Structure and Mechanics of Membrane Proteins

Abstract

Evolution has tuned membrane proteins to exist in a lipid bilayer, provide for cell-cell communication, transport solutes, and convert energies. These proteins exhibit a hydrophobic belt that interacts with the lipid bilayer. Detergents are therefore used to extract membrane proteins and keep them in solution for purification and subsequent analyses. However, most membrane proteins are unstable when solubilized and hence often not accessible to NMR or X-ray crystallography. The atomic force microscope (AFM) is a powerful tool for imaging and manipulating membrane proteins in their native state. Superb images of native membranes have been recorded, and a quantitative interpretation of the data acquired using the AFM tip has become possible. In addition, multifunctional probes to simultaneously acquire information on the topography and electrical properties of membrane proteins have been produced. This progress is discussed here and fosters expectations for future developments and applications of AFM and single-molecule force spectroscopy.

Enhanced Photocurrent in Engineered Bacteriorhodopsin Monolayer

“…53,54 AFM imaging herein of cleaned evaporated gold surfaces (rms roughness 1.1 nm) exposed to dilute solutions of wild-type BR showed clear evidence of patches which ranged from a few tens to hundreds of square nanometers (Figure 2) with typical coverage ranging from 8% to 40% (20 μM solution incubations varying from 1 hour to overnight) of the available surface. Upon removal of the lipid from the purple membrane (wild-type and mutant), the individual trimers were released.…”

Atomic force microscopy – looking at mechanosensors on the cell surface

“…The sharp tip is attached to a soft cantilever, and, as the cantilever bends, its deflection is detected by movement of a laser beam reflected from the tip. AFM topographic imaging is widely used in the life sciences, and has provided high-resolution images of biomolecules, membranes and cells in buffer at unprecedented resolution (Dufrêne, 2008a, Dufrêne, 2008bEngel and Gaub, 2008;Müller and Dufrêne, 2011;Müller et al, 2009 (Hinterdorfer and Dufrêne, 2006;Engel and Gaub, 2008;Puchner and Gaub, 2009;Müller et al, 2009;Dufrêne et al, 2011). Here, the tip is brought into proximity of and retracted from the biological sample, and the cantilever deflection measures the interaction force (Fig.…”

“…Early studies have demonstrated that proteins can be subjected to controlled forces using this approach, and have yielded details of protein unfolding pathways (Rief et al, 1997). Since then, single-molecule AFM, combined with molecular dynamics simulations and protein engineering, has greatly enhanced our understanding of the mechanical behavior of a great variety of proteins (Engel and Gaub, 2008;Puchner and Gaub, 2009;Li and Cao, 2010;Marszalek and Dufrêne, 2012). Most of these single protein manipulation experiments have been conducted in vitro, where the system under study can be tightly controlled.…”

Mechanical control of mitotic progression in single animal cells

“…Originally developed for nanoscale analysis and visualization of inorganic surfaces (11), AFM has found applications in studying the mechanics of soft biological materials, from single molecules to cells and tissues (12)(13)(14)(15)(16). However, the ability to conduct experiments on poorly adherent cells, such as nonadherent white blood cells (17) or rounded mitotic cells (18), has been hampered by cantilevers of inappropriate geometry.…”