Identification of primary and secondary UBA footprints on the surface of ubiquitin in cell-mimicking crowded solution

Ubiquitination of Alzheimer's-related tau protein affects liquid-liquid phase separation in a site- and cofactor-dependent manner

The formation of biomolecular condensates has emerged as a crucial player both in neuronal physiology and neurodegeneration. Phase separation of the Alzheimer's related protein tau into liquid condensates is facilitated by polyanions and is regulated by post-translational modifications. Given the central role of ubiquitination in proteostasis regulation and signaling, we investigated the behavior of monoubiquitinated tau during formation of condensates. We ubiquitinated the lysine-rich, four-repeat domain of tau either unspecifically via enzymatic conjugation or in a position-specific manner by semisynthesis. Ubiquitin conjugation at specific sites weakened multivalent tau/RNA interactions and disfavored tau/heparin condensation. Yet, heterogeneous ubiquitination was tolerated during phase separation and stabilized droplets against aggregation-linked dissolution. Thus, we demonstrated that cofactor chemistry and site of modification affect the mesoscopic and molecular signatures of ubiquitinated tau condensates. Our findings suggest that ubiquitination could influence the physiological states and pathological transformations of tau in cellular condensates.

Biomolecular phase separation through the lens of sodium-23 NMR

Phase separation is a fundamental physicochemical process underlying the spatial arrangement and coordination of cellular events. Detailed characterization of biomolecular phase separation requires experimental access to the internal environment of dilute and especially condensed phases at high resolution. In this study, we take advantage from the ubiquitous presence of sodium ions in biomolecular samples and present the potentials of ²³Na NMR as a proxy to report the internal fluidity of biomolecular condensed phases. After establishing the temperature and viscosity dependence of ²³Na NMR relaxation rates and translational diffusion coefficient, we demonstrate that ²³Na NMR probes of rotational and translational mobility of sodium ions are capable of capturing the increasing levels of confinement in agarose gels in dependence of agarose concentration. The ²³Na NMR approach is then applied to a gel‐forming phenylalanine‐glycine (FG)‐containing peptide, part of the nuclear pore complex involved in controlling the traffic between cytoplasm and cell nucleus. It is shown that the ²³Na NMR together with the ¹⁷O NMR provide a detailed picture of the sodium ion and water mobility within the interior of the FG peptide hydrogel. As another example, we study phase separation in water‐triethylamine (TEA) mixture and provide evidence for the presence of multiple microscopic environments within the TEA‐rich phase. Our results highlight the potentials of ²³Na NMR in combination with ¹⁷O NMR in studying biological phase separation, in particular with regards to the molecular properties of biomolecular condensates and their regulation through various physico‐ and biochemical factors.

Semisynthetic Modification of Tau Protein with Di-Ubiquitin Chains for Aggregation Studies

Ubiquitin, a protein modifier that regulates diverse essential cellular processes, is also a component of the protein inclusions characteristic of many neurodegenerative disorders. In Alzheimer’s disease, the microtubule associated tau protein accumulates within damaged neurons in the form of cross-beta structured filaments. Both mono- and polyubiquitin were found linked to several lysine residues belonging to the region of tau protein that forms the structured core of the filaments. Thus, besides priming the substrate protein for proteasomal degradation, ubiquitin could also contribute to the assembly and stabilization of tau protein filaments. To advance our understanding of the impact of ubiquitination on tau protein aggregation and function, we applied disulfide-coupling chemistry to modify tau protein at position 353 with Lys48- or Lys63-linked di-ubiquitin, two representative polyubiquitin chains that differ in topology and structure. Aggregation kinetics experiments performed on these conjugates reveal that di-ubiquitination retards filament formation and perturbs the fibril elongation rate more than mono-ubiquitination. We further show that di-ubiquitination modulates tau-mediated microtubule assembly. The effects on tau protein aggregation and microtubule polymerization are essentially independent from polyubiquitin chain topology. Altogether, our findings provide novel insight into the consequences of ubiquitination on the functional activity and disease-related behavior of tau protein.

Solution NMR insights into dynamic supramolecular assemblies of disordered amyloidogenic proteins

The extraordinary flexibility and structural heterogeneity of intrinsically disordered proteins (IDP) make them functionally versatile molecules. We have now begun to better understand their fundamental role in biology, however many aspects of their behaviour remain difficult to grasp experimentally. This is especially true for the intermolecular interactions which lead to the formation of transient or highly dynamic supramolecular self-assemblies, such as oligomers, aggregation intermediates and biomolecular condensates. Both the emerging functions and pathogenicity of these structures have stimulated great efforts to develop methodologies capable of providing useful insights. Significant progress in solution NMR spectroscopy has made this technique one of the most powerful to describe structural and dynamic features of IDPs within such assemblies at atomic resolution. Here, we review the most recent works that have illuminated key aspects of IDP assemblies and contributed significant advancements towards our understanding of the complex conformational landscape of prototypical disease-associated proteins. We also include a primer on some of the fundamental and innovative NMR methods being used in the discussed studies.

