BRCA1/BARD1 intrinsically disordered regions facilitate chromatin recruitment and ubiquitylation.

BRCA1/BARD1 is a tumor suppressor E3 ubiquitin (Ub) ligase with roles in DNA damage repair and in transcriptional regulation. BRCA1/BARD1 RING domains interact with nucleosomes to facilitate mono-ubiquitylation of distinct residues on the C-terminal tail of histone H2A. These enzymatic domains constitute a small fraction of the heterodimer, raising the possibility of functional chromatin interactions involving other regions such as the BARD1 C-terminal domains that bind nucleosomes containing the DNA damage signal H2A K15-Ub and H4 K20me0, or portions of the expansive intrinsically disordered regions found in both subunits. Herein, we reveal novel interactions that support robust H2A ubiquitylation activity mediated through a high-affinity, intrinsically disordered DNA-binding region of BARD1. These interactions support BRCA1/BARD1 recruitment to chromatin and sites of DNA damage in cells and contribute to their survival. We also reveal distinct BRCA1/BARD1 complexes that depend on the presence of H2A K15-Ub, including a complex where a single BARD1 subunit spans adjacent nucleosome units. Our findings identify an extensive network of multivalent BARD1-nucleosome interactions that serve as a platform for BRCA1/BARD1-associated functions on chromatin.

DASH/Dam1 complex mutants stabilize ploidy in histone-humanized yeast by weakening kinetochore-microtubule attachments.

Forcing budding yeast to chromatinize their DNA with human histones manifests an abrupt fitness cost. We previously proposed chromosomal aneuploidy and missense mutations as two potential modes of adaptation to histone humanization. Here, we show that aneuploidy in histone-humanized yeasts is specific to a subset of chromosomes that are defined by their centromeric evolutionary origins but that these aneuploidies are not adaptive. Instead, we find that a set of missense mutations in outer kinetochore proteins drives adaptation to human histones. Furthermore, we characterize the molecular mechanism underlying adaptation in two mutants of the outer kinetochore DASH/Dam1 complex, which reduce aneuploidy by suppression of chromosome instability. Molecular modeling and biochemical experiments show that these two mutants likely disrupt a conserved oligomerization interface thereby weakening microtubule attachments. We propose a model through which weakened microtubule attachments promote increased kinetochore-microtubule turnover and thus suppress chromosome instability. In sum, our data show how a set of point mutations evolved in histone-humanized yeasts to counterbalance human histone-induced chromosomal instability through weakening microtubule interactions, eventually promoting a return to euploidy.

A Bifunctional Role for the UHRF1 UBL Domain in the Control of Hemi-methylated DNA-Dependent Histone Ubiquitylation.

DNA methylation patterns regulate gene expression programs and are maintained through a highly coordinated process orchestrated by the RING E3 ubiquitin ligase UHRF1. UHRF1 controls DNA methylation inheritance by reading epigenetic modifications to histones and DNA to activate histone H3 ubiquitylation. Here, we find that all five domains of UHRF1, including the previously uncharacterized ubiquitin-like domain (UBL), cooperate for hemi-methylated DNA-dependent H3 ubiquitin ligation. Our structural and biochemical studies, including mutations found in cancer genomes, reveal a bifunctional requirement for the UBL in histone modification: (1) the UBL makes an essential interaction with the backside of the E2 and (2) the UBL coordinates with other UHRF1 domains that recognize epigenetic marks on DNA and histone H3 to direct ubiquitin to H3. Finally, we show UBLs from other E3s also have a conserved interaction with the E2, Ube2D, highlighting a potential prevalence of interactions between UBLs and E2s.

Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate dynamic instability.

