Effects of MTX-23, a Novel PROTAC of Androgen Receptor Splice Variant-7 and Androgen Receptor, on CRPC Resistant to Second-Line Antiandrogen Therapy

Although second-line antiandrogen therapy (SAT) is the standard of care in men with castration-resistant prostate cancer (CRPC), resistance inevitably occurs. One major proposed mechanism of resistance to SAT involves the emergence of androgen receptor (AR) splice variant-7, AR-V7. Recently, we developed MTX-23 using the principle of proteolysis targeting chimera (PROTAC) to target both AR-V7 and AR-full length (AR-FL). MTX-23 has been designed to simultaneously bind AR's DNA binding domain (DBD) and the Von Hippel-Lindau (VHL) E3 ubiquitin ligase. Immunoblots demonstrated that MTX-23's degradation concentration 50% (DC₅₀) for AR-V7 and AR-FL was 0.37 and 2 μmol/L, respectively. Further studies revealed that MTX-23 inhibited prostate cancer cellular proliferation and increased apoptosis only in androgen-responsive prostate cancer cells. The antiproliferative effect of MTX-23 was partially reversed when either AR-V7 or AR-FL was overexpressed and was completely abrogated when both were overexpressed. To assess the potential therapeutic value of MTX-23, we next generated 12 human prostate cancer cell lines that are resistant to the four FDA-approved SAT agents-abiraterone, enzalutamide, apalutamide, and darolutamide. When resistant cells were treated with MTX-23, decreased cellular proliferation and reduced tumor growth were observed both in vitro and in mice. These results collectively suggest that MTX-23 is a novel PROTAC small molecule that may be effective against SAT-resistant CRPC by degrading both AR-V7 and AR-FL.

The Cellular Location of Rad23, a Polyubiquitin Chain-Binding Protein, Plays a Key Role in Its Interaction with Substrates of the Proteasome

Well-studied structural motifs in Rad23 have been shown to bind polyubiquitin chains and the proteasome. These domains are predicted to enable Rad23 to transport polyubiquitylated (polyUb) substrates to the proteasome (Chen and Madura, 2002 [1]). The validation of this model, however, has been hindered by the lack of specific physiological substrates of Rad23. We report here that Rad23 can bind Ho-endonuclease (Ho-endo), a nuclear protein that initiates mating-type switching in Saccharomyces cerevisiae. We observed that the degradation of Ho-endo required export from the nucleus, in agreement with a previous report (Kaplun et al., 2003 [2]), and suggests that Rad23 can traffic proteins out of the nucleus. In agreement, the subcellular distribution of Rad23 is noticeably altered in genetic mutants that disrupt nucleocytoplasmic trafficking. Significantly, the location of Rad23 affected its binding to polyUb substrates. Mutations in nuclear export stabilized substrates, and caused accumulation in the nucleus. Importantly, Rad23 also accumulated in the nucleus in an export mutant, and bound to higher levels of polyUb proteins. In contrast, Rad23 is localized in the cytosol in rna1-1, a nucleocytoplasmic transport mutant, and it forms reduced binding to polyUb substrates. These and other studies indicate that substrates that are conjugated to polyubiquitin chains in the nucleus may rely on an export-dependent mechanism to be degraded by the proteasome. The evolutionary conservation of Rad23 and similar substrate-trafficking proteins predicts an important role for export in the turnover of nuclear proteins.

A role for Saccharomyces cerevisiae Centrin (Cdc31) in mitochondrial function and biogenesis

Centrins belong to a family of proteins containing calcium-binding EF-hand motifs that perform well-established roles in centrosome and spindle pole body (SPB) duplication. Yeast encodes a single Centrin protein (Cdc31) that binds components in the SPB. However, further studies revealed a role for Centrins in mRNA export, and interactions with contractile filaments and photoreceptors. In addition, human Centrin-2 can bind the DNA-lesion recognition factor XPC, and improve the efficiency of nucleotide excision repair. Similarly, we reported that yeast Cdc31 binds Rad4, a functional counterpart of the XPC DNA repair protein. We also found that Cdc31 is involved in the ubiquitin/proteasome system, and mutations interfere with intracellular protein turnover. In this report, we describe new findings that indicate a role for Cdc31 in the energy metabolism pathway. Cdc31 and cdc31 mutant proteins showed distinct interactions with proteins in energy metabolism, and mutants showed sensitivity to oxidative stress and poor growth on non-fermentable carbon. Significant alteration in mitochondrial morphology was also detected. Although it is unclear how Cdc31 contributes to so many unrelated mechanisms, we propose that by controlling SPB duplication Centrin proteins might link the cellular responses to DNA damage, oxidative load and proteotoxic stresses to growth control.

Catalytically Active Proteasomes Function Predominantly in the Cytosol

The ubiquitin/proteasome pathway is a well characterized system for degrading intracellular proteins, although many aspects remain poorly understood. There is, for instance, a conspicuous lack of understanding of the site(s) where nuclear proteins are degraded because the subcellular distribution of peptidase activity has not been investigated systematically. Although nuclear proteins could be degraded by importing proteasomes into the nucleus, it is also evident that some nuclear proteins are degraded only after export to cytosolic proteasomes. Proteasomes and substrates are mobile, and consequently, the sites of degradation might not be static. We sought to identify the location of proteasomes to provide more conclusive evidence on the sites of protein degradation. We report that catalytically active proteasomes exist almost exclusively in the cytosol. The resulting lack of nuclear peptidase activity suggests that little, if any, degradation occurs in the nucleus. These and other studies suggest that the export of proteolytic substrates could define an important regulatory step in the degradation of nuclear proteins by cytosolic proteasomes.

CD44 promotes multi-drug resistance by protecting P-glycoprotein from FBXO21-mediated ubiquitination

Here we demonstrate that a ubiquitin E3-ligase, FBXO21, targets the multidrug resistance transporter, ABCB1, also known as P-glycoprotein (P-gp), for proteasomal degradation. We also show that the Ser291-phosphorylated form of the multifunctional protein and stem cell marker, CD44, inhibits FBXO21-directed degradation of P-gp. Thus, CD44 increases P-gp mediated drug resistance and represents a potential therapeutic target in P-gp-positive cells.

