RESUMEN
Proteolysis-targeting chimeras (PROTACs) are bifunctional molecules that bind and recruit an E3 ubiquitin ligase to a targeted protein of interest, often through the utilization of a small molecule inhibitor. To expand the possible range of kinase targets that can be degraded by PROTACs, we sought to develop a PROTAC utilizing a hydrocarbon-stapled peptide as the targeting agent to bind the surface of a target protein of interest. In this study, we describe the development of a proteolysis-targeting chimera, dubbed Stapled Inhibitor Peptide - PROTAC or StIP-TAC, linking a hydrocarbon-stapled peptide with an E3 ligase ligand for targeted degradation of Protein Kinase A (PKA). This StIP-TAC molecule stimulated E3-mediated protein degradation of PKA, and this effect could be reversed by the addition of the proteasomal inhibitor MG-132. Further, StIP-TAC treatment led to a significant reduction in PKA substrate phosphorylation. Since many protein targets of interest lack structural features that make them amenable to small molecule targeting, development of StIP-TACs may broaden the potential range of protein targets using a PROTAC-mediated proteasomal degradation approach.
RESUMEN
Fibrolamellar hepatocellular carcinoma (FLC) is a rare liver cancer that is driven by the fusion of DNAJB1 and PRKACA, the catalytic subunit of protein kinase A (PKA). PKA activity is controlled through regulatory proteins that both inhibit catalytic activity and control localization, and an excess of regulatory subunits ensures PRKACA activity is inhibited. Here, we found an increase in the ratio of catalytic to regulatory units in FLC patient tumors driven by DNAJB1::PRKACA using mass spectrometry, biochemistry, and immunofluorescence, with increased nuclear localization of the kinase. Overexpression of DNAJB1::PRKACA, ATP1B1::PRKACA, or PRKACA, but not catalytically inactive kinase, caused similar transcriptomic changes in primary human hepatocytes, recapitulating the changes observed in FLC. Consistently, tumors in patients missing a regulatory subunit or harboring an ATP1B1::PRKACA fusion were indistinguishable from FLC based on the histopathological, transcriptomic, and drug-response profiles. Together, these findings indicate that the DNAJB1 domain of DNAJB1::PRKACA is not required for FLC. Instead, changes in PKA activity and localization determine the FLC phenotype. Significance: Alterations leading to unconstrained protein kinase A signaling, regardless of the presence or absence of PRKACA fusions, drive the phenotypes of fibrolamellar hepatocellular carcinoma, reshaping understanding of the pathogenesis of this rare liver cancer.
Asunto(s)
Carcinoma Hepatocelular , Subunidades Catalíticas de Proteína Quinasa Dependientes de AMP Cíclico , Proteínas del Choque Térmico HSP40 , Neoplasias Hepáticas , Humanos , Proteínas del Choque Térmico HSP40/metabolismo , Proteínas del Choque Térmico HSP40/genética , Carcinoma Hepatocelular/patología , Carcinoma Hepatocelular/genética , Carcinoma Hepatocelular/metabolismo , Neoplasias Hepáticas/patología , Neoplasias Hepáticas/genética , Neoplasias Hepáticas/metabolismo , Subunidades Catalíticas de Proteína Quinasa Dependientes de AMP Cíclico/metabolismo , Subunidades Catalíticas de Proteína Quinasa Dependientes de AMP Cíclico/genética , Proteínas Quinasas Dependientes de AMP Cíclico/metabolismo , Proteínas Quinasas Dependientes de AMP Cíclico/genética , Proteínas de Fusión Oncogénica/genética , Proteínas de Fusión Oncogénica/metabolismo , Regulación Neoplásica de la Expresión Génica , ATPasa Intercambiadora de Sodio-PotasioRESUMEN
Fibrolamellar hepatocellular carcinoma (FLC) is a usually lethal primary liver cancer driven by a somatic dysregulation of protein kinase A. We show that the proteome of FLC tumors is distinct from that of adjacent nontransformed tissue. These changes can account for some of the cell biological and pathological alterations in FLC cells, including their drug sensitivity and glycolysis. Hyperammonemic encephalopathy is a recurrent problem in these patients, and established treatments based on the assumption of liver failure are unsuccessful. We show that many of the enzymes that produce ammonia are increased and those that consume ammonia are decreased. We also demonstrate that the metabolites of these enzymes change as expected. Thus, hyperammonemic encephalopathy in FLC may require alternative therapeutics.
