The anaphase-promoting complex/cyclosome (APC/C) is a 22S ubiquitin ligase complex that initiates chromosome segregation and mitotic exit. We have used biochemical and electron microscopic analyses of Saccharomyces cerevisiae and human APC/C to address how the APC/C subunit Doc1 contributes to recruitment and processive ubiquitylation of APC/C substrates, and to understand how APC/C monomers interact to form a 36S dimeric form. We show that Doc1 interacts with Cdc27, Cdc16 and Apc1 and is located in the vicinity of the cullin-RING module Apc2-Apc11 in the inner cavity of the APC/C. Substrate proteins also bind in the inner cavity, in close proximity to Doc1 and the coactivator Cdh1, and induce conformational changes in Apc2-Apc11. Our results suggest that substrates are recruited to the APC/C by binding to a bipartite substrate receptor composed of a coactivator protein and Doc1.

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/.

Kinetochores, multisubunit protein assemblies, connect chromosomes to spindle microtubules to promote chromosome segregation. The 10-subunit KMN assembly (comprising KNL1, MIS12, and NDC80 complexes, designated KNL1C, MIS12C, and NDC80C) binds microtubules and regulates mitotic checkpoint function through NDC80C and KNL1C, respectively. MIS12C, on the other hand, connects the KMN to the chromosome-proximal domain of the kinetochore through a direct interaction with CENP-C. The structural basis for this crucial bridging function of MIS12C is unknown. Here, we report crystal structures of human MIS12C associated with a fragment of CENP-C and unveil the role of Aurora B kinase in the regulation of this interaction. The structure of MIS12:CENP-C complements previously determined high-resolution structures of functional regions of NDC80C and KNL1C and allows us to build a near-complete structural model of the KMN assembly. Our work illuminates the structural organization of essential chromosome segregation machinery that is conserved in most eukaryotes.

The meiotic chromosome axis plays key roles in meiotic chromosome organization and recombination, yet the underlying protein components of this structure are highly diverged. Here, we show that 'axis core proteins' from budding yeast (Red1), mammals (SYCP2/SYCP3), and plants (ASY3/ASY4) are evolutionarily related and play equivalent roles in chromosome axis assembly. We first identify 'closure motifs' in each complex that recruit meiotic HORMADs, the master regulators of meiotic recombination. We next find that axis core proteins form homotetrameric (Red1) or heterotetrameric (SYCP2:SYCP3 and ASY3:ASY4) coiled-coil assemblies that further oligomerize into micron-length filaments. Thus, the meiotic chromosome axis core in fungi, mammals, and plants shares a common molecular architecture, and likely also plays conserved roles in meiotic chromosome axis assembly and recombination control.

Protein transport into the nucleus is mediated by transport receptors. Import of highly charged proteins, such as histone H1 and ribosomal proteins, requires a dimer of two transport receptors. In this study, we determined the cryo-EM structure of the Imp7:Impβ:H1.0 complex, showing that the two importins form a cradle that accommodates the linker histone. The H1.0 globular domain is bound to Impβ, whereas the acidic loops of Impβ and Imp7 chaperone the positively charged C-terminal tail. Although it remains disordered, the H1 tail serves as a zipper that closes and stabilizes the structure through transient non-specific interactions with importins. Moreover, we found that the GGxxF and FxFG motifs in the Imp7 C-terminal tail are essential for Imp7:Impβ dimerization and H1 import, resembling importin interaction with nucleoporins, which, in turn, promote complex disassembly. The architecture of many other complexes might be similarly defined by rapidly exchanging electrostatic interactions mediated by disordered regions.

Cell spreading requires the coupling of actin-driven membrane protrusion and integrin-mediated adhesion to the extracellular matrix. The integrin-activating adaptor protein kindlin-2 plays a central role for cell adhesion and membrane protrusion by directly binding and recruiting paxillin to nascent adhesions. Here, we report that kindlin-2 has a dual role during initial cell spreading: it binds paxillin via the pleckstrin homology and F0 domains to activate Rac1, and it directly associates with the Arp2/3 complex to induce Rac1-mediated membrane protrusions. Consistently, abrogation of kindlin-2 binding to Arp2/3 impairs lamellipodia formation and cell spreading. Our findings identify kindlin-2 as a key protein that couples cell adhesion by activating integrins and the induction of membrane protrusions by activating Rac1 and supplying Rac1 with the Arp2/3 complex.

