Journal of Molecular Biology

Prion diseases are fatal neurodegenerative disorders driven by the conversion of the cellular prion protein (PrP) into a misfolded, pathogenic conformer. Beyond serving as a substrate for prion propagation, PrP is also thought to mediate neurotoxic signaling. Within this framework, the central region of PrP has emerged as a critical regulatory element. Notably, deletion of residues 105-125 ({Delta}CR) leads to spontaneous neurodegeneration in vivo and induces abnormal ionic currents in cultured cells and primary neurons, indicating that this region is essential for controlling the toxicity of the N-terminal domain. Current models propose that the N-terminus functions as a toxic effector whose activity is modulated by the C-terminal domain. This intramolecular interplay is likely central to the physiological role of PrP, and its disruption may contribute to neurodegeneration. Here, we investigated how deletion of the central region affects the structure and dynamics of full-length PrP. We generated membrane-bound models of full-length, diglycosylated wild-type (WT) PrP and the neurotoxic {Delta}CR mutant, and compared their conformational dynamics using molecular dynamics simulations. The two proteins exhibited markedly distinct behaviours. WT PrP adopted a more compact conformational ensemble of the N-terminal domain, consistent with stabilizing interactions between the flexible N-terminus and the globular C-terminal domain. In contrast, the {Delta}CR variant displayed more extended conformations and a substantial redistribution of intramolecular contacts, including the loss of specific interactions between the disordered N-terminal tail and the globular domain. This altered structural organization was accompanied by an increased propensity of the N-terminal domain to approach the membrane surface in the mutant. Our results provide a molecular model in which the central region engages intramolecular interaction networks that ultimately help regulate N-terminal residence at the plasma membrane, offering mechanistic insight into how CR deletion shifts the conformational ensemble toward membrane-associated states that may be associated with neurotoxic activity.

The three conventional isoforms of the Ras G-protein (H-, K-, N-Ras) function as molecular on-off switches that regulate a wide array of signaling pathways, including the Ras-PI3K-PIP3-PDK1-AKT pathway that is central to innate immunity and normal cell growth, and is dysregulated in many disease states. Activation of the pathway by Ras requires adequate Ras-PI3K binding affinity. Here we focus on the interface of known structure in the H-Ras:PI3K{gamma} co-complex essential to multiple pathways including directed pseudopod growth in leukocyte chemotaxis. At this interface 10 H-Ras residues, all 100% conserved between the H-, K- and N-Ras isomers, contact the Ras binding domain of PI3K{gamma} (PI3K{gamma}RBD). To investigate the degree to which the native H-Ras:PI3K{gamma}RBD interface is optimized by evolution for maximal binding affinity, 8 interfacial Ras mutations selected from the COSMIC database and the literature were introduced at the contact positions. All 8 Ras mutations were observed to alter the H-Ras:PI3K{gamma}RBD binding affinity, with 4 mutations yielding significant affinity increases and 4 yielding significant affinity decreases. These findings indicate that the native H-Ras:PI3K{gamma}RBD interface provides intermediate, rather than maximal, binding affinity. Such intermediate affinity is consistent with the substantial binding plasticity of the conserved H-, N-, K-Ras effector docking surface, which has evolved to bind a diverse array of effectors. Furthermore, the findings provide evidence that COSMIC-linked mutations at the H-Ras:PI3K{gamma}RBD interface frequently generate affinity increases as well as decreases, with potential implications for molecular mechanisms of disease and for tool development in cell biology.

DNA helicases are ATP-dependent motor proteins that catalyze duplex DNA unwinding and are involved in DNA repair, recombination and replication restart. Prominent members of the non-hexameric SF1A UvrD-family helicases are E. coli UvrD, Rep, B. stearothermophilus PcrA and M. tuberculosis UvrD1. SF1A monomers are processive 3 to 5 single stranded DNA translocases, but need to be activated to become DNA helicases. One mechanism of activation is dimerization. Whereas Rep, UvrD and PcrA form non-covalent dimers, the Mtb UvrD1 helicase forms a redox-dependent covalent dimer. Dimerization of Mtb UvrD1 occurs between the same regulatory domain (2B) within each subunit stabilized by a disulfide bond formed between the same cysteine (Cys451) within each subunit. Dimerization relieves an inhibitory interaction between the 2B domain and duplex DNA within the monomer-DNA complex. We show here that Rep, UvrD and PcrA dimerize using the same 2B-2B interface. By placing a Cys residue within the 2B domains of Rep, UvrD and PcrA in the structurally equivalent position occupied by Cys451 of Mtb UvrD1, all three enzymes form redox-dependent covalent dimers that are constitutively active helicases with increased processivity compared to the non-covalent dimers. Hence, the 2B domain is a general dimerization domain for UvrD-family SF1A helicases.