Asunto(s)
Encefalopatías , Carcinoma Hepatocelular , Neoplasias Hepáticas , Síndromes de Neurotoxicidad , Humanos , Carcinoma Hepatocelular/genética , Carcinoma Hepatocelular/patología , Neoplasias Hepáticas/genética , Neoplasias Hepáticas/patología , Proteoma , AmoníacoRESUMEN
AAA+ (ATPases Associated with diverse cellular Activities) proteases unfold substrate proteins by pulling the substrate polypeptide through a narrow pore. To overcome the barrier to unfolding, substrates may require extended association with the ATPase. Failed unfolding attempts can lead to a slip of grip, which may result in substrate dissociation, but how substrate sequence affects slippage is unresolved. Here, we measured single molecule dwell time using total internal reflection fluorescence microscopy, scoring time-dependent dissociation of engaged substrates from bacterial AAA+ ATPase unfoldase/translocase ClpX. Substrates comprising a stable domain resistant to unfolding and a C-terminal unstructured tail, tagged with a degron for initiating translocase insertion, were used to determine dwell time in relation to tail length and composition. We found greater tail length promoted substrate retention during futile unfolding. Additionally, we tested two tail compositions known to frustrate unfolding. A poly-glycine tract (polyG) promoted release, but only when adjacent to the folded domain, whereas glycine-alanine repeats (GAr) did not promote release. A high complexity motif containing polar and charged residues also promoted release. We further investigated the impact of these and related motifs on substrate degradation rates and ATP consumption, using the unfoldase-protease complex ClpXP. Here, substrate domain stability modulates the effects of substrate tail sequences. polyG and GAr are both inhibitory for unfolding, but act in different ways. GAr motifs only negatively affected degradation of highly stable substrates, which is accompanied by reduced ClpXP ATPase activity. Together, our results specify substrate characteristics that affect unfolding and degradation by ClpXP.
Asunto(s)
ATPasas Asociadas con Actividades Celulares Diversas , Adenosina Trifosfatasas , Endopeptidasa Clp , Imagen Individual de Molécula , Adenosina Trifosfatasas/genética , Adenosina Trifosfatasas/metabolismo , Adenosina Trifosfato/metabolismo , ATPasas Asociadas con Actividades Celulares Diversas/metabolismo , Endopeptidasa Clp/metabolismo , Glicina , Chaperonas Moleculares/metabolismo , Especificidad por SustratoRESUMEN
Fibrolamellar hepatocellular carcinoma (FLC) is a rare and often lethal liver cancer with no proven effective systemic therapy. Inhibition of the antiapoptotic protein BCL-XL was found to synergize with a variety of systemic therapies in vitro using cells dissociated from patient-derived xenografts (PDX) of FLC or cells dissociated directly from surgical patient resections. As BCL-XL is physiologically expressed in platelets, prior efforts to leverage this vulnerability in other cancers have been hampered by severe thrombocytopenia. To overcome this toxicity, we treated FLC models with DT2216, a proteolysis targeting chimera (PROTAC) that directs BCL-XL for degradation via the von Hippel-Lindau (VHL) E3 ligase, which is minimally expressed in platelets. The combination of irinotecan and DT2216 in vitro on cells directly acquired from patients or in vivo using several xenografts derived from patients with FLC demonstrated remarkable synergy and at clinically achievable doses not associated with significant thrombocytopenia.