Polycomb Repressive Complex 2 (PRC2) is essential for gene silencing, establishing transcriptional repression of specific genes by tri-methylating Lysine 27 of histone H3, a process mediated by cofactors such as AEBP2. In spite of its biological importance, little is known about PRC2 architecture and subunit organization. Here, we present the first three-dimensional electron microscopy structure of the human PRC2 complex bound to its cofactor AEBP2. Using a novel internal protein tagging-method, in combination with isotopic chemical cross-linking and mass spectrometry, we have localized all the PRC2 subunits and their functional domains and generated a detailed map of interactions. The position and stabilization effect of AEBP2 suggests an allosteric role of this cofactor in regulating gene silencing. Regions in PRC2 that interact with modified histone tails are localized near the methyltransferase site, suggesting a molecular mechanism for the chromatin-based regulation of PRC2 activity.DOI:http://dx.doi.org/10.7554/eLife.00005.001.

INO80/SWR1 family chromatin remodelers are complexes composed of >15 subunits and molecular masses exceeding 1 MDa. Their important role in transcription and genome maintenance is exchanging the histone variants H2A and H2A.Z. We report the architecture of S. cerevisiae INO80 using an integrative approach of electron microscopy, crosslinking and mass spectrometry. INO80 has an embryo-shaped head-neck-body-foot architecture and shows dynamic open and closed conformations. We can assign an Rvb1/Rvb2 heterododecamer to the head in close contact with the Ino80 Snf2 domain, Ies2, and the Arp5 module at the neck. The high-affinity nucleosome-binding Nhp10 module localizes to the body, whereas the module that contains actin, Arp4, and Arp8 maps to the foot. Structural and biochemical analyses indicate that the nucleosome is bound at the concave surface near the neck, flanked by the Rvb1/2 and Arp8 modules. Our analysis establishes a structural and functional framework for this family of large remodelers.

Kinetochores, multi-subunit complexes that assemble at the interface with centromeres, bind spindle microtubules to ensure faithful delivery of chromosomes during cell division. The configuration and function of the kinetochore-centromere interface is poorly understood. We report that a protein at this interface, CENP-M, is structurally and evolutionarily related to small GTPases but is incapable of GTP-binding and conformational switching. We show that CENP-M is crucially required for the assembly and stability of a tetramer also comprising CENP-I, CENP-H, and CENP-K, the HIKM complex, which we extensively characterize through a combination of structural, biochemical, and cell biological approaches. A point mutant affecting the CENP-M/CENP-I interaction hampers kinetochore assembly and chromosome alignment and prevents kinetochore recruitment of the CENP-T/W complex, questioning a role of CENP-T/W as founder of an independent axis of kinetochore assembly. Our studies identify a single pathway having CENP-C as founder, and CENP-H/I/K/M and CENP-T/W as CENP-C-dependent followers.DOI: http://dx.doi.org/10.7554/eLife.02978.001.

In the microRNA (miRNA) pathway, Dicer processes precursors to mature miRNAs. For efficient processing, double-stranded RNA-binding proteins support Dicer proteins. In flies, Loquacious (Loqs) interacts with Dicer1 (dmDcr1) to facilitate miRNA processing. Here, we have solved the structure of the third double-stranded RNA-binding domain (dsRBD) of Loqs and define specific structural elements that interact with dmDcr1. In addition, we show that the linker preceding dsRBD3 contributes significantly to dmDcr1 binding. Furthermore, our structural work demonstrates that the third dsRBD of Loqs forms homodimers. Mutations in the dimerization interface abrogate dmDcr1 interaction. Loqs, however, binds to dmDcr1 as a monomer using the identified dimerization surface, which suggests that Loqs might form dimers under conditions where dmDcr1 is absent or not accessible. Since critical sequence elements are conserved, we suggest that dimerization might be a general feature of dsRBD proteins in gene silencing.

The number of publications in the field of chemical cross-linking combined with mass spectrometry (XL-MS) to derive constraints for protein three-dimensional structure modeling and to probe protein-protein interactions has increased during the last years. As the technique is now becoming routine for in vitro and in vivo applications in proteomics and structural biology there is a pressing need to define protocols as well as data analysis and reporting formats. Such consensus formats should become accepted in the field and be shown to lead to reproducible results. This first, community-based harmonization study on XL-MS is based on the results of 32 groups participating worldwide. The aim of this paper is to summarize the status quo of XL-MS and to compare and evaluate existing cross-linking strategies. Our study therefore builds the framework for establishing best practice guidelines to conduct cross-linking experiments, perform data analysis, and define reporting formats with the ultimate goal of assisting scientists to generate accurate and reproducible XL-MS results.