Mutations in the low complexity domains of RNA-binding proteins are associated with neurodegenerative disease pathology. The TIA1 RNA-binding protein harbors seven such mutations linked to a clinical cohort of ALS patients. The altered low complexity domain sequence increases the number of TIA1-rich stress granules in cultured cells, delays their disassembly, and is associated with increased fibril formation. Altered molecular motions and contacts in condensed states like stress granules that result in the formation of amyloid-like fibril states is commonly observed for RNA-binding biomolecular condensates. Here we focus on the influence of the ALS mutations on fibril formation of the TIA1 low complexity domain. Repetitive seeding preparations of the seven TIA1 protein mutants all yield amyloid-like fibrils based on transmission electron microscopy images and increased thioflavin T fluorescence. Analysis of solid state nuclear magnetic resonance spectra recorded on all seven mutant fibrils reveals distinct structural differences in the relative to wild-type fibrils. Our results shed light on how the mutations affect structural conformations accessible to the TIA1 low complexity domain.

{beta}-turns are among the most common structural motifs in proteins, yet their conformational dynamics and sequence determinants remain incompletely understood. Here we present a data-driven classification and dynamic analysis of {beta}-turn conformations using large-scale molecular dynamics trajectories from the mdCATH database. Clustering of backbone dihedral angles using a cross-bond Ramachandran representation identifies six {beta}-turn types, including a previously uncharacterized hybrid I/I' cluster that combines geometric features of canonical type I and I' conformations. Time-resolved analysis indicates that this hybrid state acts as a transient intermediate state of {beta}-turns. Transitions observed in molecular dynamics simulations closely match NMR ensembles and altlocs detected in X-ray crystal structures, with the most dominant exchanges occurring between type I and II, and between type I' and II' turns. Sequence analysis shows that each turn type exhibits characteristic amino acid preferences at the central residues (i + 1 and i + 2). Within these overall preferences, specific residue pairs display distinct biases toward static or dynamic behavior. Targeted in silico substitutions that interchange dynamic- and static-enriched residue pairs shift the conformational behavior of turns accordingly, providing direct support for these sequence-dynamics relationships. Analysis of flanking secondary-structure environments reveals that structural context further modulates turn flexibility, with strand- and coil-associated turns exhibiting higher dynamic propensity than helix-associated turns. Together, these results reveal how sequence composition and structural context jointly shape the conformational landscape of {beta}-turns.

The nuclear receptor superfamily is comprised of ligand-regulated transcription factors that contain an intrinsically disordered domain at the amino-terminal end, known as the N-terminal domain (NTD). While this poorly conserved domain is known to possess ligand-independent activation function (AF-1), few NTD functions are conserved between nuclear receptors (NRs). Identified roles in other receptors include androgen receptor (AR), estrogen receptor (ER) and mineralocorticoid receptor (MR). Here, we aim to define the function of the NTD of the farnesoid X receptor (FXR), a crucial regulator of lipid and bile acid metabolism. We show that the NTD engages in interdomain contact with other FXR domains. We also observe that the NTD interacts directly with coregulator proteins. Using mutagenesis, mammalian two-hybrid assays and molecular dynamics simulations, we identify and validate a novel SXXLF motif in the NTD which mediates interactions with both coregulators and the ligand binding domain. Mutation of the motif induces large changes in conformational and allosteric coupling in FXR. Our study identifies a new nuclear receptor-interacting motif that modulates the transcriptional activity of FXR. Graphical AbstractFXR-NTD regulates transcriptional activity through interdomain communication with the LBD and is also involved in co-activator recruitment. The SENLF motif is the first defined functional element within the FXR-NTD and mediates both NTD-LBD interaction and selective co-activator engagements to drive NTD-mediated transcriptional activity. O_FIG O_LINKSMALLFIG WIDTH=135 HEIGHT=200 SRC="FIGDIR/small/724725v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@5a37aorg.highwire.dtl.DTLVardef@2fa9e1org.highwire.dtl.DTLVardef@13a19daorg.highwire.dtl.DTLVardef@1775ed2_HPS_FORMAT_FIGEXP M_FIG C_FIG

