Insights into Cotranslational Membrane Protein Insertion by Combined LILBID-Mass Spectrometry and NMR Spectroscopy.

Cotranslational insertion of membrane proteins into defined nanoparticle membranes has been developed as an efficient process to produce highly soluble samples in native-like environments and to study lipid-dependent effects on protein structure and function. Numerous examples of the structural and functional characterization of transporters, ion channels, or G-protein-coupled receptors in cotranslationally formed nanodisc complexes demonstrate the versatility of this approach, although the basic underlying mechanisms of membrane insertion are mainly unknown. We have revealed the first aspects of the insertion of proteins into nanodiscs by combining cell-free expression, noncovalent mass spectrometry, and NMR spectroscopy. We provide evidence of cooperative insertion of homo-oligomeric complexes and demonstrate the possibility to modulate their stoichiometry by modifying reaction conditions. Additionally, we show that significant amounts of lipid are released from the nanodiscs upon insertion of larger protein complexes.

High-throughput cell-free screening of eukaryotic membrane protein expression in lipidic mimetics.

Membrane proteins play essential roles in cellular function and metabolism. Nonetheless, biophysical and structural studies of membrane proteins are impeded by the difficulty of their expression in and purification from heterologous cell-based systems. As an alternative to these cell-based systems, cell-free protein synthesis has proven to be an exquisite method for screening membrane protein targets in a variety of lipidic mimetics. Here we report a high-throughput screening workflow and apply it to screen 61 eukaryotic membrane protein targets. For each target, we tested its expression in lipidic mimetics: two detergents, two liposomes, and two nanodiscs. We show that 35 membrane proteins (57%) can be expressed in a soluble fraction in at least one of the mimetics with the two detergents performing significantly better than nanodiscs and liposomes, in that order. Using the established cell-free workflow, we studied the production and biophysical assays for mitochondrial pyruvate carrier (MPC) complexes. Our studies show that the complexes produced in cell-free are functionally competent in complex formation and substrate binding. Our results highlight the utility of using cell-free systems for screening and production of eukaryotic membrane proteins.

Preparation, crystallization and preliminary X-ray diffraction analysis of two intestinal fatty-acid binding proteins in the presence of 11-(dansylamino)undecanoic acid.

Fatty-acid binding proteins (FABPs) are abundantly expressed proteins that bind a range of lipophilic molecules. They have been implicated in the import and intracellular distribution of their ligands and have been linked with metabolic and inflammatory responses in the cells in which they are expressed. Despite their high sequence identity, human intestinal FABP (hIFABP) and rat intestinal FABP (rIFABP) bind some ligands with different affinities. In order to address the structural basis of this differential binding, diffraction-quality crystals have been obtained of hIFABP and rIFABP in complex with the fluorescent fatty-acid analogue 11-(dansylamino)undecanoic acid.

Synthetic Biology-Based Solution NMR Studies on Membrane Proteins in Lipid Environments.

Although membrane proteins are in the focus of biochemical research for many decades the general knowledge of this important class is far behind soluble proteins. Despite several recent technical developments, the most challenging feature still is the generation of high-quality samples in environments suitable for the selected application. Reconstitution of membrane proteins into lipid bilayers will generate the most native-like environment and is therefore commonly desired. However, it poses tremendous problems to solution-state NMR analysis due to the dramatic increase in particle size resulting in high rotational correlation times. Nevertheless, a few promising strategies for the solution NMR analysis of membrane inserted proteins are emerging and will be discussed in this chapter. We focus on the generation of membrane protein samples in nanodisc membranes by cell-free systems and will describe the characteristic advantages of that platform in providing tailored protein expression and folding environments. We indicate frequent problems that have to be overcome in cell-free synthesis, nanodisc preparation, and customization for samples dedicated for solution-state NMR. Detailed instructions for sample preparation are given, and solution NMR approaches suitable for membrane proteins in bilayers are compiled. We further discuss the current strategies applied for signal detection from such difficult samples and describe the type of information that can be extracted from the various experiments. In summary, a comprehensive guideline for the analysis of membrane proteins in native-like membrane environments by solution-state NMR techniques will be provided.

