Molecular architecture of the 26S proteasome holocomplex determined by an integrative approach

Probing structures of large protein complexes using zero-length cross-linking

Structural mass spectrometry (MS) is a field with growing applicability for addressing complex biophysical questions regarding proteins and protein complexes. One of the major structural MS approaches involves the use of chemical cross-linking coupled with MS analysis (CX-MS) to identify proximal sites within macromolecules. Identified cross-linked sites can be used to probe novel protein-protein interactions or the derived distance constraints can be used to verify and refine molecular models. This review focuses on recent advances of "zero-length" cross-linking. Zero-length cross-linking reagents do not add any atoms to the cross-linked species due to the lack of a spacer arm. This provides a major advantage in the form of providing more precise distance constraints as the cross-linkable groups must be within salt bridge distances in order to react. However, identification of cross-linked peptides using these reagents presents unique challenges. We discuss recent efforts by our group to minimize these challenges by using multiple cycles of LC-MS/MS analysis and software specifically developed and optimized for identification of zero-length cross-linked peptides. Representative data utilizing our current protocol are presented and discussed.

xTract: software for characterizing conformational changes of protein complexes by quantitative cross-linking mass spectrometry

Chemical cross-linking in combination with mass spectrometry generates distance restraints of amino acid pairs in close proximity on the surface of native proteins and protein complexes. In this study we used quantitative mass spectrometry and chemical cross-linking to quantify differences in cross-linked peptides obtained from complexes in spatially discrete states. We describe a generic computational pipeline for quantitative cross-linking mass spectrometry consisting of modules for quantitative data extraction and statistical assessment of the obtained results. We used the method to detect conformational changes in two model systems: firefly luciferase and the bovine TRiC complex. Our method discovers and explains the structural heterogeneity of protein complexes using only sparse structural information.

ATP binding to neighbouring subunits and intersubunit allosteric coupling underlie proteasomal ATPase function

The primary functions of the proteasome are driven by a highly allosteric ATPase complex. ATP binding to only two subunits in this hexameric complex triggers substrate binding, ATPase–20S association and 20S gate opening. However, it is unclear how ATP binding and hydrolysis spatially and temporally coordinates these allosteric effects to drive substrate translocation into the 20S. Here, we use FRET to show that the proteasomal ATPases from eukaryotes (RPTs) and archaea (PAN) bind ATP with high affinity at neighbouring subunits, which complements the well-established spiral-staircase topology of the 26S ATPases. We further show that two conserved arginine fingers in PAN located at the subunit interface work together as a single allosteric unit to mediate the allosteric effects of ATP binding, without altering the nucleotide-binding pattern. Rapid kinetics analysis also shows that ring resetting of a sequential hydrolysis mechanism can be explained by thermodynamic equilibrium binding of ATP. These data support a model whereby these two functionally distinct allosteric networks cooperate to translocate polypeptides into the 20S for degradation.

The 26S proteasome contains a hexamer of ATPase subunits, which binds, unfolds and translocates substrates in an ATP-dependent manner. Kim et al. use FRET to show that ATP binding preferentially occurs at neighbouring subunits of the hexamer, and identify two allosteric systems that coordinate translocation.

CSNAP Is a Stoichiometric Subunit of the COP9 Signalosome

The highly conserved COP9 signalosome (CSN) complex is a key regulator of all cullin-RING-ubiquitin ligases (CRLs), the largest family of E3 ubiquitin ligases. Until now, it was accepted that the CSN is composed of eight canonical components. Here, we report the discovery of an additional integral and stoichiometric subunit that had thus far evaded detection, and we named it CSNAP (CSN acidic protein). We show that CSNAP binds CSN3, CSN5, and CSN6, and its incorporation into the CSN complex is mediated through the C-terminal region involving conserved aromatic residues. Moreover, depletion of this small protein leads to reduced proliferation and a flattened and enlarged morphology. Finally, on the basis of sequence and structural properties shared by both CSNAP and DSS1, a component of the related 19S lid proteasome complex, we propose that CSNAP, the ninth CSN subunit, is the missing paralogous subunit of DSS1.

A strategy for dissecting the architectures of native macromolecular assemblies

It remains particularly problematic to define the structures of native macromolecular assemblies, which are often of low abundance. Here we present a strategy for isolating complexes at endogenous levels from GFP-tagged transgenic cell lines. Using cross-linking mass spectrometry, we extracted distance restraints that allowed us to model the complexes' molecular architectures.

Molecular architecture of the yeast Mediator complex

The 21-subunit Mediator complex transduces regulatory information from enhancers to promoters, and performs an essential role in the initiation of transcription in all eukaryotes. Structural information on two-thirds of the complex has been limited to coarse subunit mapping onto 2-D images from electron micrographs. We have performed chemical cross-linking and mass spectrometry, and combined the results with information from X-ray crystallography, homology modeling, and cryo-electron microscopy by an integrative modeling approach to determine a 3-D model of the entire Mediator complex. The approach is validated by the use of X-ray crystal structures as internal controls and by consistency with previous results from electron microscopy and yeast two-hybrid screens. The model shows the locations and orientations of all Mediator subunits, as well as subunit interfaces and some secondary structural elements. Segments of 20–40 amino acid residues are placed with an average precision of 20 Å. The model reveals roles of individual subunits in the organization of the complex.

DOI:http://dx.doi.org/10.7554/eLife.08719.001

Inside a cell, proteins are made from instructions encoded by DNA. To produce a particular protein, a section of DNA within a gene is copied into a molecule of messenger ribonucleic acid (or mRNA). This process is called transcription and is carried out by an enzyme known as RNA polymerase.

Transcription begins in a region of DNA called a promoter, which is found at the start of the gene. RNA polymerase is brought to the DNA by many proteins, including the so-called Mediator complex. Mediator receives signals from within the cell and from the environment, processes the information, and instructs RNA polymerase whether to transcribe the gene or not. Mediator performs this important role in all organisms from yeast to humans, but it is not clear how it works. A crucial step towards the solution of this problem is to understand the three-dimensional structure of the complex.

Previous research using a technique called ‘electron microscopy’ showed that Mediator is composed of three modules, referred to as Head, Middle and Tail. The images from electron microscopy were not sufficiently detailed to reveal the organization of the proteins within these modules. An open-source Integrative Modeling Platform (IMP for short) was recently developed to arrive at structural models of large protein complexes from a combination of experimental data and computer models. Now, Robinson, Trnka, Pellarin et al. have used this platform to study the Mediator complex.

