of ubiquitylated substrates, which serves as an important mechanistic and regulatory component to connect the substrate from the conjugation machinery to the 26S proteasome. In this review, we discuss current knowledge and future challenges relevant to understanding the mechanism, regulation, functions and substrate specificity of proteasomal recognition mediated by a multitude of ubiquitin receptors. The mechanistic details of major recognition pathways for ubiquitylated substrates are clearly divergent within and across species, which implies functional differentiation.

Tubulin chaperone E binds microtubules and proteasomes and protects against misfolded protein stress

Mutation of tubulin chaperone E (TBCE) underlies hypoparathyroidism, retardation, and dysmorphism (HRD) syndrome with defective microtubule (MT) cytoskeleton. TBCE/yeast Pac2 comprises CAP-Gly, LRR (leucine-rich region), and UbL (ubiquitin-like) domains. TBCE folds alpha-tubulin and promotes alpha/beta dimerization. We show that Pac2 functions in MT dynamics: the CAP-Gly domain binds alpha-tubulin and MTs, and functions in suppression of benomyl sensitivity of pac2Delta mutants. Pac2 binds proteasomes: the LRR binds Rpn1, and the UbL binds Rpn10; the latter interaction mediates Pac2 turnover. The UbL also binds the Skp1-Cdc53-F-box (SCF) ubiquitin ligase complex; these competing interactions for the UbL may impact on MT dynamics. pac2Delta mutants are sensitive to misfolded protein stress. This is suppressed by ectopic PAC2 with both the CAP-Gly and UbL domains being essential. We propose a novel role for Pac2 in the misfolded protein stress response based on its ability to interact with both the MT cytoskeleton and the proteasomes.

The yeast E4 ubiquitin ligase Ufd2 interacts with the ubiquitin-like domains of Rad23 and Dsk2 via a novel and distinct ubiquitin-like binding domain

Proteins containing ubiquitin-like (UBL) and ubiquitin-associated (UBA) domains interact with various binding partners and function as hubs during ubiquitin-mediated protein degradation. A common interaction of the budding yeast UBL-UBA proteins Rad23 and Dsk2 with the E4 ubiquitin ligase Ufd2 has been described in endoplasmic reticulum-associated degradation among other pathways. The UBL domains of Rad23 and Dsk2 play a prominent role in this process by interacting with Ufd2 and different subunits of the 26 S proteasome. Here, we report crystal structures of Ufd2 in complex with the UBL domains of Rad23 and Dsk2. The N-terminal UBL-interacting region of Ufd2 exhibits a unique sequence pattern, which is distinct from any known ubiquitin- or UBL-binding domain identified so far. Residue-specific differences exist in the interactions of these UBL domains with Ufd2, which are coupled to subtle differences in their binding affinities. The molecular details of their differential interactions point to a role for adaptive evolution in shaping these interfaces.

Together, Rpn10 and Dsk2 can serve as a polyubiquitin chain-length sensor

As a signal for substrate targeting, polyubiquitin meets various layers of receptors upstream to the 26S proteasome. We obtained structural information on two receptors, Rpn10 and Dsk2, alone and in complex with (poly)ubiquitin or with each other. A hierarchy of affinities emerges with Dsk2 binding monoubiquitin tighter than Rpn10 does, whereas Rpn10 prefers the ubiquitin-like domain of Dsk2 to monoubiquitin, with increasing affinities for longer polyubiquitin chains. We demonstrated the formation of ternary complexes of both receptors simultaneously with (poly)ubiquitin and found that, depending on the ubiquitin chain length, the orientation of the resulting complex is entirely different, providing for alternate signals. Dynamic rearrangement provides a chain-length sensor, possibly explaining how accessibility of Dsk2 to the proteasome is limited unless it carries a properly tagged cargo. We propose a mechanism for a malleable ubiquitin signal that depends both on chain length and combination of receptors to produce tetraubiquitin as an efficient signal threshold.

Recognition and processing of ubiquitin-protein conjugates by the proteasome

The proteasome is an intricate molecular machine, which serves to degrade proteins following their conjugation to ubiquitin. Substrates dock onto the proteasome at its 19-subunit regulatory particle via a diverse set of ubiquitin receptors and are then translocated into an internal chamber within the 28-subunit proteolytic core particle (CP), where they are hydrolyzed. Substrate is threaded into the CP through a narrow gated channel, and thus translocation requires unfolding of the substrate. Six distinct ATPases in the regulatory particle appear to form a ring complex and to drive unfolding as well as translocation. ATP-dependent, degradation-coupled deubiquitination of the substrate is required both for efficient substrate degradation and for preventing the degradation of the ubiquitin tag. However, the proteasome also contains deubiquitinating enzymes (DUBs) that can remove ubiquitin before substrate degradation initiates, thus allowing some substrates to dissociate from the proteasome and escape degradation. Here we examine the key elements of this molecular machine and how they cooperate in the processing of proteolytic substrates.