DNA polymerase clamp loaders are AAA+ ATPases that load sliding clamps on DNA for high- speed replication. Using a platform for high-throughput mutagenesis of replication proteins in T4 bacteriophage, we carried out saturation mutagenesis of the AAA+ ATPase module of the T4 clamp loader bearing a mutation, Gln 118{lozenge}Asn (Q118N), that reduces fitness. We identified residues for which different mutations improve the fitness of the Q118N variant but are neutral in the wild-type background. These conditionally neutral "rescue hotspots" overlap with those identified earlier in another defective variant (D110C). These rescue hotspots localize to regions where the sequence is not optimal for the structure, as determined by energetic frustration analysis. We designed new sequences for three of these regions, using the protein-design algorithm ProteinMPNN. In two helical regions, several designed sequences increased the fitness of both wild-type and mutant proteins, likely due to enhanced stability. An inter-domain hinge in AAA+ module changes conformation during activation, and designs for the hinge lead to loss of fitness in the wild-type background. However, when using the active conformation as the template, designs for the hinge increase the fitness of defective variants. In contrast designs templated on the inactive conformation led to loss of fitness, suggesting that a proper conformational balance is crucial. Thus, adaptive capacity in the clamp loader resides in a network of conditionally neutral sites that enable functional tuning through shifts in stability and conformational equilibria.

In an infection, Hepatitis B Virus (HBV) core protein (HBc) normally assembles into icosahedral capsids. Capsid Assembly Modulators (CAMs) are direct acting antivirals that induce HBc mis-assembly and are the subject of active research and development. Two versions of HBc are used in structural studies of CAM-HBc complexes: Cp150 and Cp149-Y132A. Cp150 forms empty icosahedral capsids that are structurally indistinguishable from those found in virions. The Y132A mutation of Cp149 leads to an assembly defective soluble protein that crystalizes as flat hexagonal sheets, where the hexagons resemble icosahedral quasi-sixfold vertices. In this study, we compare structures of CAM-bound Cp150 to CAM-bound Cp149-Y132A. In capsids, the residues forming the CAM site shift to match the structure of bound CAMs, an induced fit. In Cp149-Y132A crystals, CAM sites show little structural adjustment in response to different CAMs binding. In turn, the array of residues that interact with CAMs varies from CAM to CAM in capsid structures but remains nearly constant in Cp149-Y132A crystals. These results illustrate important differences between CAM binding in Cp149-Y132A and Cp150 structures that will contribute to future CAM design.

The Tar-DNA Binding Protein-43 C-terminal region, TDP43LC, has been previously shown to form amyloid-like fibrils with distinct folds in ALS and FTD. In both diseases, proteinaceous inclusions contain TDP43 C-terminal protein fragments as well as phosphorylated TDP43. Here, we use solution NMR to show that soluble phosphomimetic TDP43LC, P-TDP43LC, is structurally similar to wild-type TDP43LC. Disperse P-TDP43LC, like wild-type protein, contains a central helical region flanked by long disordered regions. Despite this similarity, our turbidity measurements, imaging, and kinetic assays show that P-TDP43LC has different aggregation behavior than wild-type protein. Using solid state NMR measurements we find that that phosphomimetic mutations alter the wild-type fibril conformation. Electrostatic repulsion from negatively charged sidechains, despite having little effect on the soluble proteins structure, perturbs amyloid-like fibril formation and selects for a different conformation in vitro. These results shed light on the structural role of TDP43LC phosphorylation in fibril formation in disease. TOC Graphic O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=104 SRC="FIGDIR/small/725298v1_ufig1.gif" ALT="Figure 1"> View larger version (16K): org.highwire.dtl.DTLVardef@1c63aforg.highwire.dtl.DTLVardef@1d48ed6org.highwire.dtl.DTLVardef@1ed8fd3org.highwire.dtl.DTLVardef@17d67a8_HPS_FORMAT_FIGEXP M_FIG C_FIG SynopsisPhosphomimetic mutations at ALS and FTD neurodegeneration-associated sites in an amyloid forming protein perturbs the aggregated structure compared to wild-type protein.

