Mechanistic View of hnRNPA2 Low-Complexity Domain Structure, Interactions, and Phase Separation Altered by Mutation and Arginine Methylation.

hnRNPA2, a component of RNA-processing membraneless organelles, forms inclusions when mutated in a syndrome characterized by the degeneration of neurons (bearing features of amyotrophic lateral sclerosis [ALS] and frontotemporal dementia), muscle, and bone. Here we provide a unified structural view of hnRNPA2 self-assembly, aggregation, and interaction and the distinct effects of small chemical changes-disease mutations and arginine methylation-on these assemblies. The hnRNPA2 low-complexity (LC) domain is compact and intrinsically disordered as a monomer, retaining predominant disorder in a liquid-liquid phase-separated form. Disease mutations D290V and P298L induce aggregation by enhancing and extending, respectively, the aggregation-prone region. Co-aggregating in disease inclusions, hnRNPA2 LC directly interacts with and induces phase separation of TDP-43. Conversely, arginine methylation reduces hnRNPA2 phase separation, disrupting arginine-mediated contacts. These results highlight the mechanistic role of specific LC domain interactions and modifications conserved across many hnRNP family members but altered by aggregation-causing pathological mutations.

An intrinsically disordered motif regulates the interaction between the p47 adaptor and the p97 AAA+ ATPase.

VCP/p97, an enzyme critical to proteostasis, is regulated through interactions with protein adaptors targeting it to specific cellular tasks. One such adaptor, p47, forms a complex with p97 to direct lipid membrane remodeling. Here, we use NMR and other biophysical methods to study the structural dynamics of p47 and p47-p97 complexes. Disordered regions in p47 are shown to be critical in directing intra-p47 and p47-p97 interactions via a pair of previously unidentified linear motifs. One of these, an SHP domain, regulates p47 binding to p97 in a manner that depends on the nucleotide state of p97. NMR and electron cryomicroscopy data have been used as restraints in molecular dynamics trajectories to develop structural ensembles for p47-p97 complexes in adenosine diphosphate (ADP)- and adenosine triphosphate (ATP)-bound conformations, highlighting differences in interactions in the two states. Our study establishes the importance of intrinsically disordered regions in p47 for the formation of functional p47-p97 complexes.

TDP-43 α-helical structure tunes liquid-liquid phase separation and function.

Liquid-liquid phase separation (LLPS) is involved in the formation of membraneless organelles (MLOs) associated with RNA processing. The RNA-binding protein TDP-43 is present in several MLOs, undergoes LLPS, and has been linked to the pathogenesis of amyotrophic lateral sclerosis (ALS). While some ALS-associated mutations in TDP-43 disrupt self-interaction and function, here we show that designed single mutations can enhance TDP-43 assembly and function via modulating helical structure. Using molecular simulation and NMR spectroscopy, we observe large structural changes upon dimerization of TDP-43. Two conserved glycine residues (G335 and G338) are potent inhibitors of helical extension and helix-helix interaction, which are removed in part by variants at these positions, including the ALS-associated G335D. Substitution to helix-enhancing alanine at either of these positions dramatically enhances phase separation in vitro and decreases fluidity of phase-separated TDP-43 reporter compartments in cells. Furthermore, G335A increases TDP-43 splicing function in a minigene assay. Therefore, the TDP-43 helical region serves as a short but uniquely tunable module where application of biophysical principles can precisely control assembly and function in cellular and synthetic biology applications of LLPS.

A single N-terminal phosphomimic disrupts TDP-43 polymerization, phase separation, and RNA splicing.

TDP-43 is an RNA-binding protein active in splicing that concentrates into membraneless ribonucleoprotein granules and forms aggregates in amyotrophic lateral sclerosis (ALS) and Alzheimer's disease. Although best known for its predominantly disordered C-terminal domain which mediates ALS inclusions, TDP-43 has a globular N-terminal domain (NTD). Here, we show that TDP-43 NTD assembles into head-to-tail linear chains and that phosphomimetic substitution at S48 disrupts TDP-43 polymeric assembly, discourages liquid-liquid phase separation (LLPS) in vitro, fluidizes liquid-liquid phase separated nuclear TDP-43 reporter constructs in cells, and disrupts RNA splicing activity. Finally, we present the solution NMR structure of a head-to-tail NTD dimer comprised of two engineered variants that allow saturation of the native polymerization interface while disrupting higher-order polymerization. These data provide structural detail for the established mechanistic role of the well-folded TDP-43 NTD in splicing and link this function to LLPS. In addition, the fusion-tag solubilized, recombinant form of TDP-43 full-length protein developed here will enable future phase separation and in vitro biochemical assays on TDP-43 function and interactions that have been hampered in the past by TDP-43 aggregation.