Rad23 interaction with the proteasome is regulated by phosphorylation of its ubiquitin-like (UbL) domain

Rad23 was identified as a DNA repair protein, although a role in protein degradation has been described. The protein degradation function of Rad23 contributes to cell cycle progression, stress response, endoplasmic reticulum proteolysis, and DNA repair. Rad23 binds the proteasome through a UbL (ubiquitin-like) domain and contains UBA (ubiquitin-associated) motifs that bind multiubiquitin chains. These domains allow Rad23 to function as a substrate shuttle-factor. This property is shared by structurally similar proteins (Dsk2 and Ddi1) and is conserved among the human and mouse counterparts of Rad23. Despite much effort, the regulation of Rad23 interactions with ubiquitinated substrates and the proteasome is unknown. We report here that Rad23 is extensively phosphorylated in vivo and in vitro. Serine residues in UbL are phosphorylated and influence Rad23 interaction with proteasomes. Replacement of these serine residues with acidic residues, to mimic phosphorylation, reduced proteasome binding. We reported that when UbL is overexpressed, it can compete with Rad23 for proteasome interaction and can inhibit substrate turnover. This effect is not observed with UbL containing acidic substitutions, consistent with results that phosphorylation inhibits interaction with the proteasome. Loss of both Rad23 and Rpn10 caused pleiotropic defects that were suppressed by overexpressing either Rad23 or Rpn10. Rad23 bearing a UbL domain with acidic substitutions failed to suppress rad23Δ rpn10Δ, confirming the importance of regulated Rad23/proteasome binding. Strikingly, threonine 75 in human HR23B also regulates interaction with the proteasome, suggesting that phosphorylation is a conserved mechanism for controlling Rad23/proteasome interaction.

Yeast importin-α (Srp1) performs distinct roles in the import of nuclear proteins and in targeting proteasomes to the nucleus

Srp1 (importin-α) can translocate proteins that contain a nuclear localization signal (NLS) into the nucleus. The loss of Srp1 is lethal, although several temperature-sensitive mutants have been described. Among these mutants, srp1-31 displays the characteristic nuclear import defect of importin-α mutants, whereas srp1-49 shows a defect in protein degradation. We characterized these and additional srp1 mutants to determine whether distinct mechanisms were required for intracellular proteolysis and the import of NLS-containing proteins. We determined that srp1 mutants that failed to import NLS-containing proteins (srp1-31 and srp1-55) successfully localized proteasomes to the nucleus. In contrast, srp1 mutants that did not target proteasomes to the nucleus (srp1-49 and srp1-E402Q) were able to import NLS-containing proteins. The proteasome targeting defect of specific srp1 mutants caused stabilization of nuclear substrates and overall accumulation of multiubiquitylated proteins. Co-expression of a member of each class of srp1 mutants corrected both the proteasome localization defect and the import of NLS-containing proteins. These findings indicate that the targeting of proteasomes to the nucleus occurs by a mechanism distinct from the Srp1-mediated import of nuclear proteins.

Alteration of peripheral blood monocyte gene expression in humans following diesel exhaust inhalation

CONTEXT

Epidemiologic associations between acutely increased cardiorespiratory morbidity and mortality and particulate air pollution are well established, but the effects of acute pollution exposure on human gene expression changes are not well understood.

OBJECTIVE

In order to identify potential mechanisms underlying epidemiologic associations between air pollution and morbidity, we explored changes in gene expression in humans following inhalation of fresh diesel exhaust (DE), a model for particulate air pollution.

MATERIALS AND METHODS

Fourteen ethnically homogeneous (white males), young, healthy subjects underwent 60-min inhalation exposures on 2 separate days with clean filtered air (CA) or freshly generated and diluted DE at a concentration of 300 μg/m(3) PM(2.5). Prior to and 24 h following each session, whole blood was sampled and fractionated for peripheral blood mononuclear cell (PBMC) isolation, RNA extraction, and generation of cDNA, followed by hybridization with Agilent Whole Human Genome (4X44K) arrays.

RESULTS

Oxidative stress and the ubiquitin proteasome pathway, as well as the coagulation system, were among hypothesized pathways identified by analysis of differentially expressed genes. Nine genes from these pathways were validated using real-time polymerase chain reaction (PCR) to compare fold change in expression between DE exposed and CA days. Quantitative gene fold changes generated by real-time PCR were directionally consistent with the fold changes from the microarray analysis.

DISCUSSION AND CONCLUSION

Changes in gene expression connected with key oxidative stress, protein degradation, and coagulation pathways are likely to underlie observed physiologic and clinical outcomes and suggest specific avenues and sensitive time points for further physiologic exploration.

Proteomic identification of immunoproteasome accumulation in formalin-fixed rodent spinal cords with experimental autoimmune encephalomyelitis

Clinically relevant formalin-fixed and paraffin-embedded (FFPE) tissues have not been widely used in neuroproteomic studies because many proteins are presumed to be degraded during tissue preservation. Recent improvements in proteomics technologies, from the 2D gel analysis of intact proteins to the "shotgun" quantification of peptides and the use of isobaric tags for absolute and relative quantification (iTRAQ) method, have made the analysis of FFPE tissues possible. In recent years, iTRAQ has been one of the main methods of choice for high throughput quantitative proteomics analysis, which enables simultaneous comparison of up to eight samples in one experiment. Our objective was to assess the relative merits of iTRAQ analysis of fresh frozen versus FFPE nervous tissues by comparing experimental autoimmune encephalomyelitis (EAE)-induced proteomic changes in FFPE rat spinal cords and frozen tissues. EAE-induced proteomic changes in FFPE tissues were positively correlated with those found in the frozen tissues, albeit with ∼50% less proteome coverage. Subsequent validation of the enrichment of immunoproteasome (IP) activator 1 in EAE spinal cords led us to evaluate other proteasome and IP-specific proteins. We discovered that many IP-specific (as opposed to constitutive) proteasomal proteins were enriched in EAE rat spinal cords, and EAE-induced IP accumulation also occurred in the spinal cords of an independent mouse EAE model in a disability score-dependent manner. Therefore, we conclude that it is feasible to generate useful information from iTRAQ-based neuroproteomics analysis of archived FFPE tissues for studying neurological disease tissues.

Acute decreases in proteasome pathway activity after inhalation of fresh diesel exhaust or secondary organic aerosol

BACKGROUND

Epidemiologic studies consistently demonstrate an association between acute cardiopulmonary events and changes in air pollution; however, the mechanisms that underlie these associations are not completely understood. Oxidative stress and inflammation have been suggested to play a role in human responses to air pollution. The proteasome is an intracellular protein degradation system linked to both of these processes and may help mediate air pollution effects.

OBJECTIVES

In these studies, we determined whether acute experimental exposure to two different aerosols altered white blood cell (WBC) or red blood cell (RBC) proteasome activity in human subjects. One aerosol was fresh diesel exhaust (DE), and the other freshly generated secondary organic aerosol (SOA).