Kinesin-14 molecular motors represent an essential class of proteins that bind microtubules and walk toward their minus-ends. Previous studies have described important roles for Kinesin-14 motors at microtubule minus-ends, but their role in regulating plus-end dynamics remains controversial. Kinesin-14 motors have been shown to bind the EB family of microtubule plus-end binding proteins, suggesting that these minus-end-directed motors could interact with growing microtubule plus-ends. In this work, we explored the role of minus-end-directed Kinesin-14 motor forces in controlling plus-end microtubule dynamics. In cells, a Kinesin-14 mutant with reduced affinity to EB proteins led to increased microtubule lengths. Cell-free biophysical microscopy assays were performed using Kinesin-14 motors and an EB family marker of growing microtubule plus-ends, Mal3, which revealed that when Kinesin-14 motors bound to Mal3 at growing microtubule plus-ends, the motors subsequently walked toward the minus-end, and Mal3 was pulled away from the growing microtubule tip. Strikingly, these interactions resulted in an approximately twofold decrease in the expected postinteraction microtubule lifetime. Furthermore, generic minus-end-directed tension forces, generated by tethering growing plus-ends to the coverslip using λ-DNA, led to an approximately sevenfold decrease in the expected postinteraction microtubule growth length. In contrast, the inhibition of Kinesin-14 minus-end-directed motility led to extended tip interactions and to an increase in the expected postinteraction microtubule lifetime, indicating that plus-ends were stabilized by nonmotile Kinesin-14 motors. Together, we find that Kinesin-14 motors participate in a force balance at microtubule plus-ends to regulate microtubule lengths in cells.

The Ndc80 kinetochore complex forms load-bearing attachments to dynamic microtubule tips via biased diffusion.

Kinetochores couple chromosomes to the assembling and disassembling tips of microtubules, a dynamic behavior that is fundamental to mitosis in all eukaryotes but poorly understood. Genetic, biochemical, and structural studies implicate the Ndc80 complex as a direct point of contact between kinetochores and microtubules, but these approaches provide only a static view. Here, using techniques for manipulating and tracking individual molecules in vitro, we demonstrate that the Ndc80 complex is capable of forming the dynamic, load-bearing attachments to assembling and disassembling tips required for coupling in vivo. We also establish that Ndc80-based coupling likely occurs through a biased diffusion mechanism and that this activity is conserved from yeast to humans. Our findings demonstrate how an ensemble of Ndc80 complexes may provide the combination of plasticity and strength that allows kinetochores to maintain load-bearing tip attachments during both microtubule assembly and disassembly.

Improved Analysis of Cross-Linking Mass Spectrometry Data with Kojak 2.0, Advanced by Integration into the Trans-Proteomic Pipeline.

Fragmentation ion spectral analysis of chemically cross-linked proteins is an established technology in the proteomics research repertoire for determining protein interactions, spatial orientation, and structure. Here we present Kojak version 2.0, a major update to the original Kojak algorithm, which was developed to identify cross-linked peptides from fragment ion spectra using a database search approach. A substantially improved algorithm with updated scoring metrics, support for cleavable cross-linkers, and identification of cross-links between 15N-labeled homomultimers are among the newest features of Kojak 2.0 presented here. Kojak 2.0 is now integrated into the Trans-Proteomic Pipeline, enabling access to dozens of additional tools within that suite. In particular, the PeptideProphet and iProphet tools for validation of cross-links improve the sensitivity and accuracy of correct cross-link identifications at user-defined thresholds. These new features improve the versatility of the algorithm, enabling its use in a wider range of experimental designs and analysis pipelines. Kojak 2.0 remains open-source and multiplatform.

Discovery and Visualization of Uncharacterized Drug-Protein Adducts Using Mass Spectrometry.

Drugs are often metabolized to reactive intermediates that form protein adducts. Adducts can inhibit protein activity, elicit immune responses, and cause life-threatening adverse drug reactions. The masses of reactive metabolites are frequently unknown, rendering traditional mass spectrometry-based proteomics approaches incapable of adduct identification. Here, we present Magnum, an open-mass search algorithm optimized for adduct identification, and Limelight, a web-based data processing package for analysis and visualization of data from all existing algorithms. Limelight incorporates tools for sample comparisons and xenobiotic-adduct discovery. We validate our tools with three drug/protein combinations and apply our label-free workflow to identify novel xenobiotic-protein adducts in CYP3A4. Our new methods and software enable accurate identification of xenobiotic-protein adducts with no prior knowledge of adduct masses or protein targets. Magnum outperforms existing label-free tools in xenobiotic-protein adduct discovery, while Limelight fulfills a major need in the rapidly developing field of open-mass searching, which until now lacked comprehensive data visualization tools.

Design of a hyperstable 60-subunit protein dodecahedron. [corrected].

