SUMMARY The UCH37 deubiquitylase functions in two large and very different complexes, the 26S proteasome and the INO80 chromatin remodeler. We have performed biochemical characterization and determined crystal structures of UCH37 in complexes with RPN13 and NFRKB, which mediate its recruitment to proteasome and INO80, respectively. RPN13 and NFRKB make similar contacts to the UCH37 C-terminal domain, but quite different contacts to the catalytic UCH domain. RPN13 can activate UCH37 by disrupting dimerization, although physiologically-relevant activation likely results from stabilization of a surface competent for ubiquitin binding and modulation of the active-site crossover loop. In contrast, NFRKB inhibits UCH37 by blocking the ubiquitin-binding site and by disrupting the enzyme active site. These findings reveal remarkable commonality in mechanisms of recruitment, yet very different mechanisms of regulating enzyme activity, and provide a foundation for understanding the role of UCH37 in the unrelated proteasome and INO80 complexes.

The 26S proteasome is a large cellular assembly that mediates the selective degradation of proteins in the nucleus and cytosol and is an established target for anticancer therapeutics. Protein substrates are typically targeted to the proteasome through modification with a polyubiquitin chain, which can be recognized by several proteasome-associated ubiquitin receptors. One of these receptors, RPN13/ADRM1, is recruited to the proteasome through direct interaction with the large scaffolding protein RPN2 within the 19S regulatory particle. To better understand the interactions between RPN13, RPN2, and ubiquitin, we used human proteins to map the RPN13-binding epitope to the C-terminal 14 residues of RPN2, which, like ubiquitin, binds the N-terminal pleckstrin-like receptor of ubiquitin (PRU) domain of RPN13. We also report the crystal structures of the RPN13 PRU domain in complex with peptides corresponding to the RPN2 C terminus and ubiquitin. Through mutational analysis, we validated the RPN2-binding interface revealed by our structures and quantified binding interactions with surface plasmon resonance and fluorescence polarization. In contrast to a previous report, we find that RPN13 binds ubiquitin with an affinity similar to that of other proteasome-associated ubiquitin receptors and that RPN2, ubiquitin, and the deubiquitylase UCH37 bind to RPN13 with independent energetics. These findings provide a detailed characterization of interactions that are important for proteasome function, indicate ubiquitin affinities that are consistent with the role of RPN13 as a proteasomal ubiquitin receptor, and have major implications for the development of novel anticancer therapeutics.

TrbB, a periplasmic protein encoded by the conjugative plasmid F, has a predicted thioredoxin-like fold and possesses a C-X-X-C redox active site motif. TrbB may function in the conjugative process by serving as a disulfide bond isomerase, facilitating proper folding of a subset of F-plasmid-encoded proteins in the periplasm. Previous studies have demonstrated that a ⌬trbB F plasmid in Escherichia coli lacking DsbC E.coli , its native disulfide bond isomerase, experiences a 10-fold decrease in mating efficiency but have not provided direct evidence for disulfide bond isomerase activity. Here we demonstrate that trbB can partially restore transfer of a variant of the distantly related R27 plasmid when both chromosomal and plasmid genes encoding disulfide bond isomerases have been disrupted. In addition, we show that TrbB displays both disulfide bond isomerase and reductase activities on substrates not involved in the conjugative process. Unlike canonical members of the disulfide bond isomerase family, secondary structure predictions suggest that TrbB lacks both an N-terminal dimerization domain and an ␣-helical domain found in other disulfide bond isomerases. Phylogenetic analyses support the conclusion that TrbB belongs to a unique family of plasmid-based disulfide isomerases. Interestingly, although TrbB diverges structurally from other disulfide bond isomerases, we show that like those isomerases, TrbB relies on DsbD from E. coli for maintenance of its C-X-X-C redox active site motif.

Bacterial conjugation is the process by which a conjugative plasmid transfers from donor to recipient bacterium. During this process, single-stranded plasmid DNA is actively and specifically transported from the cytoplasm of the donor, through a large membrane-spanning assembly known as the pore complex, and into the cytoplasm of the recipient. In Gram negative bacteria, construction of the pore requires localization of a subset of structural and catalytically active proteins to the bacterial periplasm. Unlike the cytoplasm, the periplasm contains proteins that promote disulfide bond formation within or between cysteine-containing proteins. To ensure proper protein folding and assembly, bacteria employ periplasmic redox systems for thiol oxidation, disulfide bond/sulfenic acid reduction, and disulfide bond isomerization. Recent data suggest that plasmid-based proteins belonging to the disulfide bond formation family play an integral role in the conjugative process by serving as mediators in folding and/or assembly of pore complex proteins. Here we report the identification of 165 thioredoxin-like family members across 89 different plasmid systems. Using phylogenetic analysis, all but nine family members were categorized into thioredoxin-like subfamilies. In addition, we discuss the diversity, conservation, and putative roles of thioredoxin-like proteins in plasmid systems, which include homologs of DsbA, DsbB, DsbC, DsbD, DsbG, and CcmG from Escherichia coli, TlpA from Bradyrhizobium japonicum, Com1 from Coxiella burnetii, as well as TrbB and TraF from plasmid F, and the absolute conservation of a disulfide isomerase in plasmids containing homologs of the transfer proteins TraH, TraN, and TraU.