METHODS

Thirty-eight healthy subjects underwent 2-hr resting inhalation exposures to DE and separate exposures to clean air (CA); 26 subjects were exposed to DE, CA, and SOA. CA responses were subtracted from DE or SOA responses, and mixed linear models with F-tests were used to test the effect of exposure to each aerosol on WBC and RBC proteasome activity.

RESULTS

WBC proteasome activity was reduced 8% (p = 0.04) after exposure to either DE or SOA and decreased by 11.5% (p = 0.03) when SOA was analyzed alone. RBCs showed similar 8-10% declines in proteasome activity (p = 0.05 for DE alone).

CONCLUSIONS

Air pollution produces oxidative stress and inflammation in many experimental models, including humans. Two experimental aerosols caused rapid declines in proteasome activity in peripheral blood cells, supporting a key role for the proteasome in acute human responses to air pollution.

Sts1 plays a key role in targeting proteasomes to the nucleus

The evidence that nuclear proteins can be degraded by cytosolic proteasomes has received considerable experimental support. However, the presence of proteasome subunits in the nucleus also suggests that protein degradation could occur within this organelle. We determined that Sts1 can target proteasomes to the nucleus and facilitate the degradation of a nuclear protein. Specific sts1 mutants showed reduced nuclear proteasomes at the nonpermissive temperature. In contrast, high expression of Sts1 increased the levels of nuclear proteasomes. Sts1 targets proteasomes to the nucleus by interacting with Srp1, a nuclear import factor that binds nuclear localization signals. Deletion of the NLS in Sts1 prevented its interaction with Srp1 and caused proteasome mislocalization. In agreement with this observation, a mutation in Srp1 that weakened its interaction with Sts1 also reduced nuclear targeting of proteasomes. We reported that Sts1 could suppress growth and proteolytic defects of rad23Δ rpn10Δ. We show here that Sts1 suppresses a previously undetected proteasome localization defect in this mutant. Taken together, these findings explain the suppression of rad23Δ rpn10Δ by Sts1 and suggest that the degradation of nuclear substrates requires efficient proteasome localization.

Synthetic lethality of rpn11-1 rpn10Δ is linked to altered proteasome assembly and activity

An rpn11-1 temperature-sensitive mutant shows defect in proteolysis, mitochondrial function and proteasome assembly. The Rpn11 protein is a proteasome subunit that deubiquitinates proteolytic substrates. Multiubiquitinated proteins interact with proteasome receptors, such as Rpn10, which intriguingly is also required for promoting proteasome stability. We report here that Rpn10 binds Rpn11, and genetic studies revealed synthetic lethality of an rpn11-1 rpn10Δ double mutant. The carboxy-terminus of Rpn11 is critical for function, as deletion of 7 C-terminal residues prevented suppression of rpn11-1 rpn10Δ. Native gel electrophoresis showed increased levels of the proteasome 20S catalytic particle in rpn11-1 rpn10Δ, and altered assembly. The inviability of rpn11-1 rpn10Δ was suppressed by rpn10(uim), a mutant that can bind the proteasome, but not multiubiquitin chains. rpn10(uim) reduced the levels of free 20S, and increased formation of intact proteasomes. In contrast, rpn10(vwa), which binds multiubiquitin chains but not the proteasome, failed to suppress rpn11-1 rpn10Δ. Moreover, high levels of multiubiquitinated proteins were bound to rpn10(vwa), but were not delivered to the proteasome. Based on these findings, we propose that the lethality of rpn11-1 rpn10Δ results primarily from altered proteasome integrity. It is conceivable that Rpn10/Rpn11 interaction couples proteasome assembly to substrate binding.

Proteasome assembly influences interaction with ubiquitinated proteins and shuttle factors

A major fraction of intracellular protein degradation is mediated by the proteasome. Successful degradation of these substrates requires ubiquitination and delivery to the proteasome followed by protein unfolding and disassembly of the multiubiquitin chain. Enzymes, such as Rpn11, dismantle multiubiquitin chains, and mutations can affect proteasome assembly and activity. We report that different rpn11 mutations can affect proteasome interaction with ubiquitinated proteins. Moreover, proteasomes are unstable in rpn11-1 and do not form productive interactions with multiubiquitinated proteins despite high levels in cell extracts. However, increased levels of ubiquitinated proteins were found associated with shuttle factors. In contrast to rpn11-1, proteasomes expressing a catalytically inactive mutant (rpn11(AXA)) were more stable and bound very high amounts of ubiquitinated substrates. Expression of the carboxyl-terminal domain of Rpn11 partially suppressed the growth and proteasome stability defects of rpn11-1. These results indicate that ubiquitinated substrates are preferentially delivered to intact proteasome.

Proteasome inhibition decreases cardiac remodeling after initiation of pressure overload

We tested the possibility that proteasome inhibition may reverse preexisting cardiac hypertrophy and improve remodeling upon pressure overload. Mice were submitted to aortic banding and followed up for 3 wk. The proteasome inhibitor epoxomicin (0.5 mg/kg) or the vehicle was injected daily, starting 2 wk after banding. At the end of the third week, vehicle-treated banded animals showed significant (P<0.05) increase in proteasome activity (PA), left ventricle-to-tibial length ratio (LV/TL), myocyte cross-sectional area (MCA), and myocyte apoptosis compared with sham-operated animals and developed signs of heart failure, including increased lung weight-to-TL ratio and decreased ejection fraction. When compared with that group, banded mice treated with epoxomicin showed no increase in PA, a lower LV/TL and MCA, reduced apoptosis, stabilized ejection fraction, and no signs of heart failure. Because overload-mediated cardiac remodeling largely depends on the activation of the proteasome-regulated transcription factor NF-kappaB, we tested whether epoxomicin would prevent this activation. NF-kappaB activity increased significantly upon overload, which was suppressed by epoxomicin. The expression of NF-kappaB-dependent transcripts, encoding collagen types I and III and the matrix metalloprotease-2, increased (P<0.05) after banding, which was abolished by epoxomicin. The accumulation of collagen after overload, as measured by histology, was 75% lower (P<0.05) with epoxomicin compared with vehicle. Myocyte apoptosis increased by fourfold in hearts submitted to aortic banding compared with sham-operated hearts, which was reduced by half upon epoxomicin treatment. Therefore, we propose that proteasome inhibition after the onset of pressure overload rescues ventricular remodeling by stabilizing cardiac function, suppressing further progression of hypertrophy, repressing collagen accumulation, and reducing myocyte apoptosis.

p62 serves as a shuttling factor for TrkA interaction with the proteasome

The scaffold protein p62 is involved in internalization and trafficking of TrkA. The receptor is deubiquitinated by the proteasomes prior to degradation by lysosomes. Here we demonstrate that p62 serves as a shuttling protein for interaction of ubiquitinated TrkA with Rpt1, one of the six ATPases of 19S regulatory particle of the 26S proteasome. In p62(-/-) mouse brain TrkA failed to interact with the Rpt1. The interaction of TrkA with Rpt1 was reduced in proteasomes isolated from p62(-/-) brain, but was restored by addition of p62. The UBA domain of p62 interacts with TrkA and its PB1/UbL domain with AAA-ATPase cassette in the C-terminal region of Rpt1. Last, neurotrophin-dependent turnover of TrkA was impaired by reduction in the level of p62. These findings reveal that p62 serves as a shuttling factor for interaction of ubiquitinated substrates with the proteasome and could promote localized protein turnover in neurons.

