α-Helix mimetics: outwards and upwards.

α-Helices are common secondary structural elements forming key parts of the large, generally featureless interfacial regions of many therapeutically-relevant protein-protein interactions (PPIs). The rational design of helix mimetics is an appealing small-molecule strategy for the mediation of aberrant PPIs, however the first generation of scaffolds presented a relatively small number of residues on a single recognition surface. Increasingly, helices involved in PPIs are found to have more complex binding modes, utilizing two or three recognition surfaces, or binding with extended points of contact. To address these unmet needs the design and synthesis of new generations of multi-sided, extended, and supersecondary structures are underway.

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.

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.

The Saccharomyces cerevisiae DNA repair gene RAD2 is regulated in meiosis but not during the mitotic cell cycle.

The expression of the RAD2 gene of Saccharomyces cerevisiae is elevated upon DNA damage. Here, we show that RAD2 transcript levels also rise approximately eightfold during meiosis but remain constant during the mitotic cell cycle. The period of maximal RAD2 mRNA accumulation during meiosis is consistent with a possible role of RAD2 in a late stage of recombination, in mismatch repair of heteroduplexes, or both.

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.

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.

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.

Inhibition of the HIF1α-p300 interaction by quinone- and indandione-mediated ejection of structural Zn(II).

Protein-protein interactions between the hypoxia inducible factor (HIF) and the transcriptional coactivators p300/CBP are potential cancer targets due to their role in the hypoxic response. A natural product based screen led to the identification of indandione and benzoquinone derivatives that reduce the tight interaction between a HIF-1α fragment and the CH1 domain of p300. The indandione derivatives were shown to fragment to give ninhydrin, which was identified as the active species. Both the naphthoquinones and ninhydrin were observed to induce Zn(II) ejection from p300 and the catalytic domain of the histone demethylase KDM4A. Together with previous reports on the effects of related compounds on HIF-1α and other systems, the results suggest that care should be taken in interpreting biological results obtained with highly electrophilic/thiol modifying compounds.

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.

Use and effectiveness of buddy support in a self-help smoking cessation program.

Purpose. This study assesses buddy support in a community-based, minimal-contact smoking cessation program. Design. Telephone interviews with participants (n=641, response=74%) before and after (end-of-program, n=1,023, response=83%; three months n=757, response=74%; six months, n=859, response=84%; and 12 months, n=713, response=70%) intervention provided the data to be analyzed. Setting. The Chicago metropolitan area was the setting. Subjects. Subjects were a random sample of registrants for the intervention program. Intervention. A self-help smoking cessation program was used, which included a manual and complementary televised segments. Engaging a buddy was optional. Measures. Background and psychosocial characteristics of participants, characteristics of buddies, program compliance, and smoking behavior were the measures used. Results. Almost one third (30.3%) engaged a buddy. Those most likely to engage a buddy were female (33.4%), younger than 30 (37.2%), educated beyond high school (33.4%), highly determined to quit (41.8%), and more likely to need help from others (39.8%). More than half of the buddies were from outside the participant's household (55.1%), and more than half were nonsmokers (60.9%). Having a buddy was associated positively with manual use (gamma=.38), viewing televised segments (gamma=.23), recalling manual segments (gamma=.33), and recalling televised segments (gamma=.26). Among those who read the manual least, having a buddy was associated with viewing televised segments (gamma=.26, p less than .05) and with end-of-program quitting (16.8% vs. 9.8%, p less than .05). Having a buddy also was associated with higher abstinence through 12 months (5.8% vs. 2.7%, p=.013). Among those with lower determination, the end-of-program quit rate was more than three times greater (p=.013) for those with a buddy (16.1%) than without a buddy (5.2%). Participants whose buddy was their spouse or partner were more likely to quit at end-of-program (29.1% vs. 18.4%, p=.031). Conclusions. Buddy support should be promoted as an adjunct to minimal-contact smoking cessation programs. Impact of buddy support might be improved by guiding participants in choosing a buddy.

Nucleotide sequence, transcript mapping, and regulation of the RAD2 gene of Saccharomyces cerevisiae.

We determined the nucleotide sequence, mapped the 5' and 3' mRNA termini, and examined the regulation of the RAD2 gene of Saccharomyces cerevisiae. A long open reading frame within the RAD2 transcribed region encodes a protein of 1,031 amino acids with a calculated molecular weight of 117,847. A disruption of the RAD2 gene that deletes the 78 carboxyl terminal codons results in loss of RAD2 function. The 5' ends of RAD2 mRNA show considerable heterogeneity, mapping 5 to 62 nucleotides upstream of the first ATG codon of the long RAD2 open reading frame. The longest RAD2 transcripts also contain a short open reading frame of 37 codons that precedes and overlaps the 5' end of the long RAD2 open reading frame. The RAD2 3' mRNA end maps 171 nucleotides downstream of the TAA termination codon and 20 nucleotides downstream from a 12-base-pair inverted repeat that might function in transcript termination. Northern blot analysis showed a ninefold increase in steady-state levels of RAD2 mRNA after treatment of yeast cells with UV light. The 5' flanking region of the RAD2 gene contains several direct and inverted repeats and a 44-nucleotide-long purine-rich tract. The sequence T G G A G G C A T T A A found at position -167 to -156 in the RAD2 gene is similar to a sequence present in the 5' flanking regions of the RAD7 and RAD10 genes.

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.

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.

Expression of the Saccharomyces cerevisiae DNA repair gene RAD6 that encodes a ubiquitin conjugating enzyme, increases in response to DNA damage and in meiosis but remains constant during the mitotic cell cycle.

The RAD6 gene of Saccharomyces cerevisiae encodes a ubiquitin-conjugating (E2) enzyme and is required for the repair of damaged DNA, mutagenesis, and sporulation. Here, we report our studies on the regulation of RAD6 gene expression after UV damage, during the mitotic cell cycle, in meiosis, and following heat shock and starvation. RAD6 mRNA levels became elevated in cells exposed to UV light, and at all UV doses the increase in mRNA levels was rapid and occurred within 30 min after exposure to UV. RAD6 mRNA levels also increased in sporulating MATa/MAT alpha cells, and the period of maximal accumulation of RAD6 mRNA during meiosis is coincident with the time during which recombination occurs. However, RAD6 mRNA levels showed no periodic fluctuation in the mitotic cell cycle, were not elevated upon heat shock, and fell in cells in the stationary phase of growth. These observations suggest that RAD6 activity is required throughout the cell cycle rather than being restricted to a specific stage, and that during meiosis, high levels of RAD6 activity may be needed at a stage coincident with genetic recombination. The observation that RAD6 transcription is not induced by heat and starvation, treatments that activate stress responses, suggests that the primary role of RAD6 is in the repair of damaged DNA rather than in adapting cells to stress situations.

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.

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.

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.