First, Robinson, Trnka, Pellarin et al. collected experimental data on the structure of the Mediator complex using two approaches called ‘chemical cross-linking’ and ‘mass spectrometry’. This data was combined with biochemical and structural information from previous studies to generate a three-dimensional model of the structure of the entire Mediator using IMP. The model is detailed enough to show the location and orientation of all the proteins in the complex. For example, a protein called Med17 connects the Head and Middle modules, while another subunit—known as Med14—spans the entire complex and makes extensive contacts with other proteins in all three modules.

DOI:http://dx.doi.org/10.7554/eLife.08719.002

Conformation and Dynamics of the Troponin I C-Terminal Domain: Combining Single-Molecule and Computational Approaches for a Disordered Protein Region

In recent years, single-molecule Förster resonance energy transfer (smFRET) has emerged as a critical and flexible tool in structural biology, particularly in the study of highly dynamic regions and molecular assemblies. The usefulness of smFRET can be further extended by combining it with computational approaches, marrying the coarse-grained experimental data with higher-resolution in silico calculations. Here we use smFRET to determine six pairwise distances within the intrinsically disordered C-terminal domain of the troponin I subunit (TnIC) of the cardiac troponin complex. We used published conflicting structures of TnIC as starting models for molecular dynamics simulations, which were validated through successful comparison with smFRET measurements before extracting information on conformational dynamics. We find that pairwise distances between residues fluctuate widely in silico, but simulations are generally in good agreement with longer time scale smFRET measurements after averaging across time. Finally, Monte Carlo simulations establish that the lower-energy conformers of TnIC are indeed varied, but that the highest-sampled clusters resemble the published, conflicting models. In this way, we find that the controversial structures are simply stabilized local minima of this dynamic region, and a population including all three would still be consistent with spectroscopic measurements. Taken together, the combined approaches described here allow us to critically evaluate existing models of TnIC, giving insight into the conformation and dynamics of TnIC's disordered state prior to its probable disorder-order transition. Moreover, they provide a framework for combining computational and experimental methods with different time scales for the study of disordered and dynamic protein states.

Coevolutionary analyses require phylogenetically deep alignments and better null models to accurately detect inter-protein contacts within and between species

Background

When biomolecules physically interact, natural selection operates on them jointly. Contacting positions in protein and RNA structures exhibit correlated patterns of sequence evolution due to constraints imposed by the interaction, and molecular arms races can develop between interacting proteins in pathogens and their hosts. To evaluate how well methods developed to detect coevolving residues within proteins can be adapted for cross-species, inter-protein analysis, we used statistical criteria to quantify the performance of these methods in detecting inter-protein residues within 8 angstroms of each other in the co-crystal structures of 33 bacterial protein interactions. We also evaluated their performance for detecting known residues at the interface of a host-virus protein complex with a partially solved structure.

Results

Our quantitative benchmarking showed that all coevolutionary methods clearly benefit from alignments with many sequences. Methods that aim to detect direct correlations generally outperform other approaches. However, faster mutual information based methods are occasionally competitive in small alignments and with relaxed false positive rates. Two commonly used null distributions are anti-conservative and have high false positive rates in some scenarios, although the empirical distribution of scores performs reasonably well with deep alignments.

Conclusions

We conclude that coevolutionary analysis of cross-species protein interactions holds great promise but requires sequencing many more species pairs.

Electronic supplementary material

The online version of this article (doi:10.1186/s12859-015-0677-y) contains supplementary material, which is available to authorized users.

Large oligomeric complex structures can be computationally assembled by efficiently combining docked interfaces

Macromolecular oligomeric assemblies are involved in many biochemical processes of living organisms. The benefits of such assemblies in crowded cellular environments include increased reaction rates, efficient feedback regulation, cooperativity and protective functions. However, an atom-level structural determination of large assemblies is challenging due to the size of the complex and the difference in binding affinities of the involved proteins. In this study, we propose a novel combinatorial greedy algorithm for assembling large oligomeric complexes from information on the approximate position of interaction interfaces of pairs of monomers in the complex. Prior information on complex symmetry is not required but rather the symmetry is inferred during assembly. We implement an efficient geometric score, the transformation match score, that bypasses the model ranking problems of state-of-the-art scoring functions by scoring the similarity between the inferred dimers of the same monomer simultaneously with different binding partners in a (sub)complex with a set of pregenerated docking poses. We compiled a diverse benchmark set of 308 homo and heteromeric complexes containing 6 to 60 monomers. To explore the applicability of the method, we considered 48 sets of parameters and selected those three sets of parameters, for which the algorithm can correctly reconstruct the maximum number, namely 252 complexes (81.8%) in, at least one of the respective three runs. The crossvalidation coverage, that is, the mean fraction of correctly reconstructed benchmark complexes during crossvalidation, was 78.1%, which demonstrates the ability of the presented method to correctly reconstruct topology of a large variety of biological complexes.

Cryo-EM: A Unique Tool for the Visualization of Macromolecular Complexity

3D cryo-electron microscopy (cryo-EM) is an expanding structural biology technique that has recently undergone a quantum leap progression in its achievable resolution and its applicability to the study of challenging biological systems. Because crystallization is not required, only small amounts of sample are needed, and because images can be classified in a computer, the technique has the potential to deal with compositional and conformational mixtures. Therefore, cryo-EM can be used to investigate complete and fully functional macromolecular complexes in different functional states, providing a richness of biological insight. In this review, we underlie some of the principles behind the cryo-EM methodology of single particle analysis and discuss some recent results of its application to challenging systems of paramount biological importance. We place special emphasis on new methodological developments that are leading to an explosion of new studies, many of which are reaching resolutions that could only be dreamed of just a couple of years ago.

Chemical cross-linking and native mass spectrometry: A fruitful combination for structural biology

Mass spectrometry (MS) is becoming increasingly popular in the field of structural biology for analyzing protein three-dimensional-structures and for mapping protein-protein interactions. In this review, the specific contributions of chemical crosslinking and native MS are outlined to reveal the structural features of proteins and protein assemblies. Both strategies are illustrated based on the examples of the tetrameric tumor suppressor protein p53 and multisubunit vinculin-Arp2/3 hybrid complexes. We describe the distinct advantages and limitations of each technique and highlight synergistic effects when both techniques are combined. Integrating both methods is especially useful for characterizing large protein assemblies and for capturing transient interactions. We also point out the future directions we foresee for a combination of in vivo crosslinking and native MS for structural investigation of intact protein assemblies.