Examination of the role of intestinal fatty acid-binding protein in drug absorption using a parallel artificial membrane permeability assay.

Transcellular diffusion across the absorptive epithelial cells (enterocytes) of the small intestine is the main route of absorption for most orally administered drugs. The process by which lipophilic compounds transverse the aqueous environment of the cytoplasm, however, remains poorly defined. In the present study, we have identified a structurally diverse group of lipophilic drugs that display low micromolar binding affinities for a cytosolic lipid-binding protein - intestinal fatty acid-binding protein (I-FABP). Binding to I-FABP significantly enhanced the transport of lipophilic drug molecules across a model membrane, and the degree of transport enhancement was related to both drug lipophilicity and I-FABP binding affinity. These data suggest that intracellular lipid-binding proteins such as I-FABP may enhance the membrane transport of lipophilic xenobiotics and facilitate drug access to the enterocyte cytoplasm and cytoplasmic organelles.

Protein labeling strategies for liquid-state NMR spectroscopy using cell-free synthesis.

Preparation of a protein sample for liquid-state nuclear magnetic resonance (NMR) spectroscopy analysis requires optimization of many parameters. This review describes labeling strategies for obtaining assignments of protein resonances. Particular emphasis is placed on the advantages of cell-free protein production, which enables exclusive labeling of the protein of interest, thereby simplifying downstream processing steps and increasing the availability of different labeling strategies for a target protein. Furthermore, proteins can be synthesized in milligram yields, and the open nature of the cell-free system allows the addition of stabilizers, scrambling inhibitors or hydrophobic solubilization environments directly during the protein synthesis, which is especially beneficial for membrane proteins. Selective amino acid labeling of the protein of interest, the possibility of addressing scrambling issues and avoiding the need for labile amino acid precursors have been key factors in enabling the introduction of new assignment strategies based on different labeling schemes as well as on new pulse sequences. Combinatorial selective labeling methods have been developed to reduce the number of protein samples necessary to achieve a complete backbone assignment. Furthermore, selective labeling helps to decrease spectral overlap and overcome size limitations for solution NMR analysis of larger complexes, oligomers, intrinsically disordered proteins and membrane proteins.

Analyzing native membrane protein assembly in nanodiscs by combined non-covalent mass spectrometry and synthetic biology.

Membrane proteins frequently assemble into higher order homo- or hetero-oligomers within their natural lipid environment. This complex formation can modulate their folding, activity as well as substrate selectivity. Non-disruptive methods avoiding critical steps, such as membrane disintegration, transfer into artificial environments or chemical modifications are therefore essential to analyze molecular mechanisms of native membrane protein assemblies. The combination of cell-free synthetic biology, nanodisc-technology and non-covalent mass spectrometry provides excellent synergies for the analysis of membrane protein oligomerization within defined membranes. We exemplify our strategy by oligomeric state characterization of various membrane proteins including ion channels, transporters and membrane-integrated enzymes assembling up to hexameric complexes. We further indicate a lipid-dependent dimer formation of MraY translocase correlating with the enzymatic activity. The detergent-free synthesis of membrane protein/nanodisc samples and the analysis by LILBID mass spectrometry provide a versatile platform for the analysis of membrane proteins in a native environment.

From Nanodiscs to Isotropic Bicelles: A Procedure for Solution Nuclear Magnetic Resonance Studies of Detergent-Sensitive Integral Membrane Proteins.

Nanodiscs and isotropic bicelles are promising membrane mimetics in the field of solution nuclear magnetic resonance (NMR) spectroscopy of integral membrane proteins (IMPs). Despite varied challenges to solution NMR studies of IMPs, we attribute the paucity of solution NMR structures in these environments to the inability of diverse IMPs to withstand detergent treatment during standard nanodisc and bicelle preparations. Here, we present a strategy that creates small isotropic bicelles from IMPs co-translationally embedded in large nanodiscs using cell-free expression. Our results demonstrate appreciable gains in NMR spectral quality while preserving lipid-IMP contacts. We validate the approach on the detergent-sensitive LspA, which finally allowed us to perform high-quality triple-resonance NMR experiments for structural studies. Our strategy of producing bicelles from nanodiscs comprehensively avoids detergent during expression and preparation and is suitable for solution NMR spectroscopy of lipid-IMP complexes.

