Distinct ubiquitin binding modes exhibited by SH3 domains: Molecular determinants and functional implications

SH3 domains constitute a new type of ubiquitin-binding domains. We previously showed that the third SH3 domain (SH3-C) of CD2AP binds ubiquitin in an alternative orientation. We have determined the structure of the complex between first CD2AP SH3 domain and ubiquitin and performed a structural and mutational analysis to decipher the determinants of the SH3-C binding mode to ubiquitin. We found that the Phe-to-Tyr mutation in CD2AP and in the homologous CIN85 SH3-C domain does not abrogate ubiquitin binding, in contrast to previous hypothesis and our findings for the first two CD2AP SH3 domains. The similar alternative binding mode of the SH3-C domains of these related adaptor proteins is characterised by a higher affinity to C-terminal extended ubiquitin molecules. We conclude that CD2AP/CIN85 SH3-C domain interaction with ubiquitin constitutes a new ubiquitin-binding mode involved in a different cellular function and thus changes the previously established mechanism of EGF-dependent CD2AP/CIN85 mono-ubiquitination. © 2013 Ortega Roldan et al

Membrane insertion of soluble CLIC1 into active chloride channels is triggered by specific divalent cations

The CLIC family of proteins display the unique feature of altering their structure from a soluble form to a membrane-bound chloride channel. CLIC1, a member of this family, can be found in the cytoplasm or in nuclear, ER and plasma membranes, with membrane overexpression linked to tumour proliferation. The molecular switch promoting CLIC1 membrane insertion has been related to environmental factors, but still remains unclear. Here, we use solution NMR studies to confirm that both the soluble and membrane bound forms are in the same oxidation state. Our data from fluorescence assays and chloride efflux assays indicate that Ca2+ and Zn2+ trigger association to the membrane into active chloride channels. We use fluorescence microscopy to confirm that an increase of the intracellular Ca2+ leads to re-localisation of CLIC1 to both plasma and internal membranes. Finally, we show that soluble CLIC1 adopts an equilibrium of oligomeric species, and Ca2+/Zn2+ mediated membrane insertion promotes the formation of a tetrameric assembly. Thus, our results identify Ca2+ and Zn2+ binding as the molecular switch promoting CLIC1 membrane insertion.SIGNIFICANCE STATEMENT CLIC1, a member of the CLIC family of proteins, is expressed as a soluble protein in cells but can insert in the membrane forming a chloride channel. This chloride channel form is upregulated in different types of cancers including glioblastoma and promote tumour invasiveness and metastasis. The factors promoting CLIC1 membrane insertion nor the mechanism of this process are yet understood. Here, we use a combination of solution NMR, biophysics and fluorescence microscopy to identify Ca2+ and Zn2+ binding as the switch to promote CLIC1 insertion into the membrane to form active chloride channels. We also provide a simple mechanism how such transition to the membrane occurs. Such understanding will enable subsequent studies on the structure of the chloride channel form and its inhibition

Establishing a Metabolite Extraction Method to Study the Metabolome of Blastocystis Using NMR

Blastocystis is an opportunistic parasite commonly found in the intestines of humans and other animals. Despite its high prevalence, knowledge regarding Blastocystis biology within and out-side the host is limited. Analysis of the metabolites produced by this anaerobe could provide in-sights that can help map its metabolism and determine its role in both health and disease. Due to its controversial pathogenicity, these metabolites could define its deterministic role in microbi-ome’s “health” and/or subsequently resolve Blastocystis’ potential impact in gastrointestinal health. A common method for elucidating the presence of these metabolites is through 1H nuclear magnetic resonance (NMR). However, there are currently no described benchmarked methods available to extract metabolites from Blastocystis for 1H NMR analysis. Herein, several extrac-tion solvents, lysis methods and incubation temperatures were compared for their usefulness as an extraction protocol for this protozoan. Following extraction, the samples were freeze-dried, re-solubilized and analysed with 1H NMR. The results demonstrate that carrying out the proce-dure at room temperature using methanol as an extraction solvent and bead bashing as a lysis technique provides a consistent, reproducible and efficient method to extract metabolites from Blastocystis for NMR

Mutually Beneficial Combination of Molecular Dynamics Computer Simulations and Scattering Experiments