The INO80 chromatin remodeling complex plays a central role in DNA repair, transcription, and replication. Yet, a comprehensive understanding of its structural organization remains incomplete due to the dynamic nature of several of its subunits and the sharing of several subunits with related remodeling complexes. Here, we report a computational model of the three-dimensional structure of the S. cerevisiae INO80 complex using an integrative approach that combines experimental crosslinking mass spectrometry, molecular docking, and molecular dynamics simulations. Our results reveal the spatial and dynamical organization of key modules--ARP8, ARP5, NHP10, and RVB1/2--within the intact complex. The resulting structural model agrees with crosslinking constraints, highlighting the architecture of the previously uncharacterized NHP10 module. This module, including the C-terminal region of the Ino80 scaffolding protein, has remained elusive due to its intrinsic flexibility and lack of high-resolution structural data. To facilitate this integrative modeling workflow and make it broadly accessible, we presented INTEGRATOR: (INTEGRAtive TempOral and stRuctural Analysis of protein modules), a versatile workflow package designed as a tool to elucidate the structure and dynamics of large, flexible macromolecular assemblies using well-established softwares. Our findings demonstrate the power of integrative modeling in resolving the role of the highly disordered NPH10 module in recruiting other dynamic modules into INO80 large protein assemblies and offer a generalizable framework for determining the architecture of similarly complex and heterogeneous molecular machines. This work carries broad implications for understanding the structural basis of chromatin regulation in microbial organisms and the implications for the dysregulation in diseases such as cancer.

Structure Integration with Function, Taxonomy and Sequences (SIFTS) provides residue-level mappings between UniProt Knowledgebase sequences and Protein Data Bank structures and has historically been generated through internal Protein Data Bank in Europe (PDBe) pipelines. Here, PDBe-SIFTS is presented as a fully open-source, locally deployable implementation of this mapping framework. The pipeline combines fast, scalable sequence search using MMseqs2, an improved bounded scoring scheme for ranking candidate mappings, and residue-level mapping refinement based on backbone connectivity. PDBe-SIFTS is distributed as a Python package with command-line tools for 1) building a sequence search database, 2) identifying the best sequence-structure match, 3) one-to-one mapping at the residue level, and 4) generating SIFTS annotations in PDBx/mmCIF format. Benchmarking on the complete Protein Data Bank archive showed that MMseqs2 reduced archive-scale UniProtKB searches from hours with BLASTP to minutes, approximately 22-36 times faster, while curated mappings were recovered at top rank in 93.1% of cases. The remaining discrepancies mainly involved biologically ambiguous cases such as highly conserved proteins, chimeric constructs, or closely related orthologs. These results show that PDBe-SIFTS enables fast mapping, improving structural coherence in residue-level alignments while delivering the most up-to-date and accurate mappings, comparable to expert curation. Tool: https://github.com/PDBeurope/SIFTS Quick start notebook with example: https://github.com/PDBeurope/SIFTS/tree/master/notebooks Broader audience statementMatching protein sequences to their three-dimensional structures, and mapping annotations across both, is essential for understanding protein function, interactions, and molecular mechanisms. This integrated view enables richer interpretation of biological data and underpins advances in drug discovery, disease research, and protein engineering. PDBe-SIFTS provides an open and functional framework for structure-sequence mapping, allowing researchers and databases to run, inspect, and extend these mappings locally, while benefiting from faster searches, transparent scoring, and structurally informed residue-level alignments. Graphical abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=110 SRC="FIGDIR/small/721839v1_ufig1.gif" ALT="Figure 1"> View larger version (25K): org.highwire.dtl.DTLVardef@5e6ea6org.highwire.dtl.DTLVardef@1b2754dorg.highwire.dtl.DTLVardef@1334f9forg.highwire.dtl.DTLVardef@1b083a1_HPS_FORMAT_FIGEXP M_FIG C_FIG

As protein structure prediction tools become widely adopted across biology, there is a growing need for accessible methods to assess and visualize predicted protein-protein interactions (PPIs). Here we present LIVIA (Local Interaction Visualization and Analysis), a browser-based tool that computes local PPI confidence metrics across multiple prediction platforms, identifies predicted interface residues, embeds an interactive Mol* 3D viewer, and generates visualization scripts for ChimeraX and PyMOL. The tool automatically detects prediction formats; all parsing and computation occur locally on the users machine. LIVIA is freely available at https://flyark.github.io/LIVIA.