Asunto(s)
Carcinoma Hepatocelular , Neoplasias Hepáticas , Trombocitopenia , Carcinoma Hepatocelular/tratamiento farmacológico , Humanos , Neoplasias Hepáticas/tratamiento farmacológico , PiperazinasRESUMEN
Allostery-regulation at distant sites is a key concept in biology. The proteasome exhibits multiple forms of allosteric regulation. This regulatory communication can span a distance exceeding 100 Ångstroms and can modulate interactions between the two major proteasome modules: its core particle and regulatory complexes. Allostery can further influence the assembly of the core particle with regulatory particles. In this focused review, known and postulated interactions between these proteasome modules are described. Allostery may explain how cells build and maintain diverse populations of proteasome assemblies and can provide opportunities for therapeutic interventions.
Asunto(s)
Complejo de la Endopetidasa Proteasomal , Regulación Alostérica , Complejo de la Endopetidasa Proteasomal/metabolismoRESUMEN
To repurpose therapeutics for fibrolamellar carcinoma (FLC), we developed and validated patient-derived xenografts (PDX) from surgical resections. Most agents used clinically and inhibitors of oncogenes overexpressed in FLC showed little efficacy on PDX. A high-throughput functional drug screen found primary and metastatic FLC were vulnerable to clinically available inhibitors of TOPO1 and HDAC and to napabucasin. Napabucasin's efficacy was mediated through reactive oxygen species and inhibition of translation initiation, and specific inhibition of eIF4A was effective. The sensitivity of each PDX line inversely correlated with expression of the antiapoptotic protein Bcl-xL, and inhibition of Bcl-xL synergized with other drugs. Screening directly on cells dissociated from patient resections validated these results. This demonstrates that a direct functional screen on patient tumors provides therapeutically informative data within a clinically useful time frame. Identifying these novel therapeutic targets and combination therapies is an urgent need, as effective therapeutics for FLC are currently unavailable. SIGNIFICANCE: Therapeutics informed by genomics have not yielded effective therapies for FLC. A functional screen identified TOPO1, HDAC inhibitors, and napabucasin as efficacious and synergistic with inhibition of Bcl-xL. Validation on cells dissociated directly from patient tumors demonstrates the ability for functional precision medicine in a solid tumor.This article is highlighted in the In This Issue feature, p. 2355.
Asunto(s)
Carcinoma Hepatocelular/tratamiento farmacológico , Regulación Neoplásica de la Expresión Génica , Neoplasias Hepáticas/tratamiento farmacológico , Ensayos Antitumor por Modelo de Xenoinjerto , Compuestos de Anilina/uso terapéutico , Animales , Antineoplásicos/uso terapéutico , Benzofuranos/uso terapéutico , Carcinoma Hepatocelular/genética , Femenino , Humanos , Neoplasias Hepáticas/genética , Masculino , Ratones , Naftoquinonas/uso terapéutico , Sulfonamidas/uso terapéuticoRESUMEN
Proteasomal machinery performs essential regulated protein degradation in eukaryotes. Classic proteasomes are symmetric, with a regulatory ATPase docked at each end of the cylindrical 20S. Asymmetric complexes are also present in cells, either with a single ATPase or with an ATPase and non-ATPase at two opposite ends. The mechanism that populates these different proteasomal complexes is unknown. Using archaea homologs, we construct asymmetric forms of proteasomes. We demonstrate that the gate conformation of the two opposite ends of 20S are coupled: binding one ATPase opens a gate locally, and also opens the opposite gate allosterically. Such allosteric coupling leads to cooperative binding of proteasomal ATPases to 20S and promotes formation of proteasomes symmetrically configured with two identical ATPases. It may also promote formation of asymmetric complexes with an ATPase and a non-ATPase at opposite ends. We propose that in eukaryotes a similar mechanism regulates the composition of the proteasomal population.