Ubiquitin-like and ubiquitin-associated domain proteins: significance in proteasomal degradation

The ubiquitin-proteasome pathway of protein degradation is one of the major mechanisms that are involved in the maintenance of the proper levels of cellular proteins. The regulation of proteasomal degradation thus ensures proper cell functions. The family of proteins containing ubiquitin-like (UbL) and ubiquitin-associated (UBA) domains has been implicated in proteasomal degradation. UbL-UBA domain containing proteins associate with substrates destined for degradation as well as with subunits of the proteasome, thus regulating the proper turnover of proteins.

The ubiquitin receptor Rad23: at the crossroads of nucleotide excision repair and proteasomal degradation

A protein that exemplifies the intimate link between the ubiquitin/proteasome system (UPS) and DNA repair is the yeast nucleotide excision repair (NER) protein Rad23 and its human orthologs hHR23A and hHR23B. Rad23, which was originally identified as an important factor involved in the recognition of DNA lesions, also plays a central role in targeting ubiquitylated proteins for proteasomal degradation, an activity that it shares with other ubiquitin receptors like Dsk2 and Ddi1. Although the finding that Rad23 serves as a ubiquitin receptor explains to a large extent its importance in proteasomal degradation, the precise mode of action of Rad23 in NER and the possible link with the UPS is less clear. In this review, we discuss our present knowledge on the functions of Rad23 in protein degradation and DNA repair and speculate on the importance of the dual roles of Rad23 for the cell's ability to cope with stress conditions.

1H, 13C, and 15N resonance assignment of the ubiquitin-like domain from Dsk2p

The ubiquitin-like domain (UBL) of yeast protein Dsk2p is widely believed to recognize and bind to ubiquitin receptors on the proteasome and, as part of Dsk2p, to bridge polyubiquitinated substrates and proteasomal degradation machinery. Here we report NMR resonance assignment for (1)H, (15)N, and (13)C nuclei in the backbone and side chains of the UBL domain of Dsk2p. This assignment will aid in NMR studies focused on understanding of Dsk2's interactions with proteasomal receptors and its role as a polyubiquitin shuttle in the ubiquitin-dependent proteasomal degradation as well as other cellular pathways.

Extraproteasomal Rpn10 restricts access of the polyubiquitin-binding protein Dsk2 to proteasome

Polyubiquitin is a diverse signal both in terms of chain length and linkage type. Lysine 48-linked ubiquitin is essential for marking targets for proteasomal degradation, but the significance and relative abundance of different linkages remain ambiguous. Here we dissect the relationship of two proteasome-associated polyubiquitin-binding proteins, Rpn10 and Dsk2, and demonstrate how Rpn10 filters Dsk2 interactions, maintaining proper function of the ubiquitin-proteasome system. Using quantitative mass spectrometry of ubiquitin, we found that in S. cerevisiae under normal growth conditions the majority of conjugated ubiquitin was linked via lysine 48 and lysine 63. In contrast, upon DSK2 induction, conjugates accumulated primarily in the form of lysine 48 linkages correlating with impaired proteolysis and cytotoxicity. By restricting Dsk2 access to the proteasome, extraproteasomal Rpn10 was essential for alleviating the cellular stress associated with Dsk2. This work highlights the importance of polyubiquitin shuttles such as Rpn10 and Dsk2 in controlling the ubiquitin landscape.

Ubiquitin receptors and ERAD: a network of pathways to the proteasome

The elimination of misfolded proteins, known as protein quality control, is an essential cellular process. Removal of misfolded proteins from the secretory pathway depends on their recognition in the endoplasmic reticulum (ER) followed by their retrograde transport into the cytosol for degradation. The AAA-ATPase Cdc48/p97 facilitates the translocation of misfolded ER-proteins into the cytosol. Cdc48/p97 can dock onto the ER-membrane via direct interaction with ER-membrane proteins and/or indirectly via its substrate-recruiting cofactors, which interact with the ubiquitylated substrates at the membrane. This tight interaction in conjunction with the conformational changes induced upon ATP hydrolysis within Cdc48/p97 is thought to provide the driving force for the translocation reaction. Subsequently, a series of protein-protein interactions between the Cdc48/p97 complex, its cofactors, and the ubiquitylated substrates is instrumental for the proper delivery of the ER substrates to the proteasome. These protein-protein interactions are governed mainly by ubiquitin-fold and ubiquitin-binding domains.