Orthoflavivirus, such as West Nile Virus (WNV), dengue virus (DENV) and ZIKA virus (ZIKV), are globally distributed pathogens that pose substantial threats to human health. Currently, there are still no effective antiviral drugs for WNV or ZIKV. Despite the availability of two licensed DENV vaccines, their use remains limited due to potential risks, highlighting an urgent need for antiviral drug development. The highly conserved orthoflavivirus protease NS2B/NS3 is required for viral replication, making it a promising anti-flavivirus target. A major challenge, however, is the highly charged active site of this enzyme, which requires charged chemical matters with low bioavailability. An alternative and more attractive strategy is to target potential allosteric sites or folding intermediate states of the protease. In this work, we employ the topology-based coarse-grained G[o] modeling to explore the coupled binding and folding pathways of WNV NS2B/NS3 protease and study the effects of the widely used experimental construct with a G4SG4 linker between NS2B and NS3 on stability and folding. Our results provide a holistic conformational landscape of the protease binding and folding, including several key intermediate states. We find that the presence of the G4SG4 linker alters the folding pathways and destabilizes the NS2B C-terminus. The latter is consistent with experimental observations that the G4SG4 linked protease has lower activity and adopts an open state without the substrate in crystal structures. Together, these findings provide for the first time a complete picture of the binding and folding of the NS2B/NS3 protease and identify important folding intermediate states that could be targeted for allosteric antiviral drug development. TOC Figure O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=157 SRC="FIGDIR/small/722635v1_ufig1.gif" ALT="Figure 1"> View larger version (40K): org.highwire.dtl.DTLVardef@163c356org.highwire.dtl.DTLVardef@ad7b35org.highwire.dtl.DTLVardef@173ed8aorg.highwire.dtl.DTLVardef@1f026bf_HPS_FORMAT_FIGEXP M_FIG C_FIG

The KCa2.2 and KCa3.1 channels are fundamental regulator of cellular K+ concentration, and promising target to treat diseases such as spinocerebellar ataxia and cancer. To fully exploit their therapeutic potential, and to continue studying their pathophysiological role, it is crucial to develop selective modulators for each of these two channels. Here we present a computational study to identify the molecular determinants behind the selectivity of two recently reported KCa2.2 modulators. We leveraged a protocol combining in silico mutagenesis, molecular dynamics simulations, and protein-ligand docking to analyse the pockets targeted by these ligands. We identified a Ser353/Pro245 substitution to be the main driver of the distinct pocket shapes in KCa2.2 and KCa3.1 channels, ultimately defining modulator selectivity. This approach provides novel insights into the structural differences of this binding site across potassium channel subtypes, shedding light on the selectivity determinants of modulators targeting this pocket.

The brain-derived neurotrophic factor (BDNF)-tropomyosin receptor kinase B (TrkB) signalling axis is a key effector of synaptic plasticity and neuroprotection. While TrkB activation is a major objective towards preventing dysfunction of the nervous system, it cannot be reached with exogenous BDNF administration given the unfavourable physiochemical properties of BDNF. In addition, BDNF also activates a tumour necrosis factor pathway by binding to the neurotrophin receptor p75. The TrkB agonist ZEB85 provides an alternative route to the selective activation of TrkB. We report here the structural basis for the interaction between human TrkB, and both ZEB85 and BDNF, and reveal that a sulfated tyrosine modification is indispensable for ZEB85 activation of TrkB signalling. Using structure-guided BDNF- and ZEB85-binding deficient TrkB mutants, we assessed their ability to sequester ligands from full-length TrkB in cultured human neurons. We found that the BDNF binding site extends into the extracellular juxtamembrane domain of TrkB but does not require the sulfotyrosine at residue 400 to activate TrkB. Together with biophysical analysis and AlphaFold modelling these results also explain how BDNF can displace ZEB85 from TrkB through an overlapping epitope. Our findings reveal unique features of TrkB, not present in the related neurotrophin receptors TrkA and TrkC, and suggest new directions to explore the role of sulfotyrosine in TrkB signalling and identify new TrkB-specific protein ligands. One Sentence SummaryInvestigation of the mechanism of action of TrkB agonist ZEB85 extends molecular understanding of TrkB activation.