Sts1 can overcome the loss of Rad23 and Rpn10 and represents a novel regulator of the ubiquitin/proteasome pathway

A rad23Delta rpn10Delta double mutant accumulates multi-Ub proteins, is deficient in proteolysis, and displays sensitivity to drugs that generate damaged proteins. Overexpression of Sts1 restored normal growth in rad23Delta rpn10Delta but did not overcome the DNA repair defect of rad23Delta. To understand the nature of Sts1 suppression, we characterized sts1-2, a temperature-sensitive mutant. We determined that sts1-2 was sensitive to translation inhibitors, accumulated high levels of multi-Ub proteins, and caused stabilization of proteolytic substrates. Additionally, ubiquitinated proteins that were detected in proteasomes were inefficiently cleared in sts1-2. Despite these proteolytic defects, overall proteasome activity was increased in sts1-2. We propose that Sts1 is a new regulatory factor in the ubiquitin/proteasome pathway that controls the turnover of proteasome substrates.

Activation of the Cardiac Proteasome During Pressure Overload Promotes Ventricular Hypertrophy

BACKGROUND

The adaptation of cardiac mass to hemodynamic overload requires an adaptation of protein turnover, ie, the balance between protein synthesis and degradation. We tested 2 hypotheses: (1) chronic left ventricular hypertrophy (LVH) activates the proteasome system of protein degradation, especially in the myocardium submitted to the highest wall stress, ie, the subendocardium, and (2) the proteasome system is required for the development of LVH.

METHODS AND RESULTS

Gene and protein expression of proteasome subunits and proteasome activity were measured separately from left ventricular subendocardium and subepicardium, right ventricle, and peripheral tissues in a canine model of severe, chronic (2 years) LVH induced by aortic banding and then were compared with controls. Both gene and protein expressions of proteasome subunits were increased in LVH versus control (P<0.05), which was accompanied by a significant (P<0.05) increase in proteasome activity. Posttranslational modification of the proteasome was also detected by 2-dimensional gel electrophoresis. These changes were found specifically in left ventricular subendocardium but not in left ventricular subepicardium, right ventricle, or noncardiac tissues from the same animals. In a mouse model of chronic pressure overload, a 50% increase in heart mass and a 2-fold increase in proteasome activity (both P<0.05 versus sham) were induced. In that model, the proteasome inhibitor epoxomicin completely prevented LVH while blocking proteasome activation.

CONCLUSIONS

The increase in proteasome expression and activity found during chronic pressure overload in myocardium submitted to higher stress is also required for the establishment of LVH.

Proteasome dysfunction in aged human alpha-synuclein transgenic mice

A deficit in proteasome function in Parkinson's disease has been speculated. We characterized the ubiquitin-proteasome system in three regions of brain from transgenic and nontransgenic littermates. Mice expressing a doubly mutated form of human alpha-synuclein had significant impairments whereas mice expressing the wild-type gene had lesser changes compared to nontransgenic littermates. Significant abnormalities in line hm2 alpha-SYN-39 included declines in 20S-mediated proteolytic activity, the level of the 19S proteasome subunits Rpt1 and Rpn2, and the level of soluble total high MW ubiquitin cross-reacting proteins. Line hw alpha-SYN-5 had significant, but restricted proteasome abnormalities. The severity of impairment was proportional to the substantia nigra dopaminergic neuronal loss previously identified. There were significant correlations between the level of Rpn2 with the level of Rpt1, the activity of the 20S proteasome, and the level of soluble high MW ubiquitin cross-reacting proteins. These abnormalities in symptomatic line hm2 alpha-SYN-39 mice are consistent with abnormalities identified in tissue from patients with Parkinson's disease.

Evidence for distinct functions for human DNA repair factors hHR23A and hHR23B

Rad23 proteins bind ubiquitinated substrates and the proteasome, consistent with an important role in protein degradation. Although human Rad23 proteins (hHR23A and hHR23B) have redundant roles in DNA repair, we determined they formed distinct interactions with proteasomes and multiubiquitinated proteins, but similar binding to Ataxin-3. Threonine-79 contributed to the weak proteasome-binding property of hHR23A, and its conversion to proline (T79P), which is the residue present in hHR23B, increased proteasome interaction. We also determined that hHR23A and hHR23B could be co-purified with unique proteolytic and stress-responsive factors from human breast cancer tissues, indicating that they have unique functions in vivo.

Increased proteasome activity, ubiquitin-conjugating enzymes, and eEF1A translation factor detected in breast cancer tissue

The ubiquitin (Ub)/proteasome pathway facilitates the degradation of damaged proteins and regulators of growth and stress response. The activation of this pathway in various cancers and malignancies has been described, and several genetic determinants of breast cancer, including BRCA1 and BRCA2, are linked to protein degradation. To investigate the involvement of the Ub/proteasome system in breast cancer, we examined a collection of 25 patient-matched breast cancer and normal adjacent tissues and detected activation of numerous components of the Ub/proteasome pathway. The activity of the proteasome, and levels of proteasome subunits and various targeting factors, were increased in >90% of primary breast cancer tissue specimens. In contrast, no activation was observed in benign solid tumors, indicating that the response is specific to abnormal growth in neoplastic cells. Additionally, the accumulation of high levels of certain Ub-conjugating enzymes (UbcH1, UbcH2, and UbcH5), was specific to breast cancer, as no change in abundance was detected in primary colon cancer tissue extracts. Surprisingly, the Ub/proteasome system was not activated in a well-characterized cell culture-based breast cancer model system. Collectively, these findings suggest that the analysis of primary breast cancer tissue samples will be indispensable for the biochemical characterization of neoplastic growth and for the development of therapeutics.