A facile oxygen-17 NMR method to determine effective viscosity in dilute, molecularly crowded and confined aqueous media

We present an NMR method based on natural abundance 17O relaxation of water to determine effective viscosity in biological aqueous samples. The method accurately captures viscosity of dilute and crowded protein solutions and offers a fairly simple way to quantify the internal fluidity of biological condensates formed through phase separation.

Protein shape modulates crowding effects

Protein-protein interactions are usually studied in dilute buffered solutions with macromolecule concentrations of <10 g/L. In cells, however, the macromolecule concentration can exceed 300 g/L, resulting in nonspecific interactions between macromolecules. These interactions can be divided into hard-core steric repulsions and "soft" chemical interactions. Here, we test a hypothesis from scaled particle theory; the influence of hard-core repulsions on a protein dimer depends on its shape. We tested the idea using a side-by-side dumbbell-shaped dimer and a domain-swapped ellipsoidal dimer. Both dimers are variants of the B1 domain of protein G and differ by only three residues. The results from the relatively inert synthetic polymer crowding molecules, Ficoll and PEG, support the hypothesis, indicating that the domain-swapped dimer is stabilized by hard-core repulsions while the side-by-side dimer shows little to no stabilization. We also show that protein cosolutes, which interact primarily through nonspecific chemical interactions, have the same small effect on both dimers. Our results suggest that the shape of the protein dimer determines the influence of hard-core repulsions, providing cells with a mechanism for regulating protein-protein interactions.

Survival analysis of genome-wide profiles coupled with Connectivity Map database mining to identify potential therapeutic targets for cholangiocarcinoma

Cholangiocarcinoma (CCA) is one of the most common epithelial cell malignancies worldwide. However, its prognosis is poor. The aim of the present study was to examine the prognostic landscape and potential therapeutic targets for CCA. RNA sequencing data and clinical information were downloaded from The Cancer Genome Atlas (TCGA) dataset and processed. A total of 172 genes that were significantly associated with overall survival of patients with CCA were identified using the univariate Cox regression method. Bioinformatics tools were applied using the Kyoto Encyclopedia of Genes and Genomes (KEGG) and gene ontology (GO). It was identified that ‘Wnt signaling pathway’, ‘cytoplasm’ and ‘AT DNA binding’ were the three most significant GO categories of CCA survival-associated genes. ‘Transcriptional misregulation in cancer’ was the most significant pathway identified in the KEGG analysis. Using the Drug-Gene Interaction database, a drug-gene interaction network was constructed, and 31 identified genes were involved in it. The most meaningful potential therapeutic targets were selected via protein-protein and gene-drug interactions. Among these genes, polo-like kinase 1 (PLK1) was identified to be a potential target due to its significant upregulation in CCA. To rapidly find molecules that may affect these genes, the Connectivity Map was queried. A series of molecules were selected for their potential anti-CCA functions. 0297417-0002B and tribenoside exhibited the highest connection scores with PLK1 via molecular docking. These findings may offer novel insights into treatment and perspectives on the future innovative treatment of CCA.

Alzheimer's disease-associated ubiquitin mutant Ubb+1: Properties of the carboxy-terminal domain and its influence on biomolecular interactions

Ubb⁺¹, a ubiquitin (Ub) mutant protein originating from misreading of the Ub B gene, is found accumulated in brain tissues of Alzheimer's disease patients. The mutant attracts strong interest due to its possible participation in the molecular events leading to neurodegeneration. Ubb⁺¹ is composed of the globular domain of Ub, linked to a 19-residue C-terminal peptide. Based on NMR relaxation and solvent accessibility measurements we obtained new insight into the molecular properties of Ubb⁺¹. We further determined the thermal stability of Ubb⁺¹ in the monomeric form, and in Lys48- and Lys63-linked dimers. Finally, we explored the influence of the C-terminal fragment on the interactions of Ubb⁺¹ with an isolated UBA2 domain and with membrane mimics. Our data indicate that the C-terminal fragment of Ubb⁺¹ is overall highly flexible, except for a short stretch which appears less solvent-exposed. While influencing the hydrodynamic properties of the globular domain, the fragment does not establish long-lived interactions with the globular domain. It results that the structure and stability of Ub are minimally perturbed by the peptide extension. However, binding to UBA2 and to membrane mimics are both affected, exemplifying possible changes in biomolecular recognition experienced by the disease-associated Ubb⁺¹ compared to the wild-type protein.

Crowding in Cellular Environments at an Atomistic Level from Computer Simulations

The effects of crowding in biological environments on biomolecular structure, dynamics, and function remain not well understood. Computer simulations of atomistic models of concentrated peptide and protein systems at different levels of complexity are beginning to provide new insights. Crowding, weak interactions with other macromolecules and metabolites, and altered solvent properties within cellular environments appear to remodel the energy landscape of peptides and proteins in significant ways including the possibility of native state destabilization. Crowding is also seen to affect dynamic properties, both conformational dynamics and diffusional properties of macromolecules. Recent simulations that address these questions are reviewed here and discussed in the context of relevant experiments.