Mice with endogenous TDP-43 mutations exhibit gain of splicing function and characteristics of amyotrophic lateral sclerosis.

TDP-43 (encoded by the gene TARDBP) is an RNA binding protein central to the pathogenesis of amyotrophic lateral sclerosis (ALS). However, how TARDBP mutations trigger pathogenesis remains unknown. Here, we use novel mouse mutants carrying point mutations in endogenous Tardbp to dissect TDP-43 function at physiological levels both in vitro and in vivo Interestingly, we find that mutations within the C-terminal domain of TDP-43 lead to a gain of splicing function. Using two different strains, we are able to separate TDP-43 loss- and gain-of-function effects. TDP-43 gain-of-function effects in these mice reveal a novel category of splicing events controlled by TDP-43, referred to as "skiptic" exons, in which skipping of constitutive exons causes changes in gene expression. In vivo, this gain-of-function mutation in endogenous Tardbp causes an adult-onset neuromuscular phenotype accompanied by motor neuron loss and neurodegenerative changes. Furthermore, we have validated the splicing gain-of-function and skiptic exons in ALS patient-derived cells. Our findings provide a novel pathogenic mechanism and highlight how TDP-43 gain of function and loss of function affect RNA processing differently, suggesting they may act at different disease stages.

Phosphorylation of the FUS low-complexity domain disrupts phase separation, aggregation, and toxicity.

Neuronal inclusions of aggregated RNA-binding protein fused in sarcoma (FUS) are hallmarks of ALS and frontotemporal dementia subtypes. Intriguingly, FUS's nearly uncharged, aggregation-prone, yeast prion-like, low sequence-complexity domain (LC) is known to be targeted for phosphorylation. Here we map in vitro and in-cell phosphorylation sites across FUS LC We show that both phosphorylation and phosphomimetic variants reduce its aggregation-prone/prion-like character, disrupting FUS phase separation in the presence of RNA or salt and reducing FUS propensity to aggregate. Nuclear magnetic resonance spectroscopy demonstrates the intrinsically disordered structure of FUS LC is preserved after phosphorylation; however, transient domain collapse and self-interaction are reduced by phosphomimetics. Moreover, we show that phosphomimetic FUS reduces aggregation in human and yeast cell models, and can ameliorate FUS-associated cytotoxicity. Hence, post-translational modification may be a mechanism by which cells control physiological assembly and prevent pathological protein aggregation, suggesting a potential treatment pathway amenable to pharmacologic modulation.

Lysines in the RNA Polymerase II C-Terminal Domain Contribute to TAF15 Fibril Recruitment.

Many cancer-causing chromosomal translocations result in transactivating protein products encoding FET family (FUS, EWSR1, TAF15) low-complexity (LC) domains fused to a DNA binding domain from one of several transcription factors. Recent work demonstrates that higher-order assemblies of FET LC domains bind the carboxy-terminal domain of the large subunit of RNA polymerase II (RNA pol II CTD), suggesting FET oncoproteins may mediate aberrant transcriptional activation by recruiting RNA polymerase II to promoters of target genes. Here we use nuclear magnetic resonance (NMR) spectroscopy and hydrogel fluorescence microscopy localization and fluorescence recovery after photobleaching to visualize atomic details of a model of this process, interactions of RNA pol II CTD with high-molecular weight TAF15 LC assemblies. We report NMR resonance assignments of the intact degenerate repeat half of human RNA pol II CTD alone and verify its predominant intrinsic disorder by molecular simulation. By measuring NMR spin relaxation and dark-state exchange saturation transfer, we characterize the interaction of RNA pol II CTD with amyloid-like hydrogel fibrils of TAF15 and hnRNP A2 LC domains and observe that heptads far from the acidic C-terminal tail of RNA pol II CTD bind TAF15 fibrils most avidly. Mutation of CTD lysines in heptad position 7 to consensus serines reduced the overall level of TAF15 fibril binding, suggesting that electrostatic interactions contribute to complex formation. Conversely, mutations of position 7 asparagine residues and truncation of the acidic tail had little effect. Thus, weak, multivalent interactions between TAF15 fibrils and heptads throughout RNA pol II CTD collectively mediate complex formation.