Redundant Roles of Rpn10 and Rpn13 in Recognition of Ubiquitinated Proteins and Cellular Homeostasis

Intracellular proteins tagged with ubiquitin chains are targeted to the 26S proteasome for degradation. The two subunits, Rpn10 and Rpn13, function as ubiquitin receptors of the proteasome. However, differences in roles between Rpn10 and Rpn13 in mammals remains to be understood. We analyzed mice deficient for Rpn13 and Rpn10. Liver-specific deletion of either Rpn10 or Rpn13 showed only modest impairment, but simultaneous loss of both caused severe liver injury accompanied by massive accumulation of ubiquitin conjugates, which was recovered by re-expression of either Rpn10 or Rpn13. We also found that mHR23B and ubiquilin/Plic-1 and -4 failed to bind to the proteasome in the absence of both Rpn10 and Rpn13, suggesting that these two subunits are the main receptors for these UBL-UBA proteins that deliver ubiquitinated proteins to the proteasome. Our results indicate that Rpn13 mostly plays a redundant role with Rpn10 in recognition of ubiquitinated proteins and maintaining homeostasis in Mus musculus.

At least two major ubiquitin receptor subunits that directly capture ubiquitin chains have been identified in the proteasome: Rpn10 and Rpn13. Analyses in Saccharomyces cerevisiae have suggested only a modest role of Rpn10 and Rpn13 in the recruitment of ubiquitinated proteins, as double deletion of Rpn10 and Rpn13 causes very mild phenotypes. Considering that ubiquitin recognition is an essential process for protein degradation by the proteasome and that failure in degradation of ubiquitinated proteins leads to human diseases such as neurodegeneration, it is important to evaluate the role of Rpn10 and Rpn13 in mammals. Liver-specific deletion of either Rpn10 or Rpn13 showed modest impairment, but simultaneous loss of both Rpn10 and Rpn13 caused severe liver injury accompanied by massive accumulation of ubiquitin conjugates and failure in recruiting mHR23B and ubiquilin/Plic-1 and -4 proteins, which deliver ubiquitinated proteins to the proteasome. Our findings indicate that the largely redundant roles of Rpn10 and Rpn13 in ubiquitin recognition and recruitment of mHR23B and ubiquilin/Plic-1 and -4 are essential for cellular homeostasis in mammals and should provide information for understanding the mechanism of ubiquitin recognition by the 26S proteasome in mammals and for development of therapeutic agents targeting protein degradation.

Structural disorder and its role in proteasomal degradation

The ubiquitin proteasome system is responsible for the controlled degradation of a vast number of intracellular proteins. It targets misfolded or otherwise aberrant proteins as well as proteins no longer needed at a given point in time. The 26S proteasome is a large macromolecular machine comprising 33 distinct subunits as well as a number of transiently associating cofactors. Being essentially a non-specific protease, specificity is conferred by the ubiquitin system, which selects and marks substrates for degradation. Here, we review our current understanding of the structure and function of the 26S proteasome; in doing so we highlight the role of disordered protein regions. Disordered segments in substrates promote their degradation, whereas low complexity regions prevent their proteolysis. In the 26S proteasome itself a main role of disordered segments seems to be rendering the ubiquitin receptors mobile, possibly supporting recruitment of polyubiquitylated substrates. Thus, these structural features of substrates as well as of the 26S proteasome itself likely play important roles at different stages of the protein degradation process.

N-Terminal Coiled-Coil Structure of ATPase Subunits of 26S Proteasome Is Crucial for Proteasome Function

The proteasome is an essential proteolytic machine in eukaryotic cells, where it removes damaged proteins and regulates many cellular activities by degrading ubiquitinated proteins. Its heterohexameric AAA+ ATPase Rpt subunits play a central role in proteasome activity by the engagement of substrate unfolding and translocation for degradation; however, its detailed mechanism remains poorly understood. In contrast to AAA+ ATPase domains, their N-terminal regions of Rpt subunits substantially differ from each other. Here, to investigate the requirements and roles of the N-terminal regions of six Rpt subunits derived from Saccharomyces cerevisiae, we performed systematic mutational analysis using conditional knockdown yeast strains for each Rpt subunit and bacterial heterologous expression system of the base subcomplex. We showed that the formation of the coiled-coil structure was the most important for the N-terminal region of Rpt subunits. The primary role of coiled-coil structure would be the maintenance of the ring structure with the defined order. However, the coiled-coil region would be also be involved in substrate recognition and an interaction between lid and base subcomplexes.

Moonlighting and pleiotropy within two regulators of the degradation machinery: the proteasome lid and the CSN

The distinction between pleiotrotic and moonlighting roles of proteins is challenging; however, this distinction may be clearer when it comes to multiprotein complexes. Two examples are the proteasome lid and the COP9 signalosome (CSN), which are twin enzymes with 1:1 paralogy between subunits. In each complex, one out of eight subunits harbours a JAMM/MPN⁺ metalloprotease motif. This motif contributes the canonical activity of each complex: hydrolysis of covalently attached ubiquitin by Rpn11 in the proteasome lid and hydrolysis of ubiquitin-related 1 (Rub1/Nedd8) from Cullins by Csn5 in the CSN. In both complexes, executing this activity suggests pleiotropic effects and requires an assembled full complex. However, beyond canonical functions, both Rpn11 and Csn5 are involved in additional unique, complex-independent functions, herein referred to as moonlighting activities.

Validation methods for low-resolution fitting of atomic structures to electron microscopy data

Fitting of atomic-resolution structures into reconstructions from electron cryo-microscopy is routinely used to understand the structure and function of macromolecular machines. Despite the fact that a plethora of fitting methods has been developed over recent years, standard protocols for quality assessment and validation of these fits have not been established. Here, we present the general concepts underlying current validation ideas as they relate to fitting of atomic-resolution models into electron cryo-microscopy reconstructions, with an emphasis on reconstructions with resolutions below the sub-nanometer range.

Diversity of COP9 signalosome structures and functional consequences

The COP9 signalosome (CSN) is a regulator of the ubiquitin (Ub) proteasome system (UPS). It interacts with hundreds of cullin-RING ubiquitin E3 ligases (CRLs) and regulates their activity by removing the Ub-like protein Nedd8 from cullins. In mammalian cells 7 different cullins exist which form CRLs with adaptor proteins and with a large number of substrate recognition subunits such as F-box and BTB proteins. This large variety of CRL-complexes is deneddylated by the CSN. The capacity of the CSN to interact with numerous types of CRL complexes can be explained by its structural diversity, which allows different CSN variants to interact with different binding partners and substrates and enables different subunit expression profiles. Diversity of CSN complexes presumably occurs by: (1) flexibility of CSN holo complex structure; (2) formation of CSN mini complexes and free CSN subunits and (3) generation of CSN variants via integration of CSN subunit isoforms. In this review we will discuss the structural diversity of the CSN complex and possible functional consequences.