Saccharomyces cerevisiae Ub-conjugating enzyme Ubc4 binds the proteasome in the presence of translationally damaged proteins

Surveillance mechanisms that monitor protein synthesis can promote rapid elimination of misfolded nascent proteins. We showed that the translation elongation factor eEF1A and the proteasome subunit Rpt1 play a central role in the translocation of nascent-damaged proteins to the proteasome. We show here that multiubiquitinated proteins, and the ubiquitin-conjugating (E2) enzyme Ubc4, are rapidly detected in the proteasome following translational damage. However, Ubc4 levels in the proteasome were reduced significantly in a strain that expressed a mutant Rpt1 subunit. Ubc4 and Ubc5 are functionally redundant E2 enzymes that represent ideal candidates for ubiquitinating damaged nascent proteins because they lack significant substrate specificity, are required for the degradation of bulk, damaged proteins, and contribute to cellular stress-tolerance mechanisms. In agreement with this hypothesis, we determined that ubc4Delta ubc5Delta is exceedingly sensitive to protein translation inhibitors. Collectively, these studies suggest a specific role for Ubc4 and Ubc5 in the degradation of cotranslationally damaged proteins that are targeted to the proteasome.

α-Synuclein and Parkin Contribute to the Assembly of Ubiquitin Lysine 63-linked Multiubiquitin Chains

Mutations in alpha-synuclein, Parkin, and UCH-L1 cause heritable forms of Parkinson disease. Unlike alpha-synuclein, for which no precise biochemical function has been elucidated, Parkin functions as a ubiquitin E3 ligase, and UCH-L1 is a deubiquitinating enzyme. The E3 ligase activity of Parkin in Parkinson disease is poorly understood and is further obscured by the fact that multiubiquitin chains can be formed through distinct types of linkages that regulate diverse cellular processes. For instance, ubiquitin lysine 48-linked multiubiquitin chains target substrates to the proteasome, whereas ubiquitin lysine 63-linked chains control ribosome function, protein sorting and trafficking, and endocytosis of membrane proteins. It is notable in this regard that ubiquitin lysine 63-linked chains promote the degradation of membrane proteins by the lysosome. Because both Parkin and alpha-synuclein can regulate the activity of the dopamine transporter, we investigated whether they influenced ubiquitin lysine 63-linked chain assembly. These studies revealed novel biochemical activities for both Parkin and alpha-synuclein. We determined that Parkin functions with UbcH13/Uev1a, a dimeric ubiquitin-conjugating enzyme, to assemble ubiquitin lysine 63-linked chains. Our results and the results of others indicate that Parkin can promote both lysine 48- and lysine 63-linked ubiquitin chains. alpha-Synuclein also stimulated the assembly of lysine 63-linked ubiquitin chains. Because UCH-L1, a ubiquitin hydrolase, was recently reported to form lysine 63-linked conjugates, it is evident that three proteins that are genetically linked to Parkinson disease can contribute to lysine 63 multiubiquitin chain formation.

Rad23 and Rpn10: perennial wallflowers join the melee

Substrate ubiquitination is highly regulated; by contrast, substrate targeting to the proteasome has been considered a stochastic process that is mediated primarily by high-affinity interaction with multi-ubiquitin chains. However, recent findings have shown that substrate recognition by the proteasome is also regulated, and requires Rad23, Dsk2 and Rpn10. These studies suggest that the engagement of protein ubiquitination and translocation with degradation by the proteasome is coordinated by a series of regulated events.

Proteasome-mediated degradation of cotranslationally damaged proteins involves translation elongation factor 1A

Rad23 and Rpn10 play synergistic roles in the recognition of ubiquitinated proteins by the proteasome, and loss of both proteins causes growth and proteolytic defects. However, the physiological targets of Rad23 and Rpn10 have not been well defined. We report that rad23Delta rpn10Delta is unable to grow in the presence of translation inhibitors, and this sensitivity was suppressed by translation elongation factor 1A (eEF1A). This discovery suggested that Rad23 and Rpn10 perform a role in translation quality control. Certain inhibitors increase translation errors during protein synthesis and cause the release of truncated polypeptide chains. This effect can also be mimicked by ATP depletion. We determined that eEF1A interacted with ubiquitinated proteins and the proteasome following ATP depletion. eEF1A interacted with the proteasome subunit Rpt1, and the turnover of nascent damaged proteins was deficient in rpt1. An eEF1A mutant (eEF1A(D156N)) that conferred hyperresistance to translation inhibitors was much more effective at eliminating damaged proteins and was detected in proteasomes in untreated cells. We propose that eEF1A is well suited to detect and promote degradation of damaged proteins because of its central role in translation elongation. Our findings provide a mechanistic foundation for defining how cellular proteins are degraded cotranslationally.

Rad23 stabilizes Rad4 from degradation by the Ub/proteasome pathway

Rad23 protein interacts with the nucleotide excision-repair (NER) factor Rad4, and the dimer can bind damaged DNA. Rad23 also binds ubiquitinated proteins and promotes their degradation by the proteasome. Rad23/proteasome interaction is required for efficient NER, although the specific role of the Ub/proteasome system in DNA repair is unclear. We report that the availability of Rad4 contributes significantly to the cellular tolerance to UV light. Mutations in the proteasome, and in genes encoding the ubiquitin-conjugating enzymes Ubc4 and Ubc5, stabilized Rad4 and increased tolerance to UV light. A short amino acid sequence, previously identified in human Rad23, mediates the interaction between Rad23 and Rad4. We determined that this motif was required for stabilizing Rad4, and could function independently of the intact protein. A ubiquitin-like (UbL) domain in Rad23 binds the proteasome, and is required for conferring full resistance to DNA damage. However, Rad23/proteasome interaction appears unrelated to Rad23-mediated stabilization of Rad4. Specifically, simultaneous expression of a Rad23 mutant that could not bind the proteasome, with a mutant that could not interact with Rad4, fully suppressed the UV sensitivity of rad23Delta, demonstrating that Rad23 performs two independent, but concurrent roles in NER.

Rings, chains and ladders: ubiquitin goes to work in the neuron

Our understanding of neuronal cell biology in the last 10 years has exploded. In parallel, our grasp of basic cellular processes, such as protein synthesis and protein degradation has also grown exponentially. In this review, we provide an in-depth background to details of current knowledge of the Ub/proteasome pathway. We also provide examples of recent experiments in neurobiology that suggest a central role for targeted protein degradation by the Ub/proteasome pathway to ensure proper neuronal function. From the examples provided, it is clear the activity of the proteasome is required for neuronal pathfinding during development, regulation of synaptic branching and number, and synaptic plasticity. We conclude with a discussion of how defects in proteasome pathway function may lead to neuronal dysfunction, with specific emphasis on diseases characterized by the accumulation of ubiquitin (Ub)-positive inclusions. Our goal is to excite the expert neurobiologist to the myriad ways that specific neuronal functions could be regulated (or dysregulated) by mechanisms involving the Ub/proteasome pathway.