The C-terminal threonine of Aß43 nucleates toxic aggregation via structural and dynamical changes in monomers and protofibrils.

Recent studies suggest that deposition of amyloid ß (Aß) into oligomeric aggregates and fibrils, hallmarks of Alzheimer's disease, may be initiated by the aggregation of Aß species other than the well-studied 40- and 42-residue forms, Aß40 and Aß42, respectively. Here we report on key structural, dynamic, and aggregation kinetic parameters of Aß43, extended by a single threonine at the C-terminus relative to Aß42. Using aggregation time course experiments, electron microscopy, and a combination of nuclear magnetic resonance measurements including backbone relaxation, dark-state exchange saturation transfer, and quantification of chemical shift differences and scalar coupling constants, we demonstrate that the C-terminal threonine in Aß43 increases the rate and extent of protofibril aggregation and confers slow C-terminal motions in the monomeric and protofibril-bound forms of Aß43. Relative to the neighboring residues, the hydrophilic Thr43 of Aß43 favors direct contact with the protofibril surface more so than the C-terminus of Aß40 or Aß42. Taken together, these results demonstrate the potential of a small chemical modification to affect the properties of Aß structure and aggregation, providing a mechanism for the potential role of Aß43 as a primary nucleator of Aß aggregates in Alzheimer's disease.

Dual DNA/RNA-binding factor regulates dynamics of hnRNP splicing condensates.

Despite decades of research, mechanisms by which co-transcriptional alternative splicing events are targeted to the correct genomic locations to drive cell fate decisions remain unknown. By combining structural and molecular approaches, we define a new mechanism by which an essential transcription factor (TF) targets co-transcriptional splicing through physical and functional interaction with RNA and RNA binding proteins (RBPs). We show that an essential TF co-transcriptionally regulates sex-specific alternative splicing by directly interacting with a subset of target RNAs on chromatin and modulating the dynamics of hnRNPA2 homolog nuclear splicing condensates.

Differential Occupancy of Two GA-Binding Proteins Promotes Targeting of the Drosophila Dosage Compensation Complex to the Male X Chromosome.

Little is known about how variation in sequence composition alters transcription factor occupancy to precisely recruit large transcription complexes. A key model for understanding how transcription complexes are targeted is the Drosophila dosage compensation system in which the male-specific lethal (MSL) transcription complex specifically identifies and regulates the male X chromosome. The chromatin-linked adaptor for MSL proteins (CLAMP) zinc-finger protein targets MSL to the X chromosome but also binds to GA-rich sequence elements throughout the genome. Furthermore, the GAGA-associated factor (GAF) transcription factor also recognizes GA-rich sequences but does not associate with the MSL complex. Here, we demonstrate that MSL complex recruitment sites are optimal CLAMP targets. Specificity for CLAMP binding versus GAF binding is driven by variability in sequence composition within similar GA-rich motifs. Therefore, variation within seemingly similar cis elements drives the context-specific targeting of a large transcription complex.

ALS Mutations Disrupt Phase Separation Mediated by α-Helical Structure in the TDP-43 Low-Complexity C-Terminal Domain.

RNA-binding protein TDP-43 mediates essential RNA processing but forms cytoplasmic neuronal inclusions via its C-terminal domain (CTD) in amyotrophic lateral sclerosis (ALS). It remains unclear if aggregated TDP-43 is neurotoxic and if ∼50 ALS-associated missense mutations in TDP-43 CTD promote aggregation, or if loss of normal function plays a role in disease. Recent work points to the ability of related proteins to assemble into functional phase-separated ribonucleoprotein granules via their structurally disordered prion-like domains. Here, we provide atomic details on the structure and assembly of the low-complexity CTD of TDP-43 into liquid-liquid phase-separated in vitro granules and demonstrate that ALS-associated variants disrupt interactions within granules. Using nuclear magnetic resonance spectroscopy, simulation, and microscopy, we find that a subregion cooperatively but transiently folds into a helix that mediates TDP-43 phase separation. ALS-associated mutations disrupt phase separation by inhibiting interaction and helical stabilization. Therefore, ALS-associated mutations can disrupt TDP-43 interactions, affecting function beyond encouraging aggregation.