Thiostrepton interacts covalently with Rpt subunits of the 19S proteasome and proteasome substrates

Here, we report a novel mechanism of proteasome inhibition mediated by Thiostrepton (Thsp), which interacts covalently with Rpt subunits of the 19S proteasome and proteasome substrates. We identified Thsp in a cell-based high-throughput screen using a fluorescent reporter sensitive to degradation by the ubiquitin-proteasome pathway. Thiostrepton behaves as a proteasome inhibitor in several paradigms, including cell-based reporters, detection of global ubiquitination status, and proteasome-mediated labile protein degradation. In vitro, Thsp does not block the chymotrypsin activity of the 26S proteasome. In a cell-based IκBα degradation assay, Thsp is a slow inhibitor and 4 hrs of treatment achieves the same effects as MG-132 at 30 min. We show that Thsp forms covalent adducts with proteins in human cells and demonstrate their nature by mass spectrometry. Furthermore, the ability of Thsp to interact covalently with the cysteine residues is essential for its proteasome inhibitory function. We further show that a Thsp modified peptide cannot be degraded by proteasomes in vitro. Importantly, we demonstrate that Thsp binds covalently to Rpt subunits of the 19S regulatory particle and forms bridges with a proteasome substrate. Taken together, our results uncover an important role of Thsp in 19S proteasome inhibition.

Protein structure prediction guided by crosslinking restraints--A systematic evaluation of the impact of the crosslinking spacer length

Recent development of high-resolution mass spectrometry (MS) instruments enables chemical crosslinking (XL) to become a high-throughput method for obtaining structural information about proteins. Restraints derived from XL-MS experiments have been used successfully for structure refinement and protein-protein docking. However, one formidable question is under which circumstances XL-MS data might be sufficient to determine a protein's tertiary structure de novo? Answering this question will not only include understanding the impact of XL-MS data on sampling and scoring within a de novo protein structure prediction algorithm, it must also determine an optimal crosslinker type and length for protein structure determination. While a longer crosslinker will yield more restraints, the value of each restraint for protein structure prediction decreases as the restraint is consistent with a larger conformational space. In this study, the number of crosslinks and their discriminative power was systematically analyzed in silico on a set of 2055 non-redundant protein folds considering Lys-Lys, Lys-Asp, Lys-Glu, Cys-Cys, and Arg-Arg reactive crosslinkers between 1 and 60Å. Depending on the protein size a heuristic was developed that determines the optimal crosslinker length. Next, simulated restraints of variable length were used to de novo predict the tertiary structure of fifteen proteins using the BCL::Fold algorithm. The results demonstrate that a distinct crosslinker length exists for which information content for de novo protein structure prediction is maximized. The sampling accuracy improves on average by 1.0 Å and up to 2.2 Å in the most prominent example. XL-MS restraints enable consistently an improved selection of native-like models with an average enrichment of 2.1.

xVis: a web server for the schematic visualization and interpretation of crosslink-derived spatial restraints

The identification of crosslinks by mass spectrometry has recently been established as an integral part of the hybrid structural analysis of protein complexes and networks. The crosslinking analysis determines distance restraints between two covalently linked amino acids which are typically summarized in a table format that precludes the immediate and comprehensive interpretation of the topological data. xVis displays crosslinks in clear schematic representations in form of a circular, bar or network diagram. The interactive graphs indicate the linkage sites and identification scores, depict the spatial proximity of structurally and functionally annotated protein regions and the evolutionary conservation of amino acids and facilitate clustering of proteins into subcomplexes according to the crosslink density. Furthermore, xVis offers two options for the qualitative assessment of the crosslink identifications by filtering crosslinks according to identification scores or false discovery rates and by displaying the corresponding fragment ion spectrum of each crosslink for the manual validation of the mass spectrometric data. Our web server provides an easy-to-use tool for the fast topological and functional interpretation of distance information on protein complex architectures and for the evaluation of crosslink fragment ion spectra. xVis is available under a Creative Commons Attribution-ShareAlike 4.0 International license at http://xvis.genzentrum.lmu.de/.

Evolution of proteasome regulators in eukaryotes

All living organisms require protein degradation to terminate biological processes and remove damaged proteins. One such machine is the 20S proteasome, a specialized barrel-shaped and compartmentalized multicatalytic protease. The activity of the 20S proteasome generally requires the binding of regulators/proteasome activators (PAs), which control the entrance of substrates. These include the PA700 (19S complex), which assembles with the 20S and forms the 26S proteasome and allows the efficient degradation of proteins usually labeled by ubiquitin tags, PA200 and PA28, which are involved in proteolysis through ubiquitin-independent mechanisms and PI31, which was initially identified as a 20S inhibitor in vitro. Unlike 20S proteasome, shown to be present in all Eukaryotes and Archaea, the evolutionary history of PAs remained fragmentary. Here, we made a comprehensive survey and phylogenetic analyses of the four types of regulators in 17 clades covering most of the eukaryotic supergroups. We found remarkable conservation of each PA700 subunit in all eukaryotes, indicating that the current complex PA700 structure was already set up in the last eukaryotic common ancestor (LECA). Also present in LECA, PA200, PA28, and PI31 showed a more contrasted evolutionary picture, because many lineages have subsequently lost one or two of them. The paramount conservation of PA700 composition in all eukaryotes and the dynamic evolution of PA200, PA28, and PI31 are discussed in the light of current knowledge on their physiological roles.

DockStar: a novel ILP-based integrative method for structural modeling of multimolecular protein complexes

MOTIVATION

Atomic resolution modeling of large multimolecular assemblies is a key task in Structural Cell Biology. Experimental techniques can provide atomic resolution structures of single proteins and small complexes, or low resolution data of large multimolecular complexes.

RESULTS

We present a novel integrative computational modeling method, which integrates both low and high resolution experimental data. The algorithm accepts as input atomic resolution structures of the individual subunits obtained from X-ray, NMR or homology modeling, and interaction data between the subunits obtained from mass spectrometry. The optimal assembly of the individual subunits is formulated as an Integer Linear Programming task. The method was tested on several representative complexes, both in the bound and unbound cases. It placed correctly most of the subunits of multimolecular complexes of up to 16 subunits and significantly outperformed the CombDock and Haddock multimolecular docking methods.

AVAILABILITY AND IMPLEMENTATION

http://bioinfo3d.cs.tau.ac.il/DockStar

CONTACT

naamaamir@mail.tau.ac.il or wolfson@tau.ac.il

SUPPLEMENTARY INFORMATION

Supplementary data are available at Bioinformatics online.

Single-Particle Cryo-EM at Crystallographic Resolution

Until only a few years ago, single-particle electron cryo-microscopy (cryo-EM) was usually not the first choice for many structural biologists due to its limited resolution in the range of nanometer to subnanometer. Now, this method rivals X-ray crystallography in terms of resolution and can be used to determine atomic structures of macromolecules that are either refractory to crystallization or difficult to crystallize in specific functional states. In this review, I discuss the recent breakthroughs in both hardware and software that transformed cryo-microscopy, enabling understanding of complex biomolecules and their functions at atomic level.