Asunto(s)
Archaea/metabolismo , Complejo de la Endopetidasa Proteasomal/química , Complejo de la Endopetidasa Proteasomal/metabolismo , Adenosina Trifosfatasas/metabolismo , Archaea/genética , Microscopía por Crioelectrón , Cinética , Modelos Moleculares , Complejo de la Endopetidasa Proteasomal/genética , Conformación Proteica , Thermoplasma/genética , Thermoplasma/metabolismoRESUMEN
Most proteasome substrates are marked for degradation by ubiquitin conjugation, but some are targeted by other means. The properties of these exceptional cases provide insights into the general requirements for proteasomal degradation. Here the focus is on three ubiquitin-independent substrates that have been the subject of detailed study. These are Rpn4, a transcriptional regulator of proteasome homeostasis, thymidylate synthase, an enzyme required for production of DNA precursors and ornithine decarboxylase, the initial enzyme committed to polyamine biosynthesis. It can be inferred from these cases that proteasome association and the presence of an unstructured region are the sole prerequisites for degradation. Based on that inference, artificial substrates have been designed to test the proteasome's capacity for substrate processing and its limitations. Ubiquitin-independent substrates may in some cases be a remnant of the pre-ubiquitome world, but in other cases could provide optimized regulatory solutions. This article is part of a Special Issue entitled: Ubiquitin-Proteasome System. Guest Editors: Thomas Sommer and Dieter H. Wolf.
Asunto(s)
Complejo de la Endopetidasa Proteasomal/fisiología , Proteolisis , Animales , Proteínas de Unión al ADN/fisiología , Humanos , Ornitina Descarboxilasa/fisiología , Estructura Terciaria de Proteína , Desplegamiento Proteico , Proteínas de Saccharomyces cerevisiae/fisiología , Timidilato Sintasa/fisiología , Factores de Transcripción/fisiología , Ubiquitina/fisiologíaRESUMEN
The proteasomal ATPase ring, comprising Rpt1-Rpt6, associates with the heptameric α-ring of the proteasome core particle (CP) in the mature proteasome, with the Rpt carboxy-terminal tails inserting into pockets of the α-ring. Rpt ring assembly is mediated by four chaperones, each binding a distinct Rpt subunit. Here we report that the base subassembly of the Saccharomyces cerevisiae proteasome, which includes the Rpt ring, forms a high-affinity complex with the CP. This complex is subject to active dissociation by the chaperones Hsm3, Nas6 and Rpn14. Chaperone-mediated dissociation was abrogated by a non-hydrolysable ATP analogue, indicating that chaperone action is coupled to nucleotide hydrolysis by the Rpt ring. Unexpectedly, synthetic Rpt tail peptides bound α-pockets with poor specificity, except for Rpt6, which uniquely bound the α2/α3-pocket. Although the Rpt6 tail is not visualized within an α-pocket in mature proteasomes, it inserts into the α2/α3-pocket in the base-CP complex and is important for complex formation. Thus, the Rpt-CP interface is reconfigured when the lid complex joins the nascent proteasome to form the mature holoenzyme.
Asunto(s)
Chaperonas Moleculares/metabolismo , Complejo de la Endopetidasa Proteasomal/química , Complejo de la Endopetidasa Proteasomal/metabolismo , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/metabolismo , Adenosina Trifosfatasas/química , Adenosina Trifosfatasas/genética , Adenosina Trifosfatasas/metabolismo , Adenosina Trifosfato/metabolismo , Sitios de Unión , Proteínas Portadoras/metabolismo , Microscopía por Crioelectrón , Holoenzimas/química , Holoenzimas/metabolismo , Modelos Moleculares , Complejo de la Endopetidasa Proteasomal/genética , Conformación Proteica , Proteínas Recombinantes de Fusión/química , Proteínas Recombinantes de Fusión/genética , Proteínas Recombinantes de Fusión/metabolismo , Saccharomyces cerevisiae/enzimología , Saccharomyces cerevisiae/genética , Saccharomyces cerevisiae/crecimiento & desarrollo , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/genéticaRESUMEN