TraI, the F plasmid-encoded nickase, is a 1756 amino acid protein essential for conjugative transfer of plasmid DNA from one bacterium to another. While crystal structures of N- and C-terminal domains of F TraI have been determined, central domains of the protein are structurally unexplored. The central region (between residues 306 and 1520) is known to both bind single-stranded DNA (ssDNA) and unwind DNA through a highly processive helicase activity. Here, we show that the ssDNA binding site is located between residues 381 and 858, and we also present the high-resolution solution structure of the N-terminus of this region (residues 381 to 569). This fragment folds into a four-strand parallel β sheet surrounded by α helices, and resembles the structure of the N-terminus of helicases such as RecD and RecQ despite little sequence similarity. The structure supports the model that F TraI resulted from duplication of a RecD-like domain and subsequent specialization of domains into the more N-terminal ssDNA binding domain and the more C-terminal domain containing helicase motifs. In addition, we provide evidence that the nickase and ssDNA binding domains of TraI are held close together by an 80-residue linker sequence that connects the two domains. These results suggest a possible physical explanation for the apparent negative cooperativity between the nickase and ssDNA binding domain.

In the original version of the above article, Table 2 reported that the UCH37(M148A/F149A)/RPN13 mutant complex has kinetic parameters indistinguishable from those of unbound UCH37. Further investigation of the UCH37 M148A/F149A construct revealed a gene duplication event that resulted in expression of a truncated protein product. Experiments repeated with the correct UCH37 M148A/F149A construct, in addition to a UCH37 M148D/F149D double mutant, reveal only a slight effect on the kinetic parameters. These results therefore suggest a minor role for these residues in the activation of UCH37 and in subsequent turnover of the Ub-AMC substrate. All other conclusions derived from Table 2 remain unaltered by this correction. An updated Table 2 is printed below and will be corrected in the online version of the article. The correct plasmids encoding UCH37 M148A/F149A and M148D/F149D have been submitted to the Addgene plasmid repository. The list of plasmids used in this study has been updated in the original article's Supplemental Information.

Protein substrates are targeted to the 26S proteasome through several ubiquitin receptors. One of these receptors, RPN13, is recruited to the proteasome by binding of its N-terminal pleckstrin-like receptor of ubiquitin (PRU) domain to C-terminal residues of the scaffolding protein RPN2. The RPN13 PRU domain is followed by a flexible linker and a C-terminal deubiquitylase adaptor (DEUBAD) domain, which recruits and activates the deubiquitylase UCH37. Both RPN13 and UCH37 have been implicated in human cancers, and inhibitors of the RPN2–RPN13 interaction are being developed as potential therapeutic anticancer agents. Our current study builds on the recognition that a residue central to the RPN2–RPN13 interaction, RPN2 Tyr-950, is phosphorylated in Jurkat cells. We found that the Tyr-950 phosphorylation enhances binding to RPN13. The crystal structure of the RPN2–RPN13 pTyr-950–ubiquitin complex was determined at 1.76-Å resolution and reveals specific interactions with positively charged side chains in RPN13 that explain how phosphorylation increases binding affinity without inducing conformational change. Mutagenesis and quantitative binding assays were then used to validate the crystallographic interface. Our findings support a model in which RPN13 recruitment to the proteasome is enhanced by phosphorylation of RPN2 Tyr-950, have important implications for efforts to develop specific inhibitors of the RPN2–RPN13 interaction, and suggest the existence of a previously unknown stress-response pathway.

Many medical school postdoctoral fellows (postdocs) lack training in curriculum design and student-centered instruction. A team of bioscience postdocs and a medical school curriculum assistant dean co-created an experience to fill this gap. Kern's and Kirkpatrick's frameworks were used for the design and evaluation, respectively, of both the postdoc experience and the undergraduate course they developed. Postdocs taught the course using student-centered methods, especially team-based learning and Just-in-Time Teaching. Following a successful pilot phase, this low resource postdoc experience and undergraduate course are regularly offered. Participating postdocs develop the knowledge, skills, and attitudes to effectively participate in medical school education.