Explore the vast range of titles available.

The eukaryotic RNA exosome processes and degrades RNA by directing substrates to the distributive or processive 3′ to 5′ exoribonuclease activities of Rrp6 or Rrp44, respectively. The non-catalytic nine-subunit exosome core (Exo9) features a prominent central channel. Although RNA can pass through the channel to engage Rrp44, it is not clear how RNA is directed to Rrp6 or whether Rrp6 uses the central channel. Here we report a 3.3 Å crystal structure of a ten-subunit RNA exosome complex from Saccharomyces cerevisiae composed of the Exo9 core and Rrp6 bound to single-stranded poly(A) RNA. The Rrp6 catalytic domain rests on top of the Exo9 S1/KH ring above the central channel, the RNA 3′ end is anchored in the Rrp6 active site, and the remaining RNA traverses the S1/KH ring in an opposite orientation to that observed in a structure of a Rrp44-containing exosome complex. Solution studies with human and yeast RNA exosome complexes suggest that the RNA path to Rrp6 is conserved and dependent on the integrity of the S1/KH ring. Although path selection to Rrp6 or Rrp44 is stochastic in vitro , the fate of a particular RNA may be determined in vivo by the manner in which cofactors present RNA to the RNA exosome. The exosome complex contains two catalytic subunits which degrade RNA in either a distributive (Rrp6) or a processive (Rrp44) manner—previous structures indicated how RNA could be directed to Rrp44, but the path taken to Rrp6 was unclear; here the location of the Rrp6 catalytic domain and the RNA 3′ end are determined and it is found that the RNA lies in an opposite orientation from that of the Rrp44-containing exosome structure, suggesting that the fate of an RNA may be influenced by the manner in which cofactors present it. RNA fate rests on a flip The exosome complex contains two catalytic subunits that degrade RNA in either a distributive (Rrp6) or a processive (Rrp44) manner. Previous structures indicated how RNA could be directed to Rrp44, but the path taken to Rrp6 was unclear. Christopher Lima and colleagues have now solved the crystal structure of a yeast ten-subunit exosome complex (lacking Rrp44) bound to single-stranded RNA. The location of the Rrp6 catalytic domain and the RNA 3' end are determined, but more importantly, it is found that the RNA lies in an orientation opposite that from the Rrp44-containing exosome structure. This result suggests that the fate of an RNA may be a stochastic decision based on how the RNA binds to the complex.

ISG15 plays a crucial role in the innate immune response and has been well-studied due to its antiviral activity and regulation of signal transduction, apoptosis, and autophagy. ISG15 is a ubiquitin-like protein that is activated by an E1 enzyme (Uba7) and transferred to a cognate E2 enzyme (UBE2L6) to form a UBE2L6-ISG15 intermediate that functions with E3 ligases that catalyze conjugation of ISG15 to target proteins. Despite its biological importance, the molecular basis by which Uba7 catalyzes ISG15 activation and transfer to UBE2L6 is unknown as there is no available structure of Uba7. Here, we present cryo-EM structures of human Uba7 in complex with UBE2L6, ISG15 adenylate, and ISG15 thioester intermediate that are poised for catalysis of Uba7-UBE2L6-ISG15 thioester transfer. Our structures reveal a unique overall architecture of the complex compared to structures from the ubiquitin conjugation pathway, particularly with respect to the location of ISG15 thioester intermediate. Our structures also illuminate the molecular basis for Uba7 activities and for its exquisite specificity for ISG15 and UBE2L6. Altogether, our structural, biochemical, and human cell-based data provide significant insights into the functions of Uba7, UBE2L6, and ISG15 in cells. ISGylation plays a crucial role in the innate immune response and requires sequential activity of E1, E2, and E3 enzymes. Here, the authors present cyro-EM structures that reveal the molecular mechanisms underlying ISG15 activation by the E1 enzyme Uba7 and transfer to its cognate E2 enzyme UBE2L6.