BACKGROUND: ATP-dependent proteases translocate and unfold their substrates. RESULTS: A human virus sequence with only Gly and Ala residues causes similar dysfunctions of eukaryotic and prokaryotic protease motors: unfolding failure. CONCLUSION: Sequences with amino acids of simple shape and small size impair unfolding of contiguous stable domains. SIGNIFICANCE: Compartmented ATP-dependent proteases of diverse origin share conserved principles of interaction between translocase/effector and substrate/recipient. ATP-dependent proteases engage, translocate, and unfold substrate proteins. A sequence with only Gly and Ala residues (glycine-alanine repeat; GAr) encoded by the Epstein-Barr virus of humans inhibits eukaryotic proteasome activity. It causes the ATPase translocase to slip on its protein track, stalling unfolding and interrupting degradation. The bacterial protease ClpXP is structurally simpler than the proteasome but has related elements: a regulatory ATPase complex (ClpX) and associated proteolytic chamber (ClpP). In this study, GAr sequences were found to impair ClpXP function much as in proteasomes. Stalling depended on interaction between a GAr and a suitably spaced and positioned folded domain resistant to mechanical unfolding. Persistent unfolding failure results in the interruption of degradation and the production of partial degradation products that include the resistant domain. The capacity of various sequences to cause unfolding failure was investigated. Among those tested, a GAr was most effective, implying that viral selection had optimized processivity failure. More generally, amino acids of simple shape and small size promoted unfolding failure. The ClpX ATPase is a homohexamer. Partial degradation products could exit the complex through transient gaps between the ClpX monomers or, alternatively, by backing out. Production of intermediates by diverse topological forms of the hexamer was shown to be similar, excluding lateral escape. In principle, a GAr could interrupt degradation because 1) the translocase thrusts forward less effectively or because 2) the translocase retains substrate less well when resetting between forward strokes. Kinetic analysis showed that the predominant effect was through the second of these mechanisms.
Asunto(s)
Adenosina Trifosfatasas/química , Endopeptidasa Clp/química , Proteínas de Escherichia coli/química , Escherichia coli/enzimología , Chaperonas Moleculares/química , ATPasas Asociadas con Actividades Celulares Diversas , Secuencia de Aminoácidos , Sustitución de Aminoácidos , Conectina , Humanos , Interacciones Hidrofóbicas e Hidrofílicas , Cinética , Modelos Moleculares , Datos de Secuencia Molecular , Proteínas Musculares/química , Proteínas Musculares/genética , Fragmentos de Péptidos/química , Proteínas Quinasas/química , Proteínas Quinasas/genética , Estabilidad Proteica , Estructura Terciaria de Proteína , Transporte de Proteínas , Desplegamiento Proteico , Proteolisis , Secuencias Repetitivas de Aminoácido , Tetrahidrofolato Deshidrogenasa/químicaRESUMEN
Degradation by proteasomes involves coupled translocation and unfolding of its protein substrates. Six distinct but paralogous proteasome ATPase proteins, Rpt1 to -6, form a heterohexameric ring that acts on substrates. An axially positioned loop (Ar-Φ loop) moves in concert with ATP hydrolysis, engages substrate, and propels it into a proteolytic chamber. The aromatic (Ar) residue of the Ar-Φ loop in all six Rpts of S. cerevisiae is tyrosine; this amino acid is thought to have important functional contacts with substrate. Six yeast strains were constructed and characterized in which Tyr was individually mutated to Ala. The mutant cells were viable and had distinct phenotypes. rpt3, rpt4, and rpt5 Tyr/Ala mutants, which cluster on one side of the ATPase hexamer, were substantially impaired in their capacity to degrade substrates. In contrast, rpt1, rpt2, and rpt6 mutants equaled or exceeded wild type in degradation activity. However, rpt1 and rpt6 mutants had defects that limited cell growth or viability under conditions that stressed the ubiquitin proteasome system. In contrast, the rpt3 mutant grew faster than wild type and to a smaller size, a defect that has previously been associated with misregulation of G1 cyclins. This rpt3 phenotype probably results from altered degradation of cell cycle regulatory proteins. Finally, mutation of five of the Rpt subunits increased proteasome ATPase activity, implying bidirectional coupling between the Ar-Φ loop and the ATP hydrolysis site. The present observations assign specific functions to individual Rpt proteins and provide insights into the diverse roles of the axial loops of individual proteasome ATPases.
