Aggregation of proteins containing polyglutamine (polyQ) expansions characterizes many neurodegenerative disorders, including Huntington’s disease. Molecular chaperones modulate Huntingtin (Htt) aggregation and toxicity by an ill-defined mechanism. Here we determine how the chaperonin TRiC suppresses Htt aggregation. Surprisingly, TRiC does not physically block the polyQ tract itself, but rather sequesters a short Htt sequence element N-terminal to the polyQ tract, that promotes the amyloidogenic conformation. The residues of this amyloid-promoting element essential for rapid Htt aggregation are directly bound by TRiC. Our findings illustrate how molecular chaperones, which recognize hydrophobic determinants, can prevent aggregation of polar polyQ tracts associated with neurodegenerative diseases. The observation that the switch of polyQ tracts to an amyloidogenic conformation is accelerated by short endogenous sequence elements provides a novel target for therapeutic strategies to inhibit aggregation.

The study of proteins and protein complexes using chemical crosslinking followed by the MS identification of the cross-linked peptides has found increasingly widespread use in recent years. Thus far, such analyses have used almost exclusively homobifunctional, amine-reactive cross-linking reagents. Here we report the development and application of an orthogonal cross-linking chemistry specific for carboxyl groups. Chemical cross-linking of acidic residues is achieved using homobifunctional dihydrazides as cross-linking reagents and a coupling chemistry at neutral pH that is compatible with the structural integrity of most protein complexes. In addition to cross-links formed through insertion of the dihydrazides with different spacer lengths, zero-length cross-link products are also obtained, thereby providing additional structural information. We demonstrate the application of the reaction and the MS identification of the resulting cross-linked peptides for the chaperonin TRiC/CCT and the 26S proteasome. The results indicate that the targeting of acidic residues for cross-linking provides distance restraints that are complementary and orthogonal to those obtained from lysine cross-linking, thereby expanding the yield of structural information that can be obtained from cross-linking studies and used in hybrid modeling approaches. P roteins exert the majority of their functions in the form of protein complexes to control cellular signaling, protein synthesis, folding and degradation, and many more essential processes. Therefore, elucidating the composition and structure of such complexes has been a longstanding goal of biological research.MS-based proteomics has emerged as one of the main techniques to identify and quantify proteins and their modifications in biological samples such as isolated complexes, proteome fractions, or whole proteomes. Various MS methods now provide structural information on protein assemblies (1-3). Among them, chemical cross-linking and identification of cross-linked peptides by MS (XL-MS) has been increasingly applied to determine the subunit arrangements of biologically relevant complexes (4-6). Such XL-MS experiments indicate the locations of cross-linking sites and thus the spatial proximity of reactive groups that are connected by a covalent bond. This information is then used to determine the positioning of subunits or locate interacting regions, alone or in combination with other techniques such as NMR spectroscopy, electron microscopy, and X-ray crystallography.In the last few years, optimized protocols and new computational tools for the reliable analysis of XL-MS datasets resulted in significant advances of the XL-MS technology (4-6). These advances have contributed to the emergence of a robust, integrated XL-MS method that has been successfully applied for structure determination of a number of large protein complexes (7-11) and the detection of direct, physical interactions in whole cells (12)(13)(14). To date, the cross-linking chemistries applied in these studies have targ...

Summary TRiC/CCT is a highly conserved and essential chaperonin that uses ATP cycling to facilitate folding of approximately 10% of the eukaryotic proteome. This 1 MDa hetero-oligomeric complex consists of two stacked rings of eight paralogous subunits each. Previously proposed TRiC models differ substantially in their subunit arrangements and ring register. Here, we integrate chemical crosslinking, mass spectrometry and combinatorial modeling to reveal the definitive subunit arrangement of TRiC. In vivo disulfide mapping provided additional validation for the crosslinking-derived arrangement as the definitive TRiC topology. This subunit arrangement allowed the refinement of a structural model using existing X-ray diffraction data. The new structure explains all available crosslink experiments, provides a rationale for previously unexplained structural features and reveals a surprising asymmetry of charges within the chaperonin folding chamber.

