Conformations, flexibility, and interactions observed on individual membrane proteins by atomic force microscopy

Imaging and Force Spectroscopy of Single Transmembrane Proteins with the Atomic Force Microscope

The atomic force microscope (AFM) has opened avenues and provided opportunities to investigate biological soft matter and processes ranging from nanometer (nm) to millimeter (mm). The high temporal (millisecond) and spatial (nanometer) resolutions of the AFM are suited for studying many biological processes in their native conditions. The AFM cantilever-aptly termed as a "lab on a tip"-can be used as an imaging tool as well as a handle to manipulate single bonds and proteins. Recent examples have convincingly established AFM as a tool to study the mechanical properties and monitor processes of single proteins and cells with high sensitivity, thus affording insight into important mechanistic details. This chapter specifically focuses on practical and analytical protocols of single-molecule AFM methodologies related to high-resolution imaging and single-molecule force spectroscopy of transmembrane proteins in a lipid bilayer (reconstituted or native). Both these techniques are operator oriented, and require specialized working knowledge of the instrument, theory and practical skills.

Atomic force microscopy and spectroscopy to probe single membrane proteins in lipid bilayers

The atomic force microscope (AFM) has opened vast avenues hitherto inaccessible to the biological scientist. The high temporal (millisecond) and spatial (nanometer) resolutions of the AFM are suited for studying many biological processes in their native conditions. The AFM cantilever stylus is aptly termed as a "lab on a tip" owing to its versatility as an imaging tool as well as a handle to manipulate single bonds and proteins. Recent examples assert that the AFM can be used to study the mechanical properties and monitor processes of single proteins and single cells, thus affording insight into important mechanistic details. This chapter specifically focuses on practical and analytical protocols of single-molecule AFM methodologies related to high-resolution imaging and single-molecule force spectroscopy of membrane proteins. Both these techniques are operator oriented, and require specialized working knowledge of the instrument, theoretical, and practical skills.

Vertebrate membrane proteins: structure, function, and insights from biophysical approaches

Membrane proteins are key targets for pharmacological intervention because they are vital for cellular function. Here, we analyze recent progress made in the understanding of the structure and function of membrane proteins with a focus on rhodopsin and development of atomic force microscopy techniques to study biological membranes. Membrane proteins are compartmentalized to carry out extra- and intracellular processes. Biological membranes are densely populated with membrane proteins that occupy approximately 50% of their volume. In most cases membranes contain lipid rafts, protein patches, or paracrystalline formations that lack the higher-order symmetry that would allow them to be characterized by diffraction methods. Despite many technical difficulties, several crystal structures of membrane proteins that illustrate their internal structural organization have been determined. Moreover, high-resolution atomic force microscopy, near-field scanning optical microscopy, and other lower resolution techniques have been used to investigate these structures. Single-molecule force spectroscopy tracks interactions that stabilize membrane proteins and those that switch their functional state; this spectroscopy can be applied to locate a ligand-binding site. Recent development of this technique also reveals the energy landscape of a membrane protein, defining its folding, reaction pathways, and kinetics. Future development and application of novel approaches during the coming years should provide even greater insights to the understanding of biological membrane organization and function.

Purification and aqueous phase atomic force microscopic observation of recombinant P2X2 receptor

Recombinant P2X2 receptor was observed by atomic force microscope in the aqueous phase. The P2X2 receptor was expressed in an insect cell line, and recombinant proteins were prepared under native conditions. The membrane fractions were extracted, and histidine-tagged receptor protein was purified from the fractions by column chromatography. When the purified protein fraction was diluted with water and served for atomic force microscopy, dispersed particles of about 3 nm in height were observed. In the presence of 1 mM ATP, the assembly-like images of the particles were obtained. More densely assembled images of the particles were achieved when the protein was dissolved in a Tris buffer containing 1 mM ATP. Under this condition, imaging of the surface of the particles exhibited a circular structure with a diameter of about 10 nm having a pore-like structure. These results suggest that atomic force microscopy provides structural information about P2X2 receptor in aqueous phase.

Sample preparation procedures for biological atomic force microscopy

Since the late 1980s, atomic force microscopy (AFM) has been increasingly used in biological sciences and it is now established as a versatile tool to address the structure, properties and functions of biological specimens. AFM is unique in that it provides three-dimensional images of biological structures, including biomolecules, lipid films, 2D protein crystals and cells, under physiological conditions and with unprecedented resolution. A crucial prerequisite for successful, reliable biological AFM is that the samples need to be well attached to a solid substrate using appropriate, nondestructive methods. In this review, we discuss common techniques for immobilizing biological specimens for AFM studies.

Recent progress in the application of atomic force microscopy imaging and force spectroscopy to microbiology

Atomic force microscopy imaging and force spectroscopy have recently opened a range of novel applications in microbiology. During the past two years, rapid advances have been made using atomic force microscopy to visualize the surface structure of two-dimensional bacterial protein crystals, biofilms and individual cells in physiological conditions. There has also been remarkable progress in using force spectroscopy to measure biomolecular interactions and physical properties of microbial surfaces. Specific highlights include the imaging and manipulation of membrane proteins at the subnanometer level, the observation of the surface of living cells at high resolution, the mapping of local properties such as surface charges, the measurement of elastic properties of cell-surface constituents and the probing of cellular interactions using functionalized probes.