The E1 enzyme Uba6 initiates signal transduction by activating ubiquitin and the ubiquitin-like protein FAT10 in a two-step process involving sequential catalysis of adenylation and thioester bond formation. To gain mechanistic insights into these processes, we determined the crystal structure of a human Uba6/ubiquitin complex. Two distinct architectures of the complex are observed: one in which Uba6 adopts an open conformation with the active site configured for catalysis of adenylation, and a second drastically different closed conformation in which the adenylation active site is disassembled and reconfigured for catalysis of thioester bond formation. Surprisingly, an inositol hexakisphosphate (InsP6) molecule binds to a previously unidentified allosteric site on Uba6. Our structural, biochemical, and biophysical data indicate that InsP6 allosterically inhibits Uba6 activity by altering interconversion of the open and closed conformations of Uba6 while also enhancing its stability. In addition to revealing the molecular mechanisms of catalysis by Uba6 and allosteric regulation of its activities, our structures provide a framework for developing Uba6-specific inhibitors and raise the possibility of allosteric regulation of other E1s by naturally occurring cellular metabolites. Uba6 is an E1 enzyme that regulates numerous cellular processes by activating ubiquitin and FAT10 pathways. Here, the authors present crystal structures that illuminate Uba6 catalytic mechanisms and reveal inositol hexakisphosphate as a cofactor that modulates Uba6 activity.

UBA1 and UBA6 define parallel ubiquitin (Ub) activation systems that perform non-overlapping roles in Ub and ubiquitin-like protein (Ubl) signaling. Whereas UBA1 supports the canonical Ub pathway, UBA6 also activates the Ubl FAT10, linking Ub signaling to immune-regulated proteostasis. In addition to selective Ub/Ubl activation, UBA1 and UBA6 engage distinct sets of E2s, yet how these enzymes achieve selective E2 engagement has remained unclear. Using chemical trapping and high-resolution cryo-EM, we determine four structures of UBA6–E2 complexes representing the thioester-transfer step with either FAT10 or Ub, revealing how this E1 distinguishes its cognate partners. UBA6 achieves E2 specificity through coordinated contributions of the UFD and SCCH domains, a dual-domain mechanism that contrasts with the UFD-dominated selectivity of UBA1. The structures further show that an existing inositol hexakisphosphate (InsP₆)–binding site, unique to UBA6, stabilizes an expanded SCCH cleft that pre-organizes the enzyme for selective engagement of UBA6-specific E2s. These findings define principles for E1–E2 recognition and identify InsP₆ as a cofactor shaping specificity within the Ub-like conjugation network. Specificity in ubiquitin (Ub) and Ub-like protein signaling is essential. Here, the authors use cryo-EM to show how UBA6 selectively engages its cognate E2 for dual Ub and FAT10 transfer, revealing a role for an InsP₆-binding site and illuminating molecular rules governing pathway specificity.

Nuclear RNA exosomes catalyze a range of RNA processing and decay activities that are coordinated in part by cofactors, including Mpp6, Rrp47, and the Mtr4 RNA helicase. Mpp6 interacts with the nine-subunit exosome core, while Rrp47 stabilizes the exoribonuclease Rrp6 and recruits Mtr4, but it is less clear if these cofactors work together. Using biochemistry with Saccharomyces cerevisiae proteins, we show that Rrp47 and Mpp6 stimulate exosome-mediated RNA decay, albeit with unique dependencies on elements within the nuclear exosome. Mpp6-exosomes can recruit Mtr4, while Mpp6 and Rrp47 each contribute to Mtr4-dependent RNA decay, with maximal Mtr4-dependent decay observed with both cofactors. The 3.3 Å structure of a twelve-subunit nuclear Mpp6 exosome bound to RNA shows the central region of Mpp6 bound to the exosome core, positioning its Mtr4 recruitment domain next to Rrp6 and the exosome central channel. Genetic analysis reveals interactions that are largely consistent with our model.

The androgen receptor (AR) is a type I nuclear hormone receptor and the primary drug target in prostate cancer due to its role as a lineage survival factor in prostate luminal epithelium. In prostate cancer, the AR cistrome is reprogrammed relative to normal prostate epithelium and particularly in cancers driven by oncogenic ETS fusion genes. The molecular basis for this change has remained elusive. Using purified proteins, we report a minimal cell-free system that demonstrates interdomain cooperativity between the ligand (LBD) and DNA binding domains (DBD) of AR, and its autoinhibition by the N terminus of AR. Furthermore, we identify ERG as a cofactor that activates AR’s ability to bind DNA in both high and lower affinity contexts through direct interaction within a newly identified AR-interacting motif (AIM) in the ETS domain, independent of ERG’s own DNA binding ability. Finally, we present evidence that this interaction is conserved among ETS factors whose expression is altered in prostate cancer. Our work highlights, at a biochemical level, how tumor-initiating ETS translocations result in reprogramming of the AR cistrome.

