Screening of Detergents for Stabilization of Functional Membrane Proteins

eGFP as an All-in-One Tag for Purification of Membrane Proteins

Within the last decade, cryo-electron microscopy has revolutionized our understanding of membrane proteins, but they still represent challenging targets for biochemical and structural studies. The first obstacle is often to obtain high production levels of correctly folded target protein. In these cases, the use of eGFP tags is an efficient strategy, as it allows rapid screenings of expression systems, constructs, and detergents for solubilization. Additionally, eGFP tags can now be used for affinity purification with recently developed nanobodies. Here we present a series of methods based on enhanced green fluorescent protein (eGFP) fluorescence to efficiently screen for production and stabilization of detergent-solubilized eGFP-tagged membrane proteins produced in S. cerevisiae via in-gel fluorescence SDS-PAGE and fluorescence-detection size-exclusion chromatography (FSEC). Additionally, we present a protocol describing the production of affinity resin based on eGFP-binding nanobodies produced in E. coli. We showcase the purification of human ATP7B, a copper transporting P-type ATPase, as an example of the applicability of the methods.

Expression in Saccharomyces cerevisiae and Purification of a Human Phospholipid Flippase

Membrane proteins (MPs) are challenging to study from a biochemical standpoint owing to the difficulties associated with the isolation of these proteins from the membranes they are embedded in. Even for the expression of closely-related homologues, protocols often require to be adjusted. Prominently, the solubilization step and the stabilization of recombinant proteins during the purification process are key issues, and remain a serious bottleneck. Here, we present a method for the expression and the purification of the human ATP8B1/CDC50A lipid flippase complex. Selection of the right Saccharomyces cerevisiae strain proved to be a critical step for the successful purification of this complex. Likewise, the use of cholesteryl hemisuccinate, a cholesterol analogue, contributed to significantly increase the yield of purification. We hope that the simple method described here can help researchers to succeed in the expression of other mammalian difficult-to-express lipid flippases and, by extension, help in the production of other membrane proteins whose isolation has so far proven difficult.

Fluorescence-Detection Size-Exclusion Chromatography-Based Thermostability Assay for Membrane Proteins

Green fluorescent proteins (GFPs) have lightened up almost every aspect of biological research including protein sciences. In the field of membrane protein structural biology, GFPs have been used widely to monitor membrane protein localization, expression level, the purification process and yield, and the stability inside the cells and in the test tube. Of particular interest is the fluorescence-detector size-exclusion chromatography-based thermostability assay (FSEC-TS). By simple heating and FSEC, the generally applicable method allows rapid assessment of the thermostability of GFP-fused membrane proteins without purification. Here we describe the experimental details and some typical results for the FSEC-TS method.

Biophysical and functional characterization of the human TAS1R2 sweet taste receptor overexpressed in a HEK293S inducible cell line

Sweet taste perception is mediated by a heterodimeric receptor formed by the assembly of the TAS1R2 and TAS1R3 subunits. TAS1R2 and TAS1R3 are class C G-protein-coupled receptors whose members share a common topology, including a large extracellular N-terminal domain (NTD) linked to a seven transmembrane domain (TMD) by a cysteine-rich domain. TAS1R2-NTD contains the primary binding site for sweet compounds, including natural sugars and high-potency sweeteners, whereas the TAS1R2-TMD has been shown to bind a limited number of sweet tasting compounds. To understand the molecular mechanisms governing receptor–ligand interactions, we overexpressed the human TAS1R2 (hTAS1R2) in a stable tetracycline-inducible HEK293S cell line and purified the detergent-solubilized receptor. Circular dichroism spectroscopic studies revealed that hTAS1R2 was properly folded with evidence of secondary structures. Using size exclusion chromatography coupled to light scattering, we found that the hTAS1R2 subunit is a dimer. Ligand binding properties were quantified by intrinsic tryptophan fluorescence. Due to technical limitations, natural sugars have not been tested. However, we showed that hTAS1R2 is capable of binding high potency sweeteners with K_d values that are in agreement with physiological detection. This study offers a new experimental strategy to identify new sweeteners or taste modulators that act on the hTAS1R2 and is a prerequisite for structural query and biophysical studies.

Rationale for the Quantitative Reconstitution of Membrane Proteins into Proteoliposomes

Proteoliposome reconstitution is a method of choice for the investigation of membrane proteins as it allows their manipulation in the desired hydrophobic environment and allows one to tackle their study from both functional and structural points of view. Methods for their rapid and efficient reconstitution have been known for a long time but the quality and dispersity of the resulting suspensions is often overlooked. Here we describe our routine for the obtention of monodisperse populations of proteoliposomes as well as for the quantitation of protein per liposome.

An improved fluorescent tag and its nanobodies for membrane protein expression, stability assay, and purification

Green fluorescent proteins (GFPs) are widely used to monitor membrane protein expression, purification, and stability. An ideal reporter should be stable itself and provide high sensitivity and yield. Here, we demonstrate that a coral (Galaxea fascicularis) thermostable GFP (TGP) is by such reasons an improved tag compared to the conventional jellyfish GFPs. TGP faithfully reports membrane protein stability at temperatures near 90 °C (20-min heating). By contrast, the limit for the two popular GFPs is 64 °C and 74 °C. Replacing GFPs with TGP increases yield for all four test membrane proteins in four expression systems. To establish TGP as an affinity tag for membrane protein purification, several high-affinity synthetic nanobodies (sybodies), including a non-competing pair, are generated, and the crystal structure of one complex is solved. Given these advantages, we anticipate that TGP becomes a widely used tool for membrane protein structural studies.

Interaction of detergents with biological membranes: Comparison of fluorescence assays with filtration protocols and implications for the rates of detergent association, dissociation and flip-flop

The present study mainly consists of a re-evaluation of the rate at which C₁₂E₈, a typical non-ionic detergent used for membrane studies, is able to dissociate from biological membranes, with sarcoplasmic reticulum membrane vesicles being used as an example. Utilizing a brominated derivative of C₁₂E₈ and now stopped-flow fluorescence instead of rapid filtration, we found that the rate of dissociation of this detergent from these membranes, merely perturbed with non-solubilizing concentrations of detergent, was significantly faster (t_1/2 < 10 ms) than what had previously been determined (t_1/2 ~300–400 ms) from experiments based on a rapid filtration protocol using ¹⁴C-labeled C₁₂E₈ and glass fiber filters (Binding of a non-ionic detergent to membranes: flip-flop rate and location on the bilayer, by Marc le Maire, Jesper Møller and Philippe Champeil, Biochemistry (1987) Vol 26, pages 4803–4810). We here pinpoint a methodological problem of the earlier rapid filtration experiments, and we suggest that the true overall dissociation rate of C₁₂E₈ is indeed much faster than previously thought. We also exemplify the case of brominated dodecyl-maltoside, whose kinetics for overall binding to and dissociation from membranes comprise both a rapid and a sower phase, the latter being presumably due to flip-flop between the two leaflets of the membrane. Consequently, equilibrium is reached only after a few seconds for DDM. This work thereby emphasizes the interest of using the fluorescence quenching associated with brominated detergents for studying the kinetics of detergent/membrane interactions, namely association, dissociation and flip-flop rates.