Ataxin-3 interactions with rad23 and valosin-containing protein and its associations with ubiquitin chains and the proteasome are consistent with a role in ubiquitin-mediated proteolysis

Machado-Joseph disease is caused by an expansion of a trinucleotide CAG repeat in the gene encoding the protein ataxin-3. We investigated if ataxin-3 was a proteasome-associated factor that recognized ubiquitinated substrates based on the rationale that (i) it is present with proteasome subunits and ubiquitin in cellular inclusions, (ii) it interacts with human Rad23, a protein that may translocate proteolytic substrates to the proteasome, and (iii) it shares regions of sequence similarity with the proteasome subunit S5a, which can recognize multiubiquitinated proteins. We report that ataxin-3 interacts with ubiquitinated proteins, can bind the proteasome, and, when the gene harbors an expanded repeat length, can interfere with the degradation of a well-characterized test substrate. Additionally, ataxin-3 associates with the ubiquitin- and proteasome-binding factors Rad23 and valosin-containing protein (VCP/p97), findings that support the hypothesis that ataxin-3 is a proteasome-associated factor that mediates the degradation of ubiquitinated proteins.

Investigating the importance of proteasome-interaction for Rad23 function

Rad23 contributes to diverse cellular functions that include DNA repair, stress response and growth control. An amino-terminal ubiquitin-like (UbL) domain in Rad23 interacts with catalytically active proteasomes and internal sequences bind multi-ubiquitinated proteins. Rad23 regulates the assembly of substrate-linked multi-ubiquitin chains, promotes efficient degradation of model substrates, and plays an overlapping role with the proteasome subunit, Rpn10. These and other results led to the hypothesis that Rad23 translocates proteolytic substrates to the proteasome to promote degradation. It was previously shown that the UbL domain in Rad23 could be functionally replaced by ubiquitin. However, monomeric ubiquitin does not bind the proteasome efficiently, and we therefore investigated whether proteasome interaction was required for all Rad23 functions. We report here that the ubiquitin moiety in Ub-rad23 is ubiquitinated in vivo and could provide an alternate mechanism for binding the proteasome. These results suggest that the localization of Rad23 to the proteasome, either by its UbL domain, or following ubiquitination of an amino-terminal ubiquitin moiety (Ub-rad23), is necessary for full activity.

Proteolysis of a nucleotide excision repair protein by the 26 S proteasome

The 26 S proteasome degrades a broad spectrum of proteins and interacts with several nucleotide excision repair (NER) proteins, including the complex of Rad4 and Rad23 that binds preferentially to UV-damaged DNA. The rate of NER is increased in yeast strains with mutations in genes encoding subunits of the 26 S proteasome, indicating that it could negatively regulate a repair process. The specific function of the 26 S proteasome in DNA repair is unclear. It might degrade DNA repair proteins after repair is completed or act as a molecular chaperone to promote the assembly or disassembly of the repair complex. In this study, we show that Rad4 is ubiquitylated and that Rad23 can control this process. We also find that ubiquitylated Rad4 is degraded by the 26 S proteasome. However, the interaction of Rad23 with Rad4 is not only to control degradation of Rad4, but also to assist in assembling the NER incision complex at UV-induced cyclobutane pyrimidine dimers. We speculate that, following the completion of DNA repair, specific repair proteins might be degraded by the proteasome to regulate repair.

Rad23 promotes the targeting of proteolytic substrates to the proteasome

Rad23 contains a ubiquitin-like domain (UbL(R23)) that interacts with catalytically active proteasomes and two ubiquitin (Ub)-associated (UBA) sequences that bind Ub. The UBA domains can bind Ub in vitro, although the significance of this interaction in vivo is poorly understood. Rad23 can interfere with the assembly of multi-Ub chains in vitro, and high-level expression caused stabilization of proteolytic substrates in vivo. We report here that Rad23 interacts with ubiquitinated cellular proteins through the synergistic action of its UBA domains. Rad23 plays an overlapping role with Rpn10, a proteasome-associated multi-Ub chain binding protein. Mutations in the UBA domains prevent efficient interaction with ubiquitinated proteins and result in poor suppression of the growth and proteolytic defects of a rad23 Delta rpn10 Delta mutant. High-level expression of Rad23 revealed, for the first time, an interaction between ubiquitinated proteins and the proteasome. This increase was not observed in rpn10 Delta mutants, suggesting that Rpn10 participates in the recognition of proteolytic substrates that are delivered by Rad23. Overexpression of UbL(R23) caused stabilization of a model substrate, indicating that an unregulated UbL(R23)-proteasome interaction can interfere with the efficient delivery of proteolytic substrates by Rad23. Because the suppression of a rad23 Delta rpn10 Delta mutant phenotype required both UbL(R23) and UBA domains, our findings support the hypothesis that Rad23 encodes a novel regulatory factor that translocates ubiquitinated substrates to the proteasome.

Regulation of repair by the 26S proteasome

Cellular processes such as transcription and DNA repair may be regulated through diverse mechanisms, including RNA synthesis, protein synthesis, posttranslational modification and protein degradation. The 26S proteasome, which is responsible for degrading a broad spectrum of proteins, has been shown to interact with several nucleotide excision repair proteins, including xeroderma pigmentosum B protein (XPB), Rad4, and Rad23. Rad4 and Rad23 form a complex that binds preferentially to UV-damaged DNA. The 26S proteasome may regulate repair by degrading DNA repair proteins after repair is completed or, alternatively, the proteasome may act as a molecular chaperone to promote disassembly of the repair complex. In either case, the interaction between the proteasome and nucleotide excision repair depends on proteins like Rad23 that bind ubiquitin-conjugated proteins and the proteasome. While the iteration between Rad4 and Rad23 is well established, it will be interesting to determine what other proteins are regulated in a Rad23-dependent manner.

Ubiquitin-associated (UBA) domains in Rad23 bind ubiquitin and promote inhibition of multi-ubiquitin chain assembly

Rad23 is a DNA repair protein that promotes the assembly of the nucleotide excision repair complex. Rad23 can interact with the 26S proteasome through an N-terminal ubiquitin-like domain, and inhibits the assembly of substrate-linked multi-ubiquitin (multi-Ub) chains in vitro and in vivo. Significantly, Rad23 can bind a proteolytic substrate that is conjugated to a few ubiquitin (Ub) moieties. We report here that two ubiquitin-associated (UBA) domains in Rad23 form non-covalent interactions with Ub. A mutant that lacked either UBA sequence was capable of blocking the assembly of substrate-linked multi-Ub chains, although a mutant that lacked both UBA domains was significantly impaired. These studies suggest that the interaction with Ub is required for Rad23 activity, and that other UBA-containing proteins may have a similar function.