Homologous recombination (HR) fulfils a pivotal role in the repair of DNA double-strand breaks and collapsed replication forks 1 . HR depends on the products of several paralogues of RAD51 , including the tetrameric complex of RAD51B, RAD51C, RAD51D and XRCC2 (BCDX2) 2 . BCDX2 functions as a mediator of nucleoprotein filament assembly by RAD51 and single-stranded DNA (ssDNA) during HR, but its mechanism remains undefined. Here we report cryogenic electron microscopy reconstructions of human BCDX2 in apo and ssDNA-bound states. The structures reveal how the amino-terminal domains of RAD51B, RAD51C and RAD51D participate in inter-subunit interactions that underpin complex formation and ssDNA-binding specificity. Single-molecule DNA curtain analysis yields insights into how BCDX2 enhances RAD51–ssDNA nucleoprotein filament assembly. Moreover, our cryogenic electron microscopy and functional analyses explain how RAD51C alterations found in patients with cancer 3 , 4 , 5 – 6 inactivate DNA binding and the HR mediator activity of BCDX2. Our findings shed light on the role of BCDX2 in HR and provide a foundation for understanding how pathogenic alterations in BCDX2 impact genome repair. Analyses of the structure and biochemical properties of the tetrameric complex of RAD51B, RAD51C, RAD51D and XRCC2 reveal details of its role in the repair of DNA double-strand breaks.

Quality control requires discrimination between functional and aberrant species to selectively target aberrant substrates for destruction. Nuclear RNA quality control in Saccharomyces cerevisiae includes the TRAMP complex that marks RNA for decay via polyadenylation followed by helicase-dependent 3′ to 5′ degradation by the RNA exosome. Using reconstitution biochemistry, we show that polyadenylation and helicase activities of TRAMP cooperate with processive and distributive exoribonuclease activities of the nuclear RNA exosome to protect stable RNA from degradation while selectively targeting and degrading less stable RNA. Substrate discrimination is lost when the distributive exoribonuclease activity of Rrp6 is inactivated, leading to degradation of stable and unstable RNA species. These data support a proofreading mechanism in which deadenylation by Rrp6 competes with Mtr4-dependent degradation to protect stable RNA while selectively targeting and degrading unstable RNA.

The exoribonuclease Rrp6p is critical for RNA decay in the nucleus. While Rrp6p acts on a large range of diverse substrates, it does not indiscriminately degrade all RNAs. How Rrp6p accomplishes this task is not understood. Here, we measure Rrp6p–RNA binding and degradation kinetics in vitro at single-nucleotide resolution and find an intrinsic substrate selectivity that enables Rrp6p to discriminate against specific RNAs. RNA length and the four 3′-terminal nucleotides contribute most to substrate selectivity and collectively enable Rrp6p to discriminate between different RNAs by several orders of magnitude. The most pronounced discrimination is seen against RNAs ending with CCA-3′. These RNAs correspond to 3′ termini of uncharged tRNAs, which are not targeted by Rrp6p in cells. The data show that in contrast to many other proteins that use substrate selectivity to preferentially interact with specific RNAs, Rrp6p utilizes its selectivity to discriminate against specific RNAs. This ability allows Rrp6p to target diverse substrates while avoiding a subset of RNAs.

The licensing step of DNA double-strand break repair by homologous recombination entails resection of DNA ends to generate a single-stranded DNA template for assembly of the repair machinery consisting of the RAD51 recombinase and ancillary factors 1 . DNA end resection is mechanistically intricate and reliant on the tumour suppressor complex BRCA1–BARD1 (ref. 2 ). Specifically, three distinct nuclease entities—the 5′–3′ exonuclease EXO1 and heterodimeric complexes of the DNA endonuclease DNA2, with either the BLM or WRN helicase—act in synergy to execute the end resection process 3 . A major question concerns whether BRCA1–BARD1 directly regulates end resection. Here, using highly purified protein factors, we provide evidence that BRCA1–BARD1 physically interacts with EXO1, BLM and WRN. Importantly, with reconstituted biochemical systems and a single-molecule analytical tool, we show that BRCA1–BARD1 upregulates the activity of all three resection pathways. We also demonstrate that BRCA1 and BARD1 harbour stand-alone modules that contribute to the overall functionality of BRCA1–BARD1. Moreover, analysis of a BARD1 mutant impaired in DNA binding shows the importance of this BARD1 attribute in end resection, both in vitro and in cells. Thus, BRCA1–BARD1 enhances the efficiency of all three long-range DNA end resection pathways during homologous recombination in human cells. Using highly purified protein factors, we provide evidence that BRCA1–BARD1 physically interacts with EXO1, BLM and WRN and upregulates the activity of all three resection pathways.