Asunto(s)
Complejo de la Endopetidasa Proteasomal/metabolismo , Adenosina Trifosfatasas/metabolismo , Cinética , Respuesta de Proteína DesplegadaRESUMEN
Protein degradation by eukaryotic proteasomes is a multi-step process involving substrate recognition, ATP-dependent unfolding, translocation into the proteolytic core particle, and finally proteolysis. To date, most investigations of proteasome function have focused on the first and the last steps in this process. Here we examine the relationship between the stability of a folded protein domain and its degradation rate. Test proteins were targeted to the proteasome independently of ubiquitination by directly tethering them to the protease. Degradation kinetics were compared for test protein pairs whose stability was altered by either point mutation or ligand binding, but were otherwise identical. In both intact cells and in reactions using purified proteasomes and substrates, increased substrate stability led to an increase in substrate turnover time. The steady-state time for degradation ranged from â¼5 min (dihydrofolate reductase) to 40 min (I27 domain of titin). ATP turnover was 110/min./proteasome, and was not markedly changed by substrate. Proteasomes engage tightly folded substrates in multiple iterative rounds of ATP hydrolysis, a process that can be rate-limiting for degradation.
Asunto(s)
Complejo de la Endopetidasa Proteasomal/metabolismo , Pliegue de Proteína , Proteínas de Saccharomyces cerevisiae/metabolismo , Saccharomyces cerevisiae/enzimología , Adenosina Trifosfato/genética , Adenosina Trifosfato/metabolismo , Complejo de la Endopetidasa Proteasomal/genética , Estabilidad Proteica , Estructura Terciaria de Proteína , Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/genéticaRESUMEN
In this issue of Molecular Cell, Tomko et al. (2010) establish that the six distinct ATPase subunits of the eukaryotic proteasome form a heterohexameric ring and resolve how the subunits are arranged within the ring.
RESUMEN
The great majority of proteasome substrates are marked for degradation by the attachment of polyubiquitin chains. Ornithine decarboxylase is degraded by the proteasome in the absence of this modification. We previously showed that this mechanism of degradation was conserved in eukaryotic cells. Here we use a reporter destabilized by mouse ornithine decarboxylase to screen non-essential Saccharomyces cerevisiae deletion mutants. We identified novel mutants that affect both ubiquitin-dependent and -independent proteasome degradation pathways. YLR021W (IRC25/POC3) and YPL144W (POC4) encode interacting proteins that function in proteasome assembly, with putative homologues widespread among eukaryotes. Several additional mutants suffered from defects in proteasome-mediated proteolysis. These included mutants in the urmylation pathway of protein modification (but not the Urm1 modifier itself) and the Reg1 regulatory subunit of protein phosphatase 1. Finally, we noted increased rates of ornithine decarboxylase turnover in an rpn10Delta mutant in which the degradation of certain ubiquitinated substrates is impaired. Together, these results highlight the utility of a ubiquitin-independent degron in uncovering novel factors affecting general and substrate-specific proteasome function.
Asunto(s)
Ornitina Descarboxilasa/metabolismo , Complejo de la Endopetidasa Proteasomal/fisiología , Saccharomyces cerevisiae/genética , Eliminación de Secuencia , Ubiquitina/metabolismo , Secuencia de Aminoácidos , Animales , Ratones , Chaperonas Moleculares/aislamiento & purificación , Datos de Secuencia Molecular , Saccharomyces cerevisiae/fisiología , Proteínas de Saccharomyces cerevisiae/genética , Proteínas de Saccharomyces cerevisiae/aislamiento & purificación , Proteínas de Saccharomyces cerevisiae/metabolismo , Alineación de Secuencia , Especificidad por SustratoRESUMEN
Mouse ODC (ornithine decarboxylase) is quickly degraded by the 26S proteasome in mammalian and fungal cells. Its degradation is independent of ubiquitin but requires a degradation signal composed of residues 425-461 at the ODC C-terminus, cODC (the last 37 amino acids of the ODC C-terminus). Mutational analysis of cODC revealed the presence of two essential elements in the degradation signal. The first consists of cysteine and alanine at residues 441 and 442 respectively. The second element is the C-terminus distal to residue 442; it has little or no sequence specificity, but is intolerant of insertions or deletions that alter its span. Reducing conditions, which preclude all well-characterized chemical reactions of the Cys(441) thiol, are essential for in vitro degradation. These experiments imply that the degradative function of Cys(441) does not involve its participation in chemical reaction; it, instead, functions within a structural element for recognition by the 26S proteasome.