DNA damage modulates interactions between microRNAs and the 26S proteasome

26S proteasomes are known as major non-lysosomal cellular machines for coordinated and specific destruction of ubiquitinylated proteins. The proteolytic activities of proteasomes are controlled by various post-translational modifications in response to environmental cues, including DNA damage. Besides proteolysis, proteasomes also associate with RNA hydrolysis and splicing. Here, we extend the functional diversity of proteasomes by showing that they also dynamically associate with microRNAs (miRNAs) both in the nucleus and cytoplasm of cells. Moreover, DNA damage induced by an anti-cancer drug, doxorubicin, alters the repertoire of proteasome-associated miRNAs, enriching the population of miRNAs that target cell cycle checkpoint regulators and DNA repair proteins. Collectively, these data uncover yet another potential mode of action for proteasomes in the cell via their dynamic association with microRNAs.

Substrate degradation by the proteasome: a single-molecule kinetic analysis

To address how the configuration of conjugated ubiquitins determines the recognition of substrates by the proteasome, we analyzed the degradation kinetics of substrates with chemically defined ubiquitin configurations. Contrary to the view that a tetraubiquitin chain is the minimal signal for efficient degradation, we find that distributing the ubiquitins as diubiquitin chains provides a more efficient signal. To understand how the proteasome actually discriminates among ubiquitin configurations, we developed single-molecule assays that distinguished intermediate steps of degradation kinetically. The level of ubiquitin on a substrate drives proteasome-substrate interaction, whereas the chain structure of ubiquitin affects translocation into the axial channel on the proteasome. Together these two features largely determine the susceptibility of substrates for proteasomal degradation.

CyToStruct: Augmenting the Network Visualization of Cytoscape with the Power of Molecular Viewers

It can be informative to view biological data, e.g., protein-protein interactions within a large complex, in a network representation coupled with three-dimensional structural visualizations of individual molecular entities. CyToStruct, introduced here, provides a transparent interface between the Cytoscape platform for network analysis and molecular viewers, including PyMOL, UCSF Chimera, VMD, and Jmol. CyToStruct launches and passes scripts to molecular viewers from the network's edges and nodes. We provide demonstrations to analyze interactions among subunits in large protein/RNA/DNA complexes, and similarities among proteins. CyToStruct enriches the network tools of Cytoscape by adding a layer of structural analysis, offering all capabilities implemented in molecular viewers. CyToStruct is available at https://bitbucket.org/sergeyn/cytostruct/wiki/Home and in the Cytoscape App Store. Given the coordinates of a molecular complex, our web server (http://trachel-srv.cs.haifa.ac.il/rachel/ppi/) automatically generates all files needed to visualize the complex as a Cytoscape network with CyToStruct bridging to PyMOL, UCSF Chimera, VMD, and Jmol.

Unveiling Contacts within Macromolecular Assemblies by Solving Minimum Weight Connectivity Inference (MWC) Problems

Consider a set of oligomers listing the subunits involved in subcomplexes of a macromolecular assembly, obtained e.g. using native mass spectrometry or affinity purification. Given these oligomers, connectivity inference (CI) consists of finding the most plausible contacts between these subunits, and minimum connectivity inference (MCI) is the variant consisting of finding a set of contacts of smallest cardinality. MCI problems avoid speculating on the total number of contacts but yield a subset of all contacts and do not allow exploiting a priori information on the likelihood of individual contacts. In this context, we present two novel algorithms, MILP-W and MILP-WB. The former solves the minimum weight connectivity inference (MWCI), an optimization problem whose criterion mixes the number of contacts and their likelihood. The latter uses the former in a bootstrap fashion to improve the sensitivity and the specificity of solution sets.Experiments on three systems (yeast exosome, yeast proteasome lid, human eIF3), for which reference contacts are known (crystal structure, cryo electron microscopy, cross-linking), show that our algorithms predict contacts with high specificity and sensitivity, yielding a very significant improvement over previous work, typically a twofold increase in sensitivity.The software accompanying this paper is made available and should prove of ubiquitous interest whenever connectivity inference from oligomers is faced.

Trial Watch: Proteasomal inhibitors for anticancer therapy

The so-called "ubiquitin-proteasome system" (UPS) is a multicomponent molecular apparatus that catalyzes the covalent attachment of several copies of the small protein ubiquitin to other proteins that are generally (but not always) destined to proteasomal degradation. This enzymatic cascade is crucial for the maintenance of intracellular protein homeostasis (both in physiological conditions and in the course of adaptive stress responses), and regulates a wide array of signaling pathways. In line with this notion, defects in the UPS have been associated with aging as well as with several pathological conditions including cardiac, neurodegenerative, and neoplastic disorders. As transformed cells often experience a constant state of stress (as a result of the hyperactivation of oncogenic signaling pathways and/or adverse microenvironmental conditions), their survival and proliferation are highly dependent on the integrity of the UPS. This rationale has driven an intense wave of preclinical and clinical investigation culminating in 2003 with the approval of the proteasomal inhibitor bortezomib by the US Food and Drug Administration for use in multiple myeloma patients. Another proteasomal inhibitor, carfilzomib, is now licensed by international regulatory agencies for use in multiple myeloma patients, and the approved indications for bortezomib have been extended to mantle cell lymphoma. This said, the clinical activity of bortezomib and carfilzomib is often limited by off-target effects, innate/acquired resistance, and the absence of validated predictive biomarkers. Moreover, the antineoplastic activity of proteasome inhibitors against solid tumors is poor. In this Trial Watch we discuss the contribution of the UPS to oncogenesis and tumor progression and summarize the design and/or results of recent clinical studies evaluating the therapeutic profile of proteasome inhibitors in cancer patients.

The importance of dynamics in integrative modeling of supramolecular assemblies

Revealing the atomistic architecture of supramolecular complexes is a fundamental step toward a deeper understanding of cellular functioning. To date, this formidable task is facilitated by an emerging array of integrative modeling approaches that combine experimental data from different sources. One major challenge these methods have to face is the treatment of the dynamic rearrangements of the individual subunits upon assembly. While this flexibility can be sampled at different levels, integrating native dynamic determinants with available experimental inputs can provide an effective way to reveal the molecular recognition mechanisms at the basis of supramolecular assembly.