Citrullination is a charge-modifying post-translational modification whereby proteinogenic arginine is converted to the non-coded amino acid citrulline by calcium-activated protein arginine deiminases (PADs; EC 3.5.3.15). The five known PAD enzymes in humans (PADs 1, 2, 3, 4, and 6) are differentially expressed and have distinct targets, including histones. While some PAD histone citrullination sites are known, a comprehensive investigation of all histone tail arginines targeted by catalytically active PADs 1-4 is lacking. Here, we sought to identify PAD citrullination sites in histone tails, both within histone peptides and in reconstituted nucleosomes. Toward this objective, we utilized a real-time 1H-15N NMR spectroscopy-based assay. By monitoring both arginine and citrulline backbone amide peak intensities over time, we identified sites of citrullination in 15N-labeled histone tails within peptides and reconstituted nucleosome core particles. We found that PADs 1, 2, and 4 citrullinate all directly observable histone tail arginines to varying degrees. This is distinct from PAD3, which only moderately citrullinates H2A and H4 arginine residues and does not modify H3 tail arginines. Together, these data suggest a level of histone arginine specificity by each PAD. Furthermore, histone tail citrullination is altered within nucleosomes compared to isolated peptides, which we interpret to reflect changes in conformation and accessibility. We speculate that citrullination increases nucleosomal histone tail dynamics, with implications for crosstalk between sites of histone citrullination and other important sites of regulation by PTMs (including lysines) within and between tails.

The propensity of proteins to form oligomers is ultimately dictated by their structural configuration(s). Proteins that persist in a discrete conformational state may form a limited number of specific interactions while those that sample a broader structural ensemble may instead associate with a wider array of partners. These intrinsic tendencies potentially constrain the way proteins navigate wider interaction networks. In this work, we aggregated and surveyed a wide variety of biophysical, biochemical, and cellular descriptors of the S. cerevisiae proteome to identify biases in the connectivity of its protein-protein interaction network. Using mass spectrometry-based interactome measurements and various protein stability estimates, we find that a disproportionate number of abundant, yet unstable binding proteins act as network hubs. Moreover, we show that these features alone can be used to discriminate between hubs and non-hub proteins with high accuracy (AUROC = 0.898). Interestingly, we find that half-lives of hub proteins depend on whether or not they reside within static complexes and/ or whether they interact with molecular chaperones. Finally, we note that the observed connectivity biases associated with abundant, unstable proteins only pertain to network hubs, but not to the bottlenecks that connect them. Together, our findings reveal how the conformational stability of a protein may constrain its context within protein-protein interaction networks.

Eukaryotes use several distinct quality control pathways to resolve aberrant ribosomes and mRNAs. For example, the no-go decay mRNA pathway is stimulated after ribosome collisions caused by stalled ribosomes translating damaged or truncated mRNAs. Separate decay pathways for non-functional 40S and 60S subunits containing rRNA mutations affecting decoding and peptidyl transferase activity, respectively, have also been elucidated. To our knowledge, whether eukaryotes have evolved a quality control pathway to sense and process globally stalled ribosomes is unclear; however, such a pathway would be advantageous to eukaryotes during exposure to natural elongation inhibitors such as ricin and diphtheria toxin. Here, we test how prolonged robust inhibition of elongation using a high dose of cycloheximide (CHX) affects ribosome turnover. Despite no decrease in cell viability and that mammalian ribosomes have been classically characterized of having a half-life of 3-5 days, a single 24 hr high dose of CHX resulted in drastically shortened half-lives (<24 hr) of 28S and 18S rRNA in A549 cells. A [~]2-fold reduction in nearly all ribosome species was observed by polysome analysis in HeLa and A549 cells after prolonged CHX treatment. Depletion of ribosomes was also evident when assessing ribosomal proteins from both the 40S and 60S subunits by Western blot. Literature supports that ribosomes can be degraded by autophagy and the ubiquitin (Ub)-proteasome system. Upon testing inhibitors of both pathways, only proteasome inhibitors (i.e., MG132 and bortezomib) rescued both rRNA and ribosomal protein levels. Proteasome inhibitors also rescued ribosome levels in polysome profiling experiments. Remarkably, rRNA levels were not rescued during CHX treatment when co-treated with the Ub activating enzyme E1 inhibitor, TAK243. Polysome analysis also showed that the high prolonged dose of CHX did not cause robust accumulation of collided ribosomes compared to control treatments. Proteasome-dependent turnover of rRNA was also observed with high doses of other elongation inhibitors, namely anisomycin, homoharringtonine, and lactimidomycin. The recognition capabilities of the pathway were further expanded as we observed that 80S ribosomes not trapped on the mRNA were also targeted for degradation by the proteasome. Together, our findings define the framework of a regulatory pathway in mammalian cells that degrades both ribosomal subunits in response to prolonged periods of robust elongation inhibition.