Asunto(s)
Ornitina Descarboxilasa/metabolismo , Complejo de la Endopetidasa Proteasomal/metabolismo , Ubiquitina/metabolismo , Sustitución de Aminoácidos , Animales , Cinética , Ratones , Mutagénesis Sitio-Dirigida , Ornitina Descarboxilasa/genética , Polimorfismo de Nucleótido Simple , Ratas , Proteínas Recombinantes/metabolismo , Reticulocitos/enzimología , Saccharomyces cerevisiae/enzimologíaRESUMEN
The mechanism of protein quality control and elimination of misfolded proteins in the cytoplasm is poorly understood. We studied the involvement of cytoplasmic factors required for degradation of two endoplasmic reticulum (ER)-import-defective mutated derivatives of carboxypeptidase yscY (DeltassCPY* and DeltassCPY*-GFP) and also examined the requirements for degradation of the corresponding wild-type enzyme made ER-import incompetent by removal of its signal sequence (DeltassCPY). All these protein species are rapidly degraded via the ubiquitin-proteasome system. Degradation requires the ubiquitin-conjugating enzymes Ubc4p and Ubc5p, the cytoplasmic Hsp70 Ssa chaperone machinery, and the Hsp70 cochaperone Ydj1p. Neither the Hsp90 chaperones nor Hsp104 or the small heat-shock proteins Hsp26 and Hsp42 are involved in the degradation process. Elimination of a GFP fusion (GFP-cODC), containing the C-terminal 37 amino acids of ornithine decarboxylase (cODC) directing this enzyme to the proteasome, is independent of Ssa1p function. Fusion of DeltassCPY* to GFP-cODC to form DeltassCPY*-GFP-cODC reimposes a dependency on the Ssa1p chaperone for degradation. Evidently, the misfolded protein domain dictates the route of protein elimination. These data and our further results give evidence that the Ssa1p-Ydj1p machinery recognizes misfolded protein domains, keeps misfolded proteins soluble, solubilizes precipitated protein material, and escorts and delivers misfolded proteins in the ubiquitinated state to the proteasome for degradation.
Asunto(s)
Carboxipeptidasas/química , Carboxipeptidasas/metabolismo , Citoplasma/metabolismo , Retículo Endoplásmico/metabolismo , Proteínas HSP70 de Choque Térmico/metabolismo , Pliegue de Proteína , Procesamiento Proteico-Postraduccional , Proteínas de Saccharomyces cerevisiae/química , Proteínas de Saccharomyces cerevisiae/metabolismo , Adenosina Trifosfatasas/metabolismo , Catepsina A , Proteínas del Choque Térmico HSP40/metabolismo , Modelos Biológicos , Proteínas Mutantes/metabolismo , Complejo de la Endopetidasa Proteasomal/metabolismo , Transporte de Proteínas , Proteínas Recombinantes de Fusión/metabolismo , Saccharomyces cerevisiae/citología , Ubiquitina/metabolismoRESUMEN
To determine the minimum requirements for substrate recognition and processing by proteasomes, the functional elements of a ubiquitin-independent degradation tag were dissected. The 37-residue C-terminus of ornithine decarboxylase (cODC) is a native degron, which also functions when appended to diverse proteins. Mutating the cysteine 441 residue within cODC impaired its proteasome association in the context of ornithine decarboxylase and prevented the turnover of GFP-cODC in yeast cells. Degradation of GFP-cODC with C441 mutations was restored by providing an alternate proteasome association element via fusion to the Rpn10 proteasome subunit. However, Rpn10-GFP was stable, unless extended by cODC or other peptides of similar size. In vitro reconstitution experiments confirmed the requirement for both proteasome tethering and a loosely structured region. Therefore, cODC and degradation tags in general must serve two functions: proteasome association and a site, consisting of an extended peptide region, used for initiating insertion into the protease.