Maturation of the proteasome core particle induces an affinity switch that controls regulatory particle association

Proteasome assembly is a complex process, requiring 66 subunits distributed over several subcomplexes to associate in a coordinated fashion. Ten proteasome-specific chaperones have been identified that assist in this process. For two of these, the Pba1-Pba2 dimer, it is well established that they only bind immature core particles (CP) in vivo. In contrast, the regulatory particle (RP) utilizes the same binding surface but only interacts with the mature CP in vivo. It is unclear how these binding events are regulated. Here, we show that Pba1-Pba2 binds tightly to immature CP, preventing RP binding. Changes in the CP that occur upon maturation significantly reduce its affinity for Pba1-Pba2, enabling the RP to displace the chaperone. Mathematical modeling indicates that this “affinity switch” mechanism has likely evolved to improve assembly efficiency by preventing the accumulation of stable, non-productive intermediates. Our work thus provides mechanistic insights into a crucial step in proteasome biogenesis.

Proteasomes. A molecular census of 26S proteasomes in intact neurons

The 26S proteasome is a key player in eukaryotic protein quality control and in the regulation of numerous cellular processes. Here, we describe quantitative in situ structural studies of this highly dynamic molecular machine in intact hippocampal neurons. We used electron cryotomography with the Volta phase plate, which allowed high fidelity and nanometer precision localization of 26S proteasomes. We undertook a molecular census of single- and double-capped proteasomes and assessed the conformational states of individual complexes. Under the conditions of the experiment—that is, in the absence of proteotoxic stress—only 20% of the 26S proteasomes were engaged in substrate processing. The remainder was in the substrate-accepting ground state. These findings suggest that in the absence of stress, the capacity of the proteasome system is not fully used.

Xlink Analyzer: software for analysis and visualization of cross-linking data in the context of three-dimensional structures

Structural characterization of large multi-subunit protein complexes often requires integrating various experimental techniques. Cross-linking mass spectrometry (XL-MS) identifies proximal protein residues and thus is increasingly used to map protein interactions and determine the relative orientation of subunits within the structure of protein complexes. To fully adapt XL-MS as a structure characterization technique, we developed Xlink Analyzer, a software tool for visualization and analysis of XL-MS data in the context of the three-dimensional structures. Xlink Analyzer enables automatic visualization of cross-links, identifies cross-links violating spatial restraints, calculates violation statistics, maps chemically modified surfaces, and allows interactive manipulations that facilitate analysis of XL-MS data and aid designing new experiments. We demonstrate these features by mapping interaction sites within RNA polymerase I and the Rvb1/2 complex. Xlink Analyzer is implemented as a plugin to UCSF Chimera, a standard structural biology software tool, and thus enables seamless integration of XL-MS data with, e.g. fitting of X-ray structures to EM maps. Xlink Analyzer is available for download at http://www.beck.embl.de/XlinkAnalyzer.html.

xiNET: cross-link network maps with residue resolution

xiNET is a visualization tool for exploring cross-linking/mass spectrometry results. The interactive maps of the cross-link network that it generates are a type of node-link diagram. In these maps xiNET displays: (1) residue resolution positional information including linkage sites and linked peptides; (2) all types of cross-linking reaction product; (3) ambiguous results; and, (4) additional sequence information such as domains. xiNET runs in a browser and exports vector graphics which can be edited in common drawing packages to create publication quality figures.

AVAILABILITY

xiNET is open source, released under the Apache version 2 license. Results can be viewed by uploading data to http://crosslinkviewer.org/ or by downloading the software from http://github.com/colin-combe/crosslink-viewer and running it locally.

Solution structure of yeast Rpn9: insights into proteasome lid assembly

The regulatory particle (RP) of the 26 S proteasome functions in preparing polyubiquitinated substrates for degradation. The lid complex of the RP contains an Rpn8-Rpn11 heterodimer surrounded by a horseshoe-shaped scaffold formed by six proteasome-COP9/CSN-initiation factor (PCI)-containing subunits. The PCI domains are essential for lid assembly, whereas the detailed molecular mechanisms remain elusive. Recent cryo-EM studies at near-atomic resolution provided invaluable information on the RP architecture in different functional states. Nevertheless, atomic resolution structural information on the RP is still limited, and deeper understanding of RP assembly mechanism requires further studies on the structures and interactions of individual subunits or subcomplexes. Herein we report the high-resolution NMR structures of the PCI-containing subunit Rpn9 from Saccharomyces cerevisiae. The 45-kDa protein contains an all-helical N-terminal domain and a C-terminal PCI domain linked via a semiflexible hinge. The N-terminal domain mediates interaction with the ubiquitin receptor Rpn10, whereas the PCI domain mediates interaction with the neighboring PCI subunit Rpn5. The Rpn9-Rpn5 interface highlights two structural motifs on the winged helix module forming a hydrophobic center surrounded by ionic pairs, which is a common pattern for all PCI-PCI interactions in the lid. The results suggest that divergence in surface composition among different PCI pairs may contribute to the modulation of lid assembly.

Base-CP proteasome can serve as a platform for stepwise lid formation

26S proteasome, a major regulatory protease in eukaryotes, consists of a 20S proteolytic core particle (CP) capped by a 19S regulatory particle (RP). The 19S RP is divisible into base and lid sub-complexes. Even within the lid, subunits have been demarcated into two modules: module 1 (Rpn5, Rpn6, Rpn8, Rpn9 and Rpn11), which interacts with both CP and base sub-complexes and module 2 (Rpn3, Rpn7, Rpn12 and Rpn15) that is attached mainly to module 1. We now show that suppression of RPN11 expression halted lid assembly yet enabled the base and 20S CP to pre-assemble and form a base-CP. A key role for Regulatory particle non-ATPase 11 (Rpn11) in bridging lid module 1 and module 2 subunits together is inferred from observing defective proteasomes in rpn11–m1, a mutant expressing a truncated form of Rpn11 and displaying mitochondrial phenotypes. An incomplete lid made up of five module 1 subunits attached to base-CP was identified in proteasomes isolated from this mutant. Re-introducing the C-terminal portion of Rpn11 enabled recruitment of missing module 2 subunits. In vitro, module 1 was reconstituted stepwise, initiated by Rpn11–Rpn8 heterodimerization. Upon recruitment of Rpn6, the module 1 intermediate was competent to lock into base-CP and reconstitute an incomplete 26S proteasome. Thus, base-CP can serve as a platform for gradual incorporation of lid, along a proteasome assembly pathway. Identification of proteasome intermediates and reconstitution of minimal functional units should clarify aspects of the inner workings of this machine and how multiple catalytic processes are synchronized within the 26S proteasome holoenzymes.

Defective proteasome 19S regulatory particles (RPs) were identified in rpn11f–m1, a proteasomal mutant with mitochondrial phenotypes. The Rpn11 subunit initiates assembly of a five-subunit lid module competent to integrate into pre-assembled base-20S core particle (CP), with subsequent recruitment of remaining lid subunits.