The dodecahedron [corrected] is the largest of the Platonic solids, and icosahedral protein structures are widely used in biological systems for packaging and transport. There has been considerable interest in repurposing such structures for applications ranging from targeted delivery to multivalent immunogen presentation. The ability to design proteins that self-assemble into precisely specified, highly ordered icosahedral structures would open the door to a new generation of protein containers with properties custom-tailored to specific applications. Here we describe the computational design of a 25-nanometre icosahedral nanocage that self-assembles from trimeric protein building blocks. The designed protein was produced in Escherichia coli, and found by electron microscopy to assemble into a homogenous population of icosahedral particles nearly identical to the design model. The particles are stable in 6.7 molar guanidine hydrochloride at up to 80 degrees Celsius, and undergo extremely abrupt, but reversible, disassembly between 2 molar and 2.25 molar guanidinium thiocyanate. The dodecahedron [corrected] is robust to genetic fusions: one or two copies of green fluorescent protein (GFP) can be fused to each of the 60 subunits to create highly fluorescent 'standard candles' for use in light microscopy, and a designed protein pentamer can be placed in the centre of each of the 20 pentameric faces to modulate the size of the entrance/exit channels of the cage. Such robust and customizable nanocages should have considerable utility in targeted drug delivery, vaccine design and synthetic biology.

Kinetochore-associated Stu2 promotes chromosome biorientation in vivo.

Accurate segregation of chromosomes to daughter cells is a critical aspect of cell division. It requires the kinetochores on duplicated chromosomes to biorient, attaching to microtubules from opposite poles of the cell. Bioriented attachments come under tension, while incorrect attachments lack tension and must be released to allow proper attachments to form. A well-studied error correction pathway is mediated by the Aurora B kinase, which destabilizes low tension-bearing attachments. We recently discovered that in vitro, kinetochores display an additional intrinsic tension-sensing pathway that utilizes Stu2. The contribution of kinetochore-associated Stu2 to error correction in cells, however, was unknown. Here, we identify a Stu2 mutant that abolishes its kinetochore function and show that it causes biorientation defects in vivo. We also show that this Stu2-mediated pathway functions together with the Aurora B-mediated pathway. Altogether, our work indicates that cells employ multiple pathways to ensure biorientation and the accuracy of chromosome segregation.

Human Ska complex and Ndc80 complex interact to form a load-bearing assembly that strengthens kinetochore-microtubule attachments.

Accurate segregation of chromosomes relies on the force-bearing capabilities of the kinetochore to robustly attach chromosomes to dynamic microtubule tips. The human Ska complex and Ndc80 complex are outer-kinetochore components that bind microtubules and are required to fully stabilize kinetochore-microtubule attachments in vivo. While purified Ska complex tracks with disassembling microtubule tips, it remains unclear whether the Ska complex-microtubule interaction is sufficiently strong to make a significant contribution to kinetochore-microtubule coupling. Alternatively, Ska complex might affect kinetochore coupling indirectly, through recruitment of phosphoregulatory factors. Using optical tweezers, we show that the Ska complex itself bears load on microtubule tips, strengthens Ndc80 complex-based tip attachments, and increases the switching dynamics of the attached microtubule tips. Cross-linking mass spectrometry suggests the Ska complex directly binds Ndc80 complex through interactions between the Ska3 unstructured C-terminal region and the coiled-coil regions of each Ndc80 complex subunit. Deletion of the Ska complex microtubule-binding domain or the Ska3 C terminus prevents Ska complex from strengthening Ndc80 complex-based attachments. Together, our results indicate that the Ska complex can directly strengthen the kinetochore-microtubule interface and regulate microtubule tip dynamics by forming an additional connection between the Ndc80 complex and the microtubule.

Identification of Wiskott-Aldrich syndrome protein (WASP) binding sites on the branched actin filament nucleator Arp2/3 complex.