We developed a 'computational second-site suppressor' strategy to redesign specificity at a protein-protein interface and applied it to create new specifically interacting DNase-inhibitor protein pairs. We demonstrate that the designed switch in specificity holds in in vitro binding and functional assays. We also show that the designed interfaces are specific in the natural functional context in living cells, and present the first high-resolution X-ray crystallographic analysis of a computer-redesigned functional protein-protein interface with altered specificity. The approach should be applicable to the design of interacting protein pairs with novel specificities for delineating and re-engineering protein interaction networks in living cells.

Tauopathies are neurodegenerative diseases characterized by intracellular amyloid deposits of tau protein. Missense mutations in the tau gene ( MAPT ) correlate with aggregation propensity and cause dominantly inherited tauopathies, but their biophysical mechanism driving amyloid formation is poorly understood. Many disease-associated mutations localize within tau’s repeat domain at inter-repeat interfaces proximal to amyloidogenic sequences, such as 306 VQIVYK 311 . We use cross-linking mass spectrometry, recombinant protein and synthetic peptide systems, in silico modeling, and cell models to conclude that the aggregation-prone 306 VQIVYK 311 motif forms metastable compact structures with its upstream sequence that modulates aggregation propensity. We report that disease-associated mutations, isomerization of a critical proline, or alternative splicing are all sufficient to destabilize this local structure and trigger spontaneous aggregation. These findings provide a biophysical framework to explain the basis of early conformational changes that may underlie genetic and sporadic tau pathogenesis.

Tauopathies feature progressive accumulation of tau amyloids. Pathology may begin when these amplify from a protein template, or seed, whose structure is unknown. We have purified and characterized distinct forms of tau monomer—inert (Mi) and seed-competent (Ms). Recombinant Ms triggered intracellular tau aggregation, induced tau fibrillization in vitro, and self-assembled. Ms from Alzheimer’s disease also seeded aggregation and self-assembled in vitro to form seed-competent multimers. We used crosslinking with mass spectrometry to probe structural differences in Mi vs. Ms. Crosslinks informed models of local peptide structure within the repeat domain which suggest relative inaccessibility of residues that drive aggregation (VQIINK/VQIVYK) in Mi, and exposure in Ms. Limited proteolysis supported this idea. Although tau monomer has been considered to be natively unstructured, our findings belie this assumption and suggest that initiation of pathological aggregation could begin with conversion of tau monomer from an inert to a seed-competent form.

The de novo design of protein-protein interfaces is a stringent test of our understanding of the principles underlying protein-protein interactions and would enable new approaches to biological and medical challenges. Here we describe a novel motif-based method to computationally design protein-protein complexes with native-like interface composition and interaction density. Using this method we designed a pair of proteins, Prb and Pdar, that heterodimerize with a Kd of 130 nM, 1,000-fold tighter than any previously designed de novo protein-protein complex. Directed evolution identified two point mutations that improve affinity to 180 pM. Crystal structures of complexes containing designed and evolved proteins reveal binding is entirely through the designed interface, making use of specific designed interactions. Surprisingly, in the evolved complex one of the partners is rotated 180 degrees relative to the design model. This work demonstrates that current understanding of protein-protein interfaces is sufficient to rationally design interfaces de novo, and underscores remaining challenges.

Graphical Abstract Highlights d Generation and characterization of active recombinant hTRiC and hPFD d Cryo-EM, XL-MS, and modeling reveal the structure of TRiC/ CCT-PFD complex d PFD pivots around a conserved electrostatic interface with TRiC/CCT d PFD acts on TRiC/CCT-substrate complex to enhance the rate of the folding reaction In Brief Direct interactions between two chaperonins allow them to feed folding substrates bi-directionally between active sites, preventing aggregation and promoting proteostasis. o CCT4 CCT3 PFD4 PFD1/2