Deciphering preferential interactions within supramolecular protein complexes: the proteasome case

In eukaryotic cells, intracellular protein breakdown is mainly performed by the ubiquitin-proteasome system. Proteasomes are supramolecular protein complexes formed by the association of multiple sub-complexes and interacting proteins. Therefore, they exhibit a very high heterogeneity whose function is still not well understood. Here, using a newly developed method based on the combination of affinity purification and protein correlation profiling associated with high-resolution mass spectrometry, we comprehensively characterized proteasome heterogeneity and identified previously unknown preferential associations within proteasome sub-complexes. In particular, we showed for the first time that the two main proteasome subtypes, standard proteasome and immunoproteasome, interact with a different subset of important regulators. This trend was observed in very diverse human cell types and was confirmed by changing the relative proportions of both 20S proteasome forms using interferon-γ. The new method developed here constitutes an innovative and powerful strategy that could be broadly applied for unraveling the dynamic and heterogeneous nature of other biologically relevant supramolecular protein complexes.

Tools for macromolecular model building and refinement into electron cryo-microscopy reconstructions

The recent rapid development of single-particle electron cryo-microscopy (cryo-EM) now allows structures to be solved by this method at resolutions close to 3 Å. Here, a number of tools to facilitate the interpretation of EM reconstructions with stereochemically reasonable all-atom models are described. The BALBES database has been repurposed as a tool for identifying protein folds from density maps. Modifications to Coot, including new Jiggle Fit and morphing tools and improved handling of nucleic acids, enhance its functionality for interpreting EM maps. REFMAC has been modified for optimal fitting of atomic models into EM maps. As external structural information can enhance the reliability of the derived atomic models, stabilize refinement and reduce overfitting, ProSMART has been extended to generate interatomic distance restraints from nucleic acid reference structures, and a new tool, LIBG, has been developed to generate nucleic acid base-pair and parallel-plane restraints. Furthermore, restraint generation has been integrated with visualization and editing in Coot, and these restraints have been applied to both real-space refinement in Coot and reciprocal-space refinement in REFMAC.

Topological models of heteromeric protein assemblies from mass spectrometry: application to the yeast eIF3:eIF5 complex

Describing, understanding, and modulating the function of the cell require elucidation of the structures of macromolecular assemblies. Here, we describe an integrative method for modeling heteromeric complexes using as a starting point disassembly pathways determined by native mass spectrometry (MS). In this method, the pathway data and other available information are encoded as a scoring function on the positions of the subunits of the complex. The method was assessed on its ability to reproduce the native contacts in five benchmark cases with simulated MS data and two cases with real MS data. To illustrate the power of our method, we purified the yeast initiation factor 3 (eIF3) complex and characterized it by native MS and chemical crosslinking MS. We established substoichiometric binding of eIF5 and derived a model for the five-subunit eIF3 complex, at domain level, consistent with its role as a scaffold for other initiation factors.

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Integrative MS method allows topological characterization of heteromeric complexes

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Intersubunit crosslinks increase the precision of the predicted topologies

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A 3D model of eIF3:eIF5 complex was built using restraints from MS-based methods

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Integrative modeling reveals two submodules within eIF3: eIF3b:i:g and eIF3a:c

Harnessing proteasome dynamics and allostery in drug design

SIGNIFICANCE

The proteasome is the essential protease that is responsible for regulated cleavage of the bulk of intracellular proteins. Its central role in cellular physiology has been exploited in therapies against aggressive cancers where proteasome-specific competitive inhibitors that block proteasome active centers are very effectively used. However, drugs regulating this essential protease are likely to have broader clinical usefulness. The non-catalytic sites of the proteasome emerge as an attractive alternative target in search of highly specific and diverse proteasome regulators.

RECENT ADVANCES

Crystallographic models of the proteasome leave the false impression of fixed structures with minimal molecular dynamics lacking long-distance allosteric signaling. However, accumulating biochemical and structural observations strongly support the notion that the proteasome is regulated by precise allosteric interactions arising from protein dynamics, encouraging the active search for allosteric regulators. Here, we discuss properties of several promising compounds that affect substrate gating and processing in antechambers, and interactions of the catalytic core with regulatory proteins.

CRITICAL ISSUES

Given the structural complexity of proteasome assemblies, it is a painstaking process to better understand their allosteric regulation and molecular dynamics. Here, we discuss the challenges and achievements in this field. We place special emphasis on the role of atomic force microscopy imaging in probing the allostery and dynamics of the proteasome, and in dissecting the mechanisms involving small-molecule allosteric regulators.

FUTURE DIRECTIONS

New small-molecule allosteric regulators may become a next generation of drugs targeting the proteasome, which is critical to the development of new therapies in cancers and other diseases.

Characterizing the dynamics of proteasome complexes by proteomics approaches

SIGNIFICANCE

The proteasome is the degradation machine of the ubiquitin-proteasome system, which is critical in controlling many essential biological processes. Aberrant regulation of proteasome-dependent protein degradation can lead to various human diseases, and general proteasome inhibitors have shown efficacy for cancer treatments. Though clinically effective, current proteasome inhibitors have detrimental side effects and, thus, better therapeutic strategies targeting proteasomes are needed. Therefore, a comprehensive characterization of proteasome complexes will provide the molecular details that are essential for developing new and improved drugs.

RECENT ADVANCES

New mass spectrometry (MS)-based proteomics approaches have been developed to study protein interaction networks and structural topologies of proteasome complexes. The results have helped define the dynamic proteomes of proteasome complexes, thus providing new insights into the mechanisms underlying proteasome function and regulation.

CRITICAL ISSUES

The proteasome exists as heterogeneous populations in tissues/cells, and its proteome is highly dynamic and complex. In addition, proteasome complexes are regulated by various mechanisms under different physiological conditions. Consequently, complete proteomic profiling of proteasome complexes remains a major challenge for the field.

FUTURE DIRECTIONS

We expect that proteomic methodologies enabling full characterization of proteasome complexes will continue to evolve. Further advances in MS instrumentation and protein separation techniques will be needed to facilitate the detailed proteomic analysis of low-abundance components and subpopulations of proteasome complexes. The results will help us understand proteasome biology as well as provide new therapeutic targets for disease diagnostics and treatment.

Structure of mega-hemocyanin reveals protein origami in snails

Mega-hemocyanin is a 13.5 MDa oxygen transporter found in the hemolymph of some snails. Similar to typical gastropod hemocyanins, it is composed of 400 kDa building blocks but has additional 550 kDa subunits. Together, they form a large, completely filled cylinder. The structural basis for this highly complex protein packing is not known so far. Here, we report the electron cryomicroscopy (cryo-EM) structure of mega-hemocyanin complexes from two different snail species. The structures reveal that mega-hemocyanin is composed of flexible building blocks that differ in their conformation, but not in their primary structure. Like a protein origami, these flexible blocks are optimally packed, implementing different local symmetries and pseudosymmetries. A comparison between the two structures suggests a surprisingly simple evolutionary mechanism leading to these large oxygen transporters.

