Internet-based image analysis quantifies contractile behavior of individual fibroblasts inside model tissue. (9/499)

In a cell-populated collagen gel, intrinsic fiber structure visible in differential interference contrast images can provide markers for an in situ strain gauge to quantify cell-gel mechanics, while optical sections of fluorescent protein distribution capture cytoskeletal kinematics. Mechanics quantification can be derived automatically from timelapse differential interference contrast images using a Deformation Quantification and Analysis software package accessible online at http://dqa.web.cmu.edu. In our studies, fibroblast contractile machinery was observed to function entirely within pseudopods, while GFP-alpha-actinin concentrated in pseudopod tips and cortex. Complex strain patterns around individual cells showed instances of both elastic and inelastic strain transmission, suggesting a role in observed long-range alignment of cells.  (+info)

The flexibility of DNA double crossover molecules. (10/499)

Double crossover molecules are DNA structures containing two Holliday junctions connected by two double helical arms. There are several types of double crossover molecules, differentiated by the relative orientations of their helix axes, parallel or antiparallel, and by the number of double helical half-turns (even or odd) between the two crossovers. They are found as intermediates in meiosis and they have been used extensively in structural DNA nanotechnology for the construction of one-dimensional and two-dimensional arrays and in a DNA nanomechanical device. Whereas the parallel double helical molecules are usually not well behaved, we have focused on the antiparallel molecules; antiparallel molecules with an even number of half-turns between crossovers (termed DAE molecules) produce a reporter strand when ligated, facilitating their characterization in a ligation cyclization assay. Hence, we have estimated the flexibility of antiparallel DNA double crossover molecules by means of ligation-closure experiments. We are able to show that these molecules are approximately twice as rigid as linear duplex DNA.  (+info)

Water and proton conduction through carbon nanotubes as models for biological channels. (11/499)

Carbon nanotubes, unmodified (pristine) and modified through charged atoms, were simulated in water, and their water conduction rates determined. The conducted water inside the nanotubes was found to exhibit a strong ordering of its dipole moments. In pristine nanotubes the water dipoles adopt a single orientation along the tube axis with a low flipping rate between the two possible alignments. Modification can induce in nanotubes a bipolar ordering as previously observed in biological water channels. Network thermodynamics was applied to investigate proton conduction through the nanotubes.  (+info)

Functional incorporation of integrins into solid supported membranes on ultrathin films of cellulose: impact on adhesion. (12/499)

Biomimetic models of cell surfaces were designed to study the physical basis of cell adhesion. Vesicles bearing reconstituted blood platelet integrin receptors alpha(IIb)beta(