Arp2/3 complex nucleates branched actin filaments important for cellular motility and endocytosis. WASP family proteins are Arp2/3 complex activators that play multiple roles in branching nucleation, but little is known about the structural bases of these WASP functions, owing to an incomplete understanding of how WASP binds Arp2/3 complex. Recent data show WASP binds two sites, and biochemical and structural studies led to models in which the WASP C segment engages the barbed ends of the Arp3 and Arp2 subunits while the WASP A segment binds the back side of the complex on Arp3. However, electron microscopy reconstructions showed density for WASP inconsistent with these models on the opposite (front) side of Arp2/3 complex. Here we use chemical cross-linking and mass spectrometry (XL-MS) along with computational docking and structure-based mutational analysis to map the two WASP binding sites on the complex. Our data corroborate the barbed end and back side binding models and show one WASP binding site on Arp3, on the back side of the complex, and a second site on the bottom of the complex, spanning Arp2 and ARPC1. The XL-MS-identified cross-links rule out the front side binding model and show that the A segment of WASP binds along the bottom side of the ARPC1 subunit, instead of at the Arp2/ARPC1 interface, as suggested by FRET experiments. The identified binding sites support the Arp3 tail release model to explain WASP-mediated activating conformational changes in Arp2/3 complex and provide insight into the roles of WASP in branching nucleation.

Proxl (Protein Cross-Linking Database): A Public Server, QC Tools, and Other Major Updates.

Proxl is an open-source web application for sharing, visualizing, and analyzing bottom-up protein cross-linking mass spectrometry data and results. Proxl's core features include comparing data sets, structural analysis, customizable and interactive data visualizations, access to all underlying mass spectrometry data, and quality-control tools. All features of Proxl are designed to be independent of specific cross-linker chemistry or software analysis pipelines. Proxl's sharing tools allow users to share their data with the public or securely restrict access to trusted collaborators. Since being published in 2016, Proxl has continued to be expanded and improved through active development and collaboration with cross-linking researchers. Some of Proxl's new features include a centralized, public site for sharing data, greatly expanded quality-control tools and visualizations, support for stable isotope-labeled peptides, and general improvements that make Proxl easier to use, data easier to share and import, and data visualizations more customizable. Source code and more information are found at http://proxl-ms.org/ .

Regulation of outer kinetochore Ndc80 complex-based microtubule attachments by the central kinetochore Mis12/MIND complex.

Multiple protein subcomplexes of the kinetochore cooperate as a cohesive molecular unit that forms load-bearing microtubule attachments that drive mitotic chromosome movements. There is intriguing evidence suggesting that central kinetochore components influence kinetochore-microtubule attachment, but the mechanism remains unclear. Here, we find that the conserved Mis12/MIND (Mtw1, Nsl1, Nnf1, Dsn1) and Ndc80 (Ndc80, Nuf2, Spc24, Spc25) complexes are connected by an extensive network of contacts, each essential for viability in cells, and collectively able to withstand substantial tensile load. Using a single-molecule approach, we demonstrate that an individual MIND complex enhances the microtubule-binding affinity of a single Ndc80 complex by fourfold. MIND itself does not bind microtubules. Instead, MIND binds Ndc80 complex far from the microtubule-binding domain and confers increased microtubule interaction of the complex. In addition, MIND activation is redundant with the effects of a mutation in Ndc80 that might alter its ability to adopt a folded conformation. Together, our results suggest a previously unidentified mechanism for regulating microtubule binding of an outer kinetochore component by a central kinetochore complex.

ProXL (Protein Cross-Linking Database): A Platform for Analysis, Visualization, and Sharing of Protein Cross-Linking Mass Spectrometry Data.

ProXL is a Web application and accompanying database designed for sharing, visualizing, and analyzing bottom-up protein cross-linking mass spectrometry data with an emphasis on structural analysis and quality control. ProXL is designed to be independent of any particular software pipeline. The import process is simplified by the use of the ProXL XML data format, which shields developers of data importers from the relative complexity of the relational database schema. The database and Web interfaces function equally well for any software pipeline and allow data from disparate pipelines to be merged and contrasted. ProXL includes robust public and private data sharing capabilities, including a project-based interface designed to ensure security and facilitate collaboration among multiple researchers. ProXL provides multiple interactive and highly dynamic data visualizations that facilitate structural-based analysis of the observed cross-links as well as quality control. ProXL is open-source, well-documented, and freely available at https://github.com/yeastrc/proxl-web-app .

Structure of γ-tubulin small complex based on a cryo-EM map, chemical cross-links, and a remotely related structure.