Disassembly of Lys11 and mixed linkage polyubiquitin conjugates provides insights into function of proteasomal deubiquitinases Rpn11 and Ubp6

Protein homeostasis is largely dependent on proteolysis by the ubiquitin-proteasome system. Diverse polyubiquitin modifications are reported to target cellular proteins to the proteasome. At the proteasome, deubiquitination is an essential preprocessing event that contributes to degradation efficiency. We characterized the specificities of two proteasome-associated deubiquitinases (DUBs), Rpn11 and Ubp6, and explored their impact on overall proteasome DUB activity. This was accomplished by constructing a panel of well defined ubiquitin (Ub) conjugates, including homogeneous linkages of varying lengths as well as a heterogeneously modified target. Rpn11 and Ubp6 processed Lys(11) and Lys(63) linkages with comparable efficiencies that increased with chain length. In contrast, processing of Lys(48) linkages by proteasome was inversely correlated to chain length. Fluorescently labeled tetra-Ub chains revealed endo-chain preference for Ubp6 acting on Lys(48) and random action for Rpn11. Proteasomes were more efficient at deconjugating identical substrates than their constituent DUBs by roughly 2 orders of magnitude. Incorporation into proteasomes significantly enhanced enzymatic efficiency of Rpn11, due in part to alleviation of the autoinhibitory role of its C terminus. The broad specificity of Rpn11 could explain how proteasomes were more effective at disassembling a heterogeneously modified conjugate compared with homogeneous Lys(48)-linked chains. The reduced ability to disassemble homogeneous Lys(48)-linked chains longer than 4 Ub units may prolong residency time on the proteasome.

Cross-talk between redox regulation and the ubiquitin-proteasome system in mammalian cell differentiation

BACKGROUND

Embryogenesis and stem cell differentiation are complex and orchestrated signaling processes. Reactive oxygen species (ROS) act as essential signal transducers in cellular differentiation, as has been shown through recent discoveries. On the other hand, the ubiquitin-proteasome system (UPS) has long been known to play an important role in all cellular regulated processes, including differentiation.

SCOPE OF REVIEW

In the present review, we focus on findings that highlight the interplay between redox signaling and the UPS regarding cell differentiation. Through systems biology analyses, we highlight major routes during cardiomyocyte differentiation based on redox signaling and UPS modulation.

MAJOR CONCLUSION

Oxygen availability and redox signaling are fundamental regulators of cell fate upon differentiation. The UPS plays an important role in the maintenance of pluripotency and the triggering of differentiation.

GENERAL SIGNIFICANCE

Cellular differentiation has been a matter of intense investigation mainly because of its potential therapeutic applications. Understanding regulatory mechanisms underlying cell differentiation is an important issue. Correspondingly, the role of UPS and regulation of redox processes have been emerged as essential factors to control the fate of cells upon differentiation. This article is part of a Special Issue entitled Redox regulation of differentiation and de-differentiation.

Determining protein complex structures based on a Bayesian model of in vivo Förster resonance energy transfer (FRET) data

The use of in vivo Förster resonance energy transfer (FRET) data to determine the molecular architecture of a protein complex in living cells is challenging due to data sparseness, sample heterogeneity, signal contributions from multiple donors and acceptors, unequal fluorophore brightness, photobleaching, flexibility of the linker connecting the fluorophore to the tagged protein, and spectral cross-talk. We addressed these challenges by using a Bayesian approach that produces the posterior probability of a model, given the input data. The posterior probability is defined as a function of the dependence of our FRET metric FRETR on a structure (forward model), a model of noise in the data, as well as prior information about the structure, relative populations of distinct states in the sample, forward model parameters, and data noise. The forward model was validated against kinetic Monte Carlo simulations and in vivo experimental data collected on nine systems of known structure. In addition, our Bayesian approach was validated by a benchmark of 16 protein complexes of known structure. Given the structures of each subunit of the complexes, models were computed from synthetic FRETR data with a distance root-mean-squared deviation error of 14 to 17 Å. The approach is implemented in the open-source Integrative Modeling Platform, allowing us to determine macromolecular structures through a combination of in vivo FRETR data and data from other sources, such as electron microscopy and chemical cross-linking.

Crystal structure of archaeal homolog of proteasome-assembly chaperone PbaA

Formation of the eukaryotic proteasome is not a spontaneous process but a highly ordered process assisted by several assembly chaperones. In contrast, archaeal proteasome subunits can spontaneously assemble into an active form. Recent bioinformatic analysis identified the proteasome-assembly chaperone-like proteins, PbaA and PbaB, in archaea. Our previous study showed that the PbaB homotetramer functions as a proteasome activator through its tentacle-like C-terminal segments. However, a functional role of the other homolog PbaA has remained elusive. Here we determined the 2.25-Å resolution structure of PbaA, illustrating its disparate tertiary and quaternary structures compared with PbaB. PbaA forms a homopentamer in which the C-terminal segments, with a putative proteasome-activating motif, are packed against the core. These findings offer deeper insights into the molecular evolution relationships between the proteasome-assembly chaperones and the proteasome activators.

¹H, ¹³C and ¹⁵N resonance assignments of the VWA domain of Saccharomyces cerevisiae Rpn10, a regulatory subunit of 26S proteasome

Rpn10 is a ubiquitin receptor of the 26S proteasome, and plays an important role in poly-ubiquitinated proteins recognition in the ubiquitin-proteasome protein degradation pathway. It is located in the 19S regulatory particle and interacts with several subunits of both lid and base complexes. Bioinformatics analysis of yeast Rpn10 suggests that it contains a von Willebrand (VWA domain) and a C-terminal tail containing a Ub-interacting motif. Studies of Saccharomyces cerevisiae Rpn10 suggested that its VWA domain might participate in interactions with subunit from both lid and base subcomplexes of the 19S regulatory particle. Herein, we report the chemical shift assignments of (1)H, (13)C and (15)N atoms of the VWA domain of S. cerevisiae Rpn10, which provide the basis for further structural and functional studies of Rpn10 by solution NMR technique.

Uncertainty in integrative structural modeling

Integrative structural modeling uses multiple types of input information and proceeds in four stages: (i) gathering information, (ii) designing model representation and converting information into a scoring function, (iii) sampling good-scoring models, and (iv) analyzing models and information. In the first stage, uncertainty originates from data that are sparse, noisy, ambiguous, or derived from heterogeneous samples. In the second stage, uncertainty can originate from a representation that is too coarse for the available information or a scoring function that does not accurately capture the information. In the third stage, the major source of uncertainty is insufficient sampling. In the fourth stage, clustering, cross-validation, and other methods are used to estimate the precision and accuracy of the models and information.