RNA helicases encoded by positive-strand RNA viruses are essential for genome replication, yet the specific biological functions and mechanochemical basis underlying these functions remain poorly defined. Progress has been limited by the difficulty of resolving individual catalytic steps under single-turnover conditions, which are often experimentally inaccessible for viral enzymes. Alphaviruses replicate within membrane-bound spherules that may alter local metabolite concentrations, raising the possibility that the enzymatic properties of alphaviral proteins differ from those of viruses with greater cytosolic exposure. Here, we present a kinetic and binding analysis of full-length non-structural protein 2 (nsP2) from Chikungunya virus, a multifunctional superfamily 1B NTPase and RNA helicase. Purified nsP2 binds nucleoside triphosphates with high affinity, exhibiting equilibrium dissociation constants in the single digit micromolar range. This property enabled single-turnover, pre-steady-state, and isotope-trapping experiments that are rarely feasible for viral helicases. These analyses identified two sequential conformational-change steps required for nucleotide hydrolysis. Molecular dynamics simulations suggest tightening of the RecA1 and RecA2 domains upon ATP binding followed by compaction of the enzyme mediated by interactions between the 1B subdomain and RecA2 domain. Product inhibition patterns support random release of ADP and inorganic phosphate, with relative binding affinities indicating that ADP dissociates first. The reaction is irreversible. Although nsP2 binds RNA tightly, strand separation under single-turnover conditions is too slow to represent ATP-driven unwinding, instead likely reflecting formation of an unwinding-competent nsP2-RNA complex. Together, these findings establish a quantitative framework for nsP2 function and provide a roadmap for mechanistic studies of alphaviral helicases. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=63 SRC="FIGDIR/small/723793v1_ufig1.gif" ALT="Figure 1"> View larger version (18K): org.highwire.dtl.DTLVardef@13899a1org.highwire.dtl.DTLVardef@ee1aadorg.highwire.dtl.DTLVardef@1991e1org.highwire.dtl.DTLVardef@b877f6_HPS_FORMAT_FIGEXP M_FIG C_FIG

Multifunctional proteins encode specificity through nuanced molecular interactions that operate in the presence of abundant wild-type molecules. Previous studies on multifunctional proteins have shown how certain regulated interactions parse complex biological activities into discrete functional modules. Discrete interaction edges can function as regulatory nodes whose perturbations selectively remodel proteostasis output, especially under disease conditions. However, whether such interaction specificity can be harnessed to selectively manipulate functional modules and reveal residue-level control principles, remains largely unexplored. Here, we screened for dominant negative mutations that impact partner-selective functions linked to Leu8, a critical binding residue on ubiquitin. Our results reveal that the Leu8Ala mutation specifically leads to accumulation of polyubiquitinated proteins and decreased levels of free ubiquitin suggesting loss of deubiquitinase function in yeast cells. Cellular and biochemical analyses establish that the Leu8Ala variant of ubiquitin specifically inhibits the yeast deubiquitinase, Ubp14, and its human homolog, USP5. We further demonstrate remodelling of the binding interface with increased interface contacts for the variant and Ubp14 complex. The variant shows a higher inhibitory potency compared to the wild-type ubiquitin and can inhibit Ubp14 both as unconjugated and as conjugated ubiquitin chain indicating the strength of its inhibitory function. Our results provide mechanistic insight into how edge perturbations in ubiquitin can reveal critical nodes that impact selective functions and thus fine-tune the cellular proteostasis network for therapeutic benefit.