Controlled immobilization of membrane proteins to surfaces for fourier transform infrared investigations

Smart polymer brush nanostructures guide the self-assembly of pore-spanning lipid bilayers with integrated membrane proteins

Nanopores in arrays on silicon chips are functionalized with pH-responsive poly(methacrylic acid) (PMAA) brushes and used as supports for pore-spanning lipid bilayers with integrated membrane proteins. Robust platforms are created by the covalent grafting of polymer brushes using surface-initiated atom transfer radical polymerization (ATRP), resulting in sensor chips that can be successfully reused over several assays. His-tagged proteins are selectively and reversibly bound to the nitrilotriacetic acid (NTA) functionalization of the PMAA brush, and consequently lipid bilayer membranes are formed. The enhanced membrane resistance as determined by electrochemical impedance spectroscopy and free diffusion of dyed lipids observed as fluorescence recovery after photobleaching confirmed the presence of lipid bilayers. Immobilization of the His-tagged membrane proteins on the NTA-modified PMAA brush near the pore edges is characterized by fluorescence microscopy. This system allows us to adjust the protein density in free-standing bilayers, which are stabilized by the polymer brush underneath. The potential application of the integrated platform for ion channel protein assays is demonstrated.

Biomimetic silica microspheres in biosensing

Lipid vesicles spontaneously fuse and assemble into a lipid bilayer on planar or spherical silica surfaces and other substrates. The supported lipid bilayers (SLBs) maintain characteristics of biological membranes, and are thus considered to be biomembrane mimetic systems that are stable because of the underlying substrate. Examples of their shared characteristics with biomembranes include lateral fluidity, barrier formation to ions and molecules, and their ability to incorporate membrane proteins into them. Biomimetic silica microspheres consisting of SLBs on solid or porous silica microspheres have been utilized for different biosensing applications. The advantages of such biomimetic microspheres for biosensing include their increased surface area to volume ratio which improves the detection limits of analytes, and their amenability for miniaturization, multiplexing and high throughput screening. This review presents examples and formats of using such biomimetic solid or porous silica microspheres in biosensing.

Techniques for phosphopeptide enrichment prior to analysis by mass spectrometry

Mass spectrometry is the tool of choice to investigate protein phosphorylation, which plays a vital role in cell regulation and diseases such as cancer. However, low abundances of phosphopeptides and low degrees of phosphorylation typically necessitate isolation and concentration of phosphopeptides prior to MS analysis. This review discusses the enrichment of phosphopeptides with immobilized metal affinity chromatography, reversible covalent binding, and metal oxide affinity chromatography. Capture of phosphopeptides on TiO(2) seems especially promising in terms of selectivity and recovery, but the success of all methods depends on careful selection of binding, washing, and elution solutions. Enrichment techniques are complementary, such that a combination of methods greatly enhances the number of phosphopeptides isolated from complex samples. Development of a standard series of phosphopeptides in a highly complex mixture of digested proteins would greatly aid the comparison of different enrichment methods. Phosphopeptide binding to magnetic beads and on-plate isolation prior to MALDI-MS are emerging as convenient methods for purification of small (microL) samples. On-plate enrichment can yield >70% recoveries of phosphopeptides in mixtures of a few digested proteins and can avoid sample-handling steps, but this technique is likely limited to relatively simple samples such as immunoprecipitates. With recent advances in enrichment techniques in hand, MS analysis should provide important insights into phosphorylation pathways.

A giant liposome for single-molecule observation of conformational changes in membrane proteins

We present an experimental system that allows visualization of conformational changes in membrane proteins at the single-molecule level. The target membrane protein is reconstituted in a giant liposome for independent control of the aqueous environments on the two sides of the membrane. For direct observation of conformational changes, an extra-liposomal site(s) of the target protein is bound to a glass surface, and a probe that is easily visible under a microscope, such as a micron-sized plastic bead, is attached to another site on the intra-liposomal side. A conformational change, or an angular motion in the tiny protein molecule, would manifest as a visible motion of the probe. The attachment of the protein on the glass surface also immobilizes the liposome, greatly facilitating its manipulation such as the probe injection. As a model system, we reconstituted ATP synthase (F(O)F(1)) in liposomes tens of mum in size, attached the protein specifically to a glass surface, and demonstrated its ATP-driven rotation in the membrane through the motion of a submicron bead.

Infrared spectroscopy of proteins

This review discusses the application of infrared spectroscopy to the study of proteins. The focus is on the mid-infrared spectral region and the study of protein reactions by reaction-induced infrared difference spectroscopy.