Modeling protein complex structures based on distantly related homologues can be challenging due to poor sequence and structure conservation. Therefore, utilizing even low-resolution experimental data can significantly increase model precision and accuracy. Here, we present models of the two key functional states of the yeast γ-tubulin small complex (γTuSC): one for the low-activity "open" state and another for the higher-activity "closed" state. Both models were computed based on remotely related template structures and cryo-EM density maps at 6.9Šand 8.0Šresolution, respectively. For each state, extensive sampling of alignments and conformations was guided by the fit to the corresponding cryo-EM density map. The resulting good-scoring models formed a tightly clustered ensemble of conformations in most regions. We found significant structural differences between the two states, primarily in the γ-tubulin subunit regions where the microtubule binds. We also report a set of chemical cross-links that were found to be consistent with equilibrium between the open and closed states. The protocols developed here have been incorporated into our open-source Integrative Modeling Platform (IMP) software package (http://integrativemodeling.org), and can therefore be applied to many other systems.

Integrative phenomics reveals insight into the structure of phenotypic diversity in budding yeast.

To better understand the quantitative characteristics and structure of phenotypic diversity, we measured over 14,000 transcript, protein, metabolite, and morphological traits in 22 genetically diverse strains of Saccharomyces cerevisiae. More than 50% of all measured traits varied significantly across strains [false discovery rate (FDR) = 5%]. The structure of phenotypic correlations is complex, with 85% of all traits significantly correlated with at least one other phenotype (median = 6, maximum = 328). We show how high-dimensional molecular phenomics data sets can be leveraged to accurately predict phenotypic variation between strains, often with greater precision than afforded by DNA sequence information alone. These results provide new insights into the spectrum and structure of phenotypic diversity and the characteristics influencing the ability to accurately predict phenotypes.

Microtubule nucleating gamma-TuSC assembles structures with 13-fold microtubule-like symmetry.

Microtubules are nucleated in vivo by gamma-tubulin complexes. The 300-kDa gamma-tubulin small complex (gamma-TuSC), consisting of two molecules of gamma-tubulin and one copy each of the accessory proteins Spc97 and Spc98, is the conserved, essential core of the microtubule nucleating machinery. In metazoa multiple gamma-TuSCs assemble with other proteins into gamma-tubulin ring complexes (gamma-TuRCs). The structure of gamma-TuRC indicated that it functions as a microtubule template. Because each gamma-TuSC contains two molecules of gamma-tubulin, it was assumed that the gamma-TuRC-specific proteins are required to organize gamma-TuSCs to match 13-fold microtubule symmetry. Here we show that Saccharomyces cerevisiae gamma-TuSC forms rings even in the absence of other gamma-TuRC components. The yeast adaptor protein Spc110 stabilizes the rings into extended filaments and is required for oligomer formation under physiological buffer conditions. The 8-A cryo-electron microscopic reconstruction of the filament reveals 13 gamma-tubulins per turn, matching microtubule symmetry, with plus ends exposed for interaction with microtubules, implying that one turn of the filament constitutes a microtubule template. The domain structures of Spc97 and Spc98 suggest functions for conserved sequence motifs, with implications for the gamma-TuRC-specific proteins. The gamma-TuSC filaments nucleate microtubules at a low level, and the structure provides a strong hypothesis for how nucleation is regulated, converting this less active form to a potent nucleator.

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.

Kojak: efficient analysis of chemically cross-linked protein complexes.

Protein chemical cross-linking and mass spectrometry enable the analysis of protein-protein interactions and protein topologies; however, complicated cross-linked peptide spectra require specialized algorithms to identify interacting sites. The Kojak cross-linking software application is a new, efficient approach to identify cross-linked peptides, enabling large-scale analysis of protein-protein interactions by chemical cross-linking techniques. The algorithm integrates spectral processing and scoring schemes adopted from traditional database search algorithms and can identify cross-linked peptides using many different chemical cross-linkers with or without heavy isotope labels. Kojak was used to analyze both novel and existing data sets and was compared to existing cross-linking algorithms. The algorithm provided increased cross-link identifications over existing algorithms and, equally importantly, the results in a fraction of computational time. The Kojak algorithm is open-source, cross-platform, and freely available. This software provides both existing and new cross-linking researchers alike an effective way to derive additional cross-link identifications from new or existing data sets. For new users, it provides a simple analytical resource resulting in more cross-link identifications than other methods.