Chromatin structure and function are determined by a plethora of proteins whose genome-wide distribution is typically assessed by immunoprecipitation (ChIP). Here, we developed a novel tool to investigate the local chromatin environment at specific DNA sequences. We combined the programmable DNA binding of dCas9 with the promiscuous biotin ligase BirA* (CasID) to biotinylate proteins in the direct vicinity of specific loci. Subsequent streptavidin-mediated precipitation and mass spectrometry identified both known and previously unknown chromatin factors associated with repetitive telomeric, major satellite and minor satellite DNA. With super-resolution microscopy, we confirmed the localization of the putative transcription factor ZNF512 at chromocenters. The versatility of CasID facilitates the systematic elucidation of functional protein complexes and locus-specific chromatin composition.

The molecular understanding of cellular processes requires the identification and characterization of the involved protein complexes. Affinity-purification and mass spectrometric analysis (AP-MS) are performed on a routine basis to detect proteins assembled in complexes. In particular, protein abundances obtained by quantitative mass spectrometry and direct protein contacts detected by crosslinking and mass spectrometry (XL-MS) provide complementary datasets for revealing the composition, topology and interactions of modules in a protein network. Here, we aim to combine quantitative and connectivity information by a webserver tool in order to infer protein complexes. In a first step, modeling protein abundances and functional annotations from Gene Ontology (GO) results in a network which, in a second step, is integrated with connectivity data from XL-MS analysis in order to complement and validate the protein complexes in the network. The output of our integrative approach is a quantitative protein interaction map which is supplemented with topological information of the detected protein complexes. compleXView is built up by two independent modules which are dedicated to the analysis of label-free AP-MS data and to the visualization of the detected complexes in a network together with crosslink-derived distance restraints. compleXView is available to all users without login requirements at http://xvis.genzentrum.lmu.de/compleXView.

Kinetochores are macromolecular protein assemblies at centromeres that mediate accurate chromosome segregation during cell division. The outer kinetochore KNL1SPC105, MIS12MTW1, and NDC80NDC80 complexes assemble the KMN network, which harbors the sites of microtubule binding and spindle assembly checkpoint signaling. The buildup of the KMN network that transmits microtubule pulling forces to budding yeast point centromeres is poorly understood. Here, we identify 225 inter-protein crosslinks by mass spectrometry on KMN complexes isolated from Saccharomyces cerevisiae that delineate the KMN subunit connectivity for outer kinetochore assembly. C-Terminal motifs of Nsl1 and Mtw1 recruit the SPC105 complex through Kre28, and both motifs aid tethering of the NDC80 complex by the previously reported Dsn1 C terminus. We show that a hub of three C-terminal MTW1 subunit motifs mediates the cooperative stabilization of the KMN network, which is augmented by a direct NDC80-SPC105 association.

Kinetochores form the link between chromosomes and microtubules of the mitotic spindle. The heterodecameric Dam1 complex (Dam1c) is a major component of the Saccharomyces cerevisiae outer kinetochore, assembling into 3 MDa-sized microtubule-embracing rings, but how ring assembly is specifically initiated in vivo remains to be understood. Here, we describe a molecular pathway that provides local control of ring assembly during the establishment of sister kinetochore bi-orientation. We show that Dam1c and the general microtubule plus end-associated protein (+TIP) Bim1/EB1 form a stable complex depending on a conserved motif in the Duo1 subunit of Dam1c. EM analyses reveal that Bim1 crosslinks protrusion domains of adjacent Dam1c heterodecamers and promotes the formation of oligomers with defined curvature. Disruption of the Dam1c-Bim1 interaction impairs kinetochore localization of Dam1c in metaphase and delays mitosis. Phosphorylation promotes Dam1c-Bim1 binding by relieving an intramolecular inhibition of the Dam1 C-terminus. In addition, Bim1 recruits Bik1/CLIP-170 to Dam1c and induces formation of full rings even in the absence of microtubules. Our data help to explain how new kinetochore end-on attachments are formed during the process of attachment error correction.

Accurate chromosome segregation during cell division is crucial for propagating life and protects from cellular transformation. The SKAP:Astrin heterodimer localizes to spindle microtubules and to mature microtubule-kinetochore attachments during mitosis. Depletion of either subunit disrupts spindle structure and destabilizes kinetochore-microtubule attachments. Here, we identify molecular requirements for the inter-subunit interaction of SKAP and Astrin, and discuss requirements for their kinetochore recruitment. We also identify and characterize a microtubule-binding domain in SKAP, distinct from the SXIP motif that mediates end binding (EB) protein binding and plus end tracking, and show that it stimulates the growth-rate of microtubules, possibly through a direct interaction with tubulin. Mutations targeting this microtubule-binding domain impair microtubule plus-end tracking but not kinetochore targeting, and recapitulate many effects observed during depletion of SKAP. Collectively, our studies represent the first thorough mechanistic analysis of SKAP and Astrin, and significantly advance our functional understanding of these important mitotic proteins.