Membrane protein selectively oriented on solid support and reconstituted into a lipid membrane

Mimetic functional membranes on solid support are now emerging for the development of membrane biosensor or for the study of membrane-mediated processes and should have an important impact on biodiagnostics. We established a method to reconstitute a membrane protein into a lipid membrane in a selective orientation on a solid support. Membrane protein OprM, a component of OprM-MexA-MexB multidrug efflux pump, solubilized in detergent was immobilized via its extracellular domain on aminosilane-modified silica surface. The oriented protein was reconstituted into a lipid membrane by detergent removal. The membrane protein reconstitution process carried out on silica nanoparticles and on planar silica surfaces was followed by cryo-electron microscopy (cryo-EM) and quartz crystal microbalance with dissipation monitoring (QCM-D) respectively. The selective protein orientation on aminosilane-modified silica surface was assessed by cryo-EM and was compared to the nonspecific protein deposition on silica surface. Finally, the binding of MexA, a periplasmic component of the tripartite efflux complex, was monitored with QCM-D on the oriented OprM protein monolayer. The large adsorbed mass gave a direct evidence of the high affinity of MexA with the periplasmic helical part of OprM.

A Model System To Study the Insertion of Cholesterol into a Phospholipid Monolayer

Colloidal probe atomic force microscopy (AFM) was used to study the interaction between a surface bearing tethered cholesterol groups and an egg phosphatidylcholine (egg-PC) monolayer. The cholesterol bearing surface was comprised of a mixed self-assembled monolayer comprised of O-cholesteryl N-(8'-mecapto-3',6'-dioxaoctyl)carbamate (CPEO3) molecules and beta-mercaptoethanol formed on a 20 mum diameter gold-coated silica particle. The egg-PC monolayer was adsorbed onto an octadecylthiol monolayer formed on template-stripped gold. The force between the surfaces, as a function of separation, was measured for surface concentrations of CPEO3 from 0 to 100 mol %. At all concentrations there was a long-range repulsive double-layer force due to weak surface charges. At surface concentrations of CPEO3 from 1 to 29 mol % the interaction on the approach of the surfaces showed a maximum in the repulsive force, followed by a small (2-5 nm) jump into a force minimum corresponding to adhesion of the surfaces. On separation, a normalized pull-off force of 1.0-1.6 mN m(-1) was measured. Over the same concentration range, the calculated interaction energy per CPEO3 molecule decreased from 1.1 +/- 0.2 kT to 0.04 kT. At surface concentrations of 35 mol % and above there was no reproducible adhesion between the cholesterol-bearing surface and the phospholipid monolayer. We attribute the occurrence of short-range attraction and adhesion in the 1-29 mol % regime to the insertion of (some) cholesterol groups into the phospholipid monolayer. At higher surface concentrations the efficiency of insertion is reduced due to steric effects. We discuss the experimental results in the light of the energetics of the insertion of a cholesterol molecule into a lipid bilayer.

High-resolution AFM of membrane proteins directly incorporated at high density in planar lipid bilayer

The heterologous expression and purification of membrane proteins represent major limitations for their functional and structural analysis. Here we describe a new method of incorporation of transmembrane proteins in planar lipid bilayer starting from 1 pmol of solubilized proteins. The principle relies on the direct incorporation of solubilized proteins into a preformed planar lipid bilayer destabilized by dodecyl-beta-maltoside or dodecyl-beta-thiomaltoside, two detergents widely used in membrane biochemistry. Successful incorporations are reported at 20 degrees C and at 4 degrees C with three bacterial photosynthetic multi-subunit membrane proteins. Height measurements by atomic force microscopy (AFM) of the extramembraneous domains protruding from the bilayer demonstrate that proteins are unidirectionally incorporated within the lipid bilayer through their more hydrophobic domains. Proteins are incorporated at high density into the bilayer and on incubation diffuse and segregate into protein close-packing areas. The high protein density allows high-resolution AFM topographs to be recorded and protein subunits organization delineated. This approach provides an alternative experimental platform to the classical methods of two-dimensional crystallization of membrane proteins for the structural analysis by AFM. Furthermore, the versatility and simplicity of the method are important intrinsic properties for the conception of biosensors and nanobiomaterials involving membrane proteins.

Surface engineering approaches to micropattern surfaces for cell-based assays

The ability to produce patterns of single or multiple cells through precise surface engineering of cell culture substrates has promoted the development of cellular bioassays that provide entirely new insights into the factors that control cell adhesion to material surfaces, cell proliferation, differentiation and molecular signaling pathways. The ability to control shape and spreading of attached cells and cell-cell contacts through the form and dimension of the cell-adhesive patches with high precision is important. Commitment of stem cells to different specific lineages depends strongly on cell shape, implying that controlled microenvironments through engineered surfaces may not only be a valuable approach towards fundamental cell-biological studies, but also of great importance for the design of cell culture substrates for tissue engineering. Furthermore, cell patterning is an important tool for organizing cells on transducers for cell-based sensing and cell-based drug discovery concepts. From a material engineering standpoint, patterning approaches have greatly profited by combining microfabrication technologies, such as photolithography, with biochemical functionalization to present to the cells biological cues in spatially controlled regions where the background is rendered non-adhesive ("non-fouling") by suitable chemical modification. The focus of this review is on the surface engineering aspects of biologically motivated micropatterning of two-dimensional (flat) surfaces with the aim to provide an introductory overview and critical assessment of the many techniques described in the literature. In particular, the importance of non-fouling surface chemistries, the combination of hard and soft lithography with molecular assembly techniques as well as a number of less well known, but useful patterning approaches, including direct cell writing, are discussed.