Labeling of membrane proteins by cell-free expression.

The particular advantage of the cell-free reaction is that it allows a plethora of supplementation during protein expression and offers complete control over the available amino acid pool in view of concentration and composition. In combination with the fast and reliable production efficiencies of cell-free systems, the labeling and subsequent structural evaluation of very challenging targets, such as membrane proteins, comes into focus. We describe current methods for the isotopic labeling of cell-free synthesized membrane proteins and we review techniques available to the practitioner pursuing structural studies by nuclear magnetic resonance spectroscopy. Though isotopic labeling of individual amino acid types appears to be relatively straightforward, an ongoing critical issue in most labeling schemes for structural approaches is the selective substitution of deuterons for protons. While few options are available, the continuous refinement of labeling schemes in combination with improved pulse sequences and optimized instrumentation gives promising perspectives for extended applications in the structural evaluation of cell-free synthesized membrane.

Characterization of two distinct modes of drug binding to human intestinal fatty acid binding protein.

The aqueous cytoplasm of cells poses a potentially significant barrier for many lipophilic drugs to reach their sites of action. Fatty acid binding proteins (FABPs) bind to poorly water-soluble fatty acids (FAs) and lipophilic compounds and facilitate their intracellular transport. Several structures of FA in complex with FABPs have been described, but data describing the binding sites of other lipophilic ligands including drugs are limited. Here the environmentally sensitive fluorophores, 1-anilinonapthalene 8-sulfonic acid (ANS), and 11-dansylamino undecanoic acid (DAUDA) were used to investigate drug binding to human intestinal FABP (hIFABP). Most drugs that bound hIFABP were able to displace both ANS and DAUDA. A notable exception was ketorolac, a non-steroidal anti-inflammatory drug that bound to hIFABP and displaced DAUDA but failed to displace ANS. Isothermal titration calorimetry revealed that for the majority of ligands including FA, ANS, and DAUDA, binding to hIFABP was exothermic. In contrast, ketorolac binding to hIFABP was endothermic and entropy-driven. The X-ray crystal structure of DAUDA-hIFABP revealed a FA-like binding mode where the carboxylate of DAUDA formed a network of hydrogen bonds with residues at the bottom of the binding cavity and the dansyl group interacted with residues in the portal region. In contrast, NMR chemical shift perturbation (CSP) data suggested that ANS bound only toward the bottom of the hIFABP cavity, whereas ketorolac occupied only the portal region. The CSP data further suggested that ANS and ketorolac were able to bind simultaneously to hIFABP, consistent with the lack of displacement of ANS observed by fluorescence and supported by a model of the ternary complex. The NMR solution structure of the ketorolac-hIFABP complex therefore describes a newly characterized, hydrophobic ligand binding site in the portal region of hIFABP.

Time-shared experiments for efficient assignment of triple-selectively labeled proteins.

Combinatorial triple-selective labeling facilitates the NMR assignment process for proteins that are subject to signal overlap and insufficient signal-to-noise in standard triple-resonance experiments. Aiming at maximum amino-acid type and sequence-specific information, the method represents a trade-off between the number of selectively labeled samples that have to be prepared and the number of spectra to be recorded per sample. In order to address the demand of long measurement times, we here propose pulse sequences in which individual phase-shifted transients are stored separately and recombined later to produce several 2D HN(CX) type spectra that are usually acquired sequentially. Sign encoding by the phases of (13)C 90° pulses allows to either select or discriminate against (13)C' or (13)C(α) spins coupled to (15)N. As a result, (1)H-(15)N correlation maps of the various isotopomeric species present in triple-selectively labeled proteins are deconvoluted which in turn reduces problems due to spectral overlap. The new methods are demonstrated with four different membrane proteins with rotational correlation times ranging from 18 to 52 ns.