The 26S proteasome negatively regulates the level of overall genomic nucleotide excision repair

Regulation of protein expression can be achieved through destruction of proteins by the 26S: proteasome. Cellular processes that are regulated by proteolysis include cell cycle progression, stress responses and differentiation. Several nucleotide excision repair proteins in yeast and humans, such as Rad23, Rad4 and XPB, have been shown to co-purify with Cim3 and Cim5, AAA ATPases of the 19S: proteasome regulatory subunit. However, it has not been determined if nucleotide excision repair is regulated through protein destruction. We measured nucleotide excision repair in yeast mutants that are defective in proteasome function and found that the repair of the transcribed and non-transcribed strands of an RNA polymerase II-transcribed reporter gene was increased in the absence of proteasome function. Additionally, overexpression of the Rad4 repair protein, which is bound to the repair/proteolytic factor Rad23, conferred higher rates of nucleotide excision repair. Based on our data we suggest that a protein (or proteins) involved in nucleotide excision repair or in regulation of repair is degraded by the 26S proteasome. We propose that decreased proteasome function enables increased DNA repair, due to the transient accumulation of a specific repair factor, perhaps Rad4.

The DNA repair protein rad23 is a negative regulator of multi-ubiquitin chain assembly

Rad23 is a nucleotide-excision repair protein with a previously unknown biochemical function. We determined that yeast and human Rad23 inhibited multi-ubiquitin (Ub) chain formation and the degradation of proteolytic substrates. Significantly, Rad23 could be co-precipitated with a substrate that contained a short multi-Ub chain. The UV sensitivity of rad23Delta was reduced in mutants lacking the E2 enzyme Ubc4, or the multi-Ub chain-promoting factor Ufd2. These studies suggest that the stability of proteolytic substrates is governed by the competing action of multi-Ub chain-promoting and chain-inhibiting factors. The stabilization of DNA repair and stress factors could represent an important biological function of Rad23.

Evidence for an interaction between ubiquitin-conjugating enzymes and the 26S proteasome

The targeting of proteolytic substrates is accomplished by a family of ubiquitin-conjugating (E2) enzymes and a diverse set of substrate recognition (E3) factors. The ligation of a multiubiquitin chain to a substrate can promote its degradation by the proteasome. However, the mechanism that facilitates the translocation of a substrate to the proteasome in vivo is poorly understood. We have discovered that E2 proteins, including Ubc1, Ubc2, Ubc4, and Ubc5, can interact with the 26S proteasome. Significantly, the interaction between Ubc4 and the proteasome is strongly induced by heat stress, consistent with the requirement for this E2 for efficient stress tolerance. A catalytically inactive derivative of Ubc4 (Ubc4(C86A)), which causes toxicity in yeast cells, can also bind the proteasome. Purified proteasomes can ligate ubiquitin to a test substrate without the addition of exogenous E2 protein, suggesting that the ubiquitylation of some proteolytic substrates might be directly coupled to degradation by the proteasome.

Pleiotropic defects caused by loss of the proteasome-interacting factors Rad23 and Rpn10 of Saccharomyces cerevisiae

Rad23 is a member of a novel class of proteins that contain unprocessed ubiquitin-like (UbL) domains. We showed recently that a small fraction of Rad23 can form an interaction with the 26S proteasome. Similarly, a small fraction of Rpn10 is a component of the proteasome. Rpn10 can bind multiubiquitin chains in vitro, but genetic studies have not clarified its role in vivo. We report here that the loss of both Rad23 and Rpn10 results in pleiotropic defects that are not observed in either single mutant. rad23Delta rpn10Delta displays slow growth, cold sensitivity, and a pronounced G2/M phase delay, implicating overlapping roles for Rad23 and Rpn10. Although rad23Delta rpn10Delta displays similar sensitivity to DNA damage as a rad23Delta single mutant, deletion of RAD23 in rpn10Delta significantly increased sensitivity to canavanine, a phenotype associated with an rpn10Delta single mutant. A mutant Rad23 that is unable to bind the proteasome ((DeltaUbL)rad23) does not suppress the canavanine or cold-sensitive defects of rad23Delta rpn10Delta, demonstrating that Rad23/proteasome interaction is related to these effects. Finally, the accumulation of multiubiquitinated proteins and the stabilization of a specific proteolytic substrate in rad23Delta rpn10Delta suggest that proteasome function is altered.

Characterization of a temperature-sensitive mutant of a ubiquitin-conjugating enzyme and its use as a heat-inducible degradation signal

The ubiquitin/proteasome pathway is a highly conserved mechanism of proteolysis in all eukaryotes. Ubiquitin (Ub) is conjugated to proteolytic substrates through the sequential action of ubiquitin-activating (E1/Uba) and ubiquitin-conjugating (E2/Ubc) enzymes. The mechanism of substrate recognition and ubiquitination is an area of active investigation, and we have begun a site-directed mutagenesis approach to define the biochemical and biophysical properties of ubiquitin-conjugating enzymes. We have characterized a specific mutation in Ubc4 (Ubc4(P62S)) which was previously shown to cause a temperature-sensitive growth defect in several other Ubc's. Ubc4(P62S) was rapidly degraded in vivo, contributing to the loss of function. However, reconstitution experiments revealed that the catalytic activity of Ubc4(P62S) was reversibly inactivated at 37 degrees C, demonstrating that the primary defect of Ubc4(P62S) is its inability to form a ubiquitin thioester bond at high temperature. The in vivo defect is compounded by increased susceptibility of Ubc4(P62S) to degradation by the ubiquitin/proteasome pathway. We have exploited the temperature-dependent degradation of the P62S mutant to destabilize an otherwise stable test protein (glutathione S-transferase). The use of this mutant may provide a useful cis-acting temperature-inducible degradation signal.

Reconstituting ubiquitination reactions with affinity-purified components and 32P-ubiquitin

The discovery of protein ubiquitination in a broad range of organisms and experimental settings has raised the need for a straightforward way to characterize the mechanism of substrate targeting, using purified components. The mechanism of ubiquitin conjugation to proteins has been extensively studied and is mediated by a family of evolutionarily conserved proteins. We have used previously described expression systems to purify the relevant targeting components of the ubiquitin system. These methods yielded substantial amounts of highly purified and catalytically active enzymes that permitted their use in reconstituting protein ubiquitination. We monitored ubiquitination reactions with 32P-ubiquitin rather than 125I-ubiquitin. This advance makes the procedure accessible to a broader range of experimentalists, since it eliminates the additional concerns involved in handling 125I-isotope. Furthermore, the strategies described here can be used to investigate the effects of specific mutations introduced into ubiquitin or the targeting components (E1, Ubc/E2, and E3) of this pathway.