We showcase the combination of experimental neutron scattering data and molecular dynamics (MD) simulations for exemplary phospholipid membrane systems. Neutron and X-ray reflectometry and small-angle scattering measurements are determined by the scattering length density profile in real space, but it is not usually possible to retrieve this profile unambiguously from the data alone. MD simulations predict these density profiles, but they require experimental control. Both issues can be addressed simultaneously by cross-validating scattering data and MD results. The strengths and weaknesses of each technique are discussed in detail with the aim of optimizing the opportunities provided by this combination

Structure-based discovery and in vitro validation of inhibitors of chloride intracellular channel 4 protein

The use of computer-aided methods have continued to propel accelerated drug discovery across various disease models, interestingly allowing the specific inhibition of pathogenic targets. Chloride Intracellular Channel Protein 4 (CLIC4) is a novel class of intracellular ion channel highly implicated in tumor and vascular biology. It regulates cell proliferation, apoptosis and angiogenesis; and is involved in multiple pathologic signaling pathways. Absence of specific inhibitors however impedes its advancement to translational research. Here, we integrate structural bioinformatics and experimental research approaches for the discovery and validation of small-molecule inhibitors of CLIC4. High-affinity allosteric binders were identified from a library of 1615 Food and Drug Administration (FDA)-approved drugs via a high-performance computing-powered blind-docking approach, resulting in the selection of amphotericin B and rapamycin. NMR assays confirmed the binding and conformational disruptive effects of both drugs while they also reversed stress-induced membrane translocation of CLIC4 and inhibited endothelial cell migration. Structural and dynamics simulation studies further revealed that the inhibitory mechanisms of these compounds were hinged on the allosteric modulation of the catalytic glutathione (GSH)-like site loop and the extended catalytic β loop which may elicit interference with the catalytic activities of CLIC4. Structure-based insights from this study provide the basis for the selective targeting of CLIC4 to treat the associated pathologies

A Zn2+-triggered two-step mechanism of CLIC1 membrane insertion and activation into chloride channels

The chloride intracellular channel (CLIC) protein family displays the unique feature of altering its structure from a soluble form to a membrane-bound chloride channel. CLIC1, a member of this family, is found in the cytoplasm or in internal and plasma membranes, with membrane relocalisation linked to endothelial disfunction, tumour proliferation and metastasis. The molecular switch promoting CLIC1 activation remains under investigation. Here, cellular Cl− efflux assays and immunofluorescence microscopy studies have identified intracellular Zn2+ release as the trigger for CLIC1 activation and membrane insertion. Biophysical assays confirmed specific binding to Zn2+, inducing membrane association and enhancing Cl− efflux in a pH-dependent manner. Together, our results identify a two- step mechanism with Zn2+ binding as the molecular switch promoting CLIC1 membrane insertion, followed by pH-mediated activation of Cl− efflux

Discovery and Rational Mutagenesis of Methionine Sulfoxide Reductase Biocatalysts To Expand the Substrate Scope of the Kinetic Resolution of Chiral Sulfoxides

Methionine sulfoxide reductase A (MsrA) enzymes have recently found applications as nonoxidative biocatalysts in the enantioselective kinetic resolution of racemic sulfoxides. This work describes the identification of selective and robust MsrA biocatalysts able to catalyze the enantioselective reduction of a variety of aromatic and aliphatic chiral sulfoxides at 8–64 mM concentration with high yields and excellent ees (up to 99%). Moreover, with the aim to expand the substrate scope of MsrA biocatalysts, a library of mutant enzymes has been designed via rational mutagenesis utilizing in silico docking, molecular dynamics, and structural nuclear magnetic resonance (NMR) studies. The mutant enzyme MsrA33 was found to catalyze the kinetic resolution of bulky sulfoxide substrates bearing non-methyl substituents on the sulfur atom with ees up to 99%, overcoming a significant limitation of the currently available MsrA biocatalysts

A quantitative assay to study the lipid selectivity of membrane-associated systems using solution NMR

The activity of membrane proteins and compounds that interact with the membrane is modulated by the surrounding lipid composition. However, there are no simple methods that determine the composition of these annular phospholipids in eukaryotic systems. Herein, we describe a simple methodology that enables the identification and quantification of the lipid composition around membrane-associated compounds using SMA-nanodiscs and routine $^1H-^{31}P NMR$