Kinetochores are macromolecular assemblies that connect chromosomes to spindle microtubules (MTs) during mitosis. The metazoan-specific ≈800-kD ROD-Zwilch-ZW10 (RZZ) complex builds a fibrous corona that assembles on mitotic kinetochores before MT attachment to promote chromosome alignment and robust spindle assembly checkpoint signaling. In this study, we combine biochemical reconstitutions, single-particle electron cryomicroscopy, cross-linking mass spectrometry, and structural modeling to build a complete model of human RZZ. We find that RZZ is structurally related to self-assembling cytosolic coat scaffolds that mediate membrane cargo trafficking, including Clathrin, Sec13-Sec31, and αβ'ε-COP. We show that Spindly, a dynein adaptor, is related to BicD2 and binds RZZ directly in a farnesylation-dependent but membrane-independent manner. Through a targeted chemical biology approach, we identify ROD as the Spindly farnesyl receptor. Our results suggest that RZZ is dynein's cargo at human kinetochores.

Kinetochores are chromatin-bound multi-protein complexes that allow high-fidelity chromosome segregation during mitosis and meiosis. Kinetochore assembly is exclusively initiated at chromatin containing Cse4/CENP-A nucleosomes. The molecular mechanisms ensuring that subcomplexes assemble efficiently into kinetochores only at centromeres, but not anywhere else, are incompletely understood. Here, we combine biochemical and genetic experiments to demonstrate that auto-inhibition of the conserved kinetochore subunit Mif2/CENP-C contributes to preventing unscheduled kinetochore assembly in budding yeast cells. We show that wild-type Mif2 is attenuated in its ability to bind a key downstream component in the assembly pathway, the Mtw1 complex, and that addition of Cse4 nucleosomes overcomes this inhibition. By exchanging the N-terminus of Mif2 with its functional counterpart from Ame1/CENP-U, we have created a Mif2 mutant which bypasses the Cse4 requirement for Mtw1 binding in vitro, thereby shortcutting kinetochore assembly. Expression of this Mif2 mutant in cells leads to mis-localization of the Mtw1 complex and causes pronounced chromosome segregation defects. We propose that auto-inhibition of Mif2/CENP-C constitutes a key concept underlying the molecular logic of kinetochore assembly.

In endothelial cells (ECs), stimulation of Toll-like receptor 4 (TLR4) by the endotoxin lipopolysaccharide (LPS) induces the release of diverse pro-inflammatory mediators, beneficial in controlling bacterial infections. However, their systemic secretion is a main driver of sepsis and chronic inflammatory diseases. Since distinct and rapid induction of TLR4 signaling is difficult to achieve with LPS due to the specific and non-specific affinity to other surface molecules and receptors, we engineered new light-oxygen-voltage-sensing (LOV)-domain-based optogenetic endothelial cell lines (opto-TLR4-LOV LECs and opto-TLR4-LOV HUVECs) that allow fast, precise temporal, and reversible activation of TLR4 signaling pathways. Using quantitative mass-spectrometry, RT-qPCR, and Western blot analysis, we show that pro-inflammatory proteins were not only expressed differently, but also had a different time course when the cells were stimulated with light or LPS. Additional functional assays demonstrated that light induction promoted chemotaxis of THP-1 cells, disruption of the EC monolayer and transmigration. In contrast, ECs incorporating a truncated version of the TLR4 extracellular domain (opto-TLR4 ΔECD2-LOV LECs) revealed high basal activity with fast depletion of the cell signaling system upon illumination. We conclude that the established optogenetic cell lines are well suited to induce rapid and precise photoactivation of TLR4, allowing receptor-specific studies.

Chemical cross-links identified by mass spectrometry generate distance restraints that reveal low-resolution structural information on proteins and protein complexes. The technology to reliably generate such data has become mature and robust enough to shift the focus to the question of how these distance restraints can be best integrated into molecular modeling calculations. Here, we introduce three workflows for incorporating distance restraints generated by chemical cross-linking and mass spectrometry into ROSETTA protocols for comparative and de novo modeling and protein-protein docking. We demonstrate that the cross-link validation and visualization software Xwalk facilitates successful cross-link data integration. Besides the protocols we introduce XLdb, a database of chemical cross-links from 14 different publications with 506 intra-protein and 62 inter-protein cross-links, where each cross-link can be mapped on an experimental structure from the Protein Data Bank. Finally, we demonstrate on a protein-protein docking reference data set the impact of virtual cross-links on protein docking calculations and show that an inter-protein cross-link can reduce on average the RMSD of a docking prediction by 5.0 Å. The methods and results presented here provide guidelines for the effective integration of chemical cross-link data in molecular modeling calculations and should advance the structural analysis of particularly large and transient protein complexes via hybrid structural biology methods.