Sequence elements that contribute to the degradation of yeast G alpha

BACKGROUND

Gpa1 is the alpha subunit of the yeast G-protein that regulates signal transduction during mating. The stability of Galpha/Gpa1 is influenced by the ubiquitin-dependent N-end rule pathway, suggesting that the regulation of G alpha levels may be important for effective mating response and recovery.

RESULTS

The G alpha sequences that confer sensitivity to degradation by the N-end rule pathway were identified. The insertion of this degradation signal (G1-Deg) into the ordinarily stable Gpa2 protein conferred proteolytic targeting. We examined G alpha degradation under different conditions and found that it was efficiently degraded in haploid and diploid cells, but was stable if it was synthesized prior to expression of the N-end rule pathway. Interestingly, a specific mutation in G alpha that is believed to promote the GTP-bound form (N388K) caused accelerated degradation.

CONCLUSION

A region encompassing a putative effector-binding domain (G1-Deg) is required for G alpha degradation via the N-end rule pathway. Our studies have shown that G alpha is susceptible to proteolysis soon after synthesis. These results are in agreement with the idea that G alpha is more unstable in the GTP-bound form, which is the predominant state of monomeric/free G alpha soon after synthesis. It is likely that the signal transduced by Gbetagamma can be regulated by adjusting the levels of G alpha through proteolysis.

Rad23 links DNA repair to the ubiquitin/proteasome pathway

Rad23 is an evolutionarily conserved protein that is important for nucleotide excision repair. A regulatory role has been proposed for Rad23 because rad23 mutants are sensitive to ultraviolet light but are still capable of incising damaged DNA. Here we show that Rad23 interacts with the 26S proteasome through an amino-terminal ubiquitin-like domain (UbL[R23]). The carboxy terminus of Rad23 binds to the Rad4 DNA repair protein and creates a link between the DNA repair and proteasome pathways. The ultraviolet sensitivity caused by deletion of the UbL(R23) domain may therefore arise from its inability to interact with the proteasome. The fusion proteins glutathione S-transferase (GST)-Rad23 and Rad4-haemagglutinin (HA), and the proteasome subunits Cim3 and Cim5, cofractionate through consecutive chromatography steps. The ubiquitin-like domain of human Rad23 (UbL[HRB]) also interacts with the human proteasome. These results demonstrate that ubiquitin-like domains (UbLs) represent a new class of proteasome-interacting motifs.

Degradation of G alpha by the N-end rule pathway

The N-end rule relates the in vivo half-life of a protein to the identity of its amino-terminal residue. Overexpression of targeting components of the N-end rule pathway in Saccharomyces cerevisiae inhibited the growth of haploid but not diploid cells. This ploidy-dependent toxicity was shown to result from enhanced degradation of Gpa1, the alpha subunit (G alpha) of a heterotrimeric guanine nucleotide-binding protein (G protein) that regulates cell differentiation in response to mating pheromones. Sst2, a protein whose absence renders cells hypersensitive to pheromone, was essential for degradation of G alpha but not other N-end rule substrates, suggesting the involvement of an indirect, or trans-, targeting mechanism. G alpha degradation by the N-end rule pathway adds another regulatory dimension to the multitude of signaling functions mediated by G proteins.

N-recognin/Ubc2 interactions in the N-end rule pathway

The N-end rule relates the in vivo half-life of a protein to the identity of its N-terminal residue. In the yeast Saccharomyces cerevisiae, substrates of the N-end rule pathway are targeted for degradation by a complex that includes the 225-kDa N-recognin, encoded by UBR1, and the 20-kDa ubiquitin-conjugating enzyme encoded by UBC2. We report that both physical stability and functional activity of the N-recognin.Ubc2 complex require the presence of a highly acidic 23-residue region at the C terminus of Ubc2. Ubc2-C88A, an inactive variant of Ubc2 in which the active-site Cys-88 has been replaced by Ala, is shown to retain the affinity for N-recognin. Expression of Ubc2-C88A inhibits the N-end rule pathway, apparently as a result of competition between Ubc2 and Ubc2-C88A for binding to N-recognin. The two-hybrid (interaction cloning) technique was used to identify a approximately 170-residue C-terminal fragment of the 1,950-residue N-recognin as a Ubc2-interacting domain. We also show that the level of UBR1 mRNA decreases upon overexpression of UBC2. This effect of UBC2 is observed with cells whose UBR1 is expressed from an unrelated promoter but is not observed if UBR1 contains a frameshift mutation, or if the Ubc2 protein lacks its C-terminal acidic region. The N-recognin.Ubc2 complex appears to regulate the expression of N-recognin through changes in the metabolic stability of its mRNA.

The N-end rule is mediated by the UBC2(RAD6) ubiquitin-conjugating enzyme

The N-end rule relates the in vivo half-life of a protein to the identity of its amino-terminal residue. Distinct versions of the N-end rule operate in all organisms examined, from mammals to bacteria. We show that UBC2(RAD6), one of at least seven ubiquitin-conjugating enzymes in the yeast Saccharomyces cerevisiae, is essential for multiubiquitination and degradation of the N-end rule substrates. We also show that UBC2 is physically associated with UBR1, the recognition component of the N-end rule pathway. These results indicate that some of the UBC2 functions, which include DNA repair, induced mutagenesis, sporulation, and regulation of retrotransposition, are mediated by protein degradation via the N-end rule pathway.

Transcript levels of the Saccharomyes cerevisiae DNA repair gene RAD23 increase in response to UV light and in meiosis but remain constant in the mitotic cell cycle

The RAD23 gene of Saccharomyces cerevisiae is required for excision-repair of UV damaged DNA. In this paper, we determine the location of the RAD23 gene in a cloned DNA fragment, identify the 1.6 kb RAD23 transcript, and examine RAD23 transcript levels in UV damaged cells, during the mitotic cell cycle, and in meiosis. The RAD23 mRNA levels are elevated 5-fold between 30 to 60 min after 37 J/m2 of UV light. RAD23 mRNA levels rise over 6-fold during meiosis at a stage coincident with high levels of genetic recombination. This response is specific to sporulation competent MATa/MAT alpha diploid cells, and is not observed in asporogenous MATa/MATa diploids. RAD23 mRNA levels, however, remain constant during the mitotic cell cycle.