Mechanical design of proteins studied by single-molecule force spectroscopy and protein engineering

Binding of cross-linked glycosylphosphatidylinositol-anchored proteins to discrete actin-associated sites and cholesterol-dependent domains

The mechanism by which cross-linked glycosylphosphatidylinositol (GPI)-anchored proteins are immobilized has been a mystery because both the binding to a transmembrane protein and attachment to a rigid cytoskeleton are needed. Using laser tweezers surface scanning resistance (SSR) technology, we obtained physical evidence for cross-linked GPI-anchored protein, Qa-2, binding to a transmembrane protein and for diffusion to discrete cytoskeleton attachment sites. At low levels of cross-linking of Qa-2 molecules, the resistance to lateral movement was that expected of monomeric lipid-anchored proteins, and no specific binding to cytoskeleton-attached structures was observed. When aggregates of the GPI-anchored protein, Qa-2, were scanned across plasma membranes, the background resistance was much higher than expected for a GPI-anchored protein alone and submicron domains of even higher resistance were observed (designated as elastic or non-elastic barriers) at a density of 82 (61 elastic and 21 small non-elastic barriers) per 100 microm(2). Elastic barriers involved weak but specific bonds to the actin cytoskeleton (broken by forces of 2 or 4 pN and were removed by cytochalasin D). Small non-elastic barriers (50-100 nm) depended upon membrane cholesterol and were closely correlated with caveolae density. Thus, cross-linked GPI-anchored proteins can diffuse through the membrane in complex with a transmembrane protein and bind weakly to discrete cytoskeleton attachment sites either associated with flexible actin networks or sphingolipid-cholesterol rich microdomains in live cell membranes. Our SSR measurements provide the first description of the physical characteristics of the interactions between rafts and stable membrane structures.

Scanning force microscopy in the applied biological sciences

Fifteen years after its invention, the scanning force microscope (SFM) is rooted deep in the biological sciences. Here we discuss the use of SFM in biotechnology and biomedical research. The spectrum of applications reviewed includes imaging, force spectroscopy and mapping, as well as sensor applications. It is our hope that this review will be useful for researchers considering the use of SFM in their studies but are uncertain about its scope of capabilities. For the benefit of readers unfamiliar with SFM technology, the fundamentals of SFM imaging and force measurement are also briefly introduced.

Force measurement and inhibitor binding assay of monomer and engineered dimer of bovine carbonic anhydrase B

We applied atomic force microscopy (AFM) to study the intramolecular mechanics of the globular protein molecule, bovine carbonic anhydrase B. The immobilized protein on an amino-functionalized silicon wafer was pulled from its N- and C-termini after being covalently cross-linked to the AFM tip, and the relationship between the tensile force applied on the protein and its extension was recorded. The native enzyme (having 261 residues with two Cys added at its ends, and in a theoretical stretching length of 96 nm) was extended only to 13 +/- 2 nm under physiological conditions before disruption of the covalent cross-linking system. Contrary to the above observation, an engineered dimer was extended to about 110 nm even in the absence of the denaturant. The difference was ascribed to the presence or presumed absence of a "knot" structure at the C-terminal end of the two forms, respectively. When a specific inhibitor was added to the experimental solution, native monomers (sp activity = 88% of the wild type enzyme) were extended to 28 +/- 4 nm, whereas dimers (sp activity = 46%) were extended to about 56 +/- 3 nm, suggesting that both monomeric units in the dimer could bind inhibitor molecules, which was further corroborated by a titration experiment using a fluorescent inhibitor. Thus, one of the monomeric units in the engineered dimer was concluded to be enzymatically inactive but capable of binding inhibitors.

Single molecule force spectroscopy in biology using the atomic force microscope

The importance of forces in biology has been recognized for quite a while but only in the past decade have we acquired instrumentation and methodology to directly measure interactive forces at the level of single biological macromolecules and/or their complexes. This review focuses on force measurements performed with the atomic force microscope. A general introduction to the principle of action is followed by review of the types of interactions being studied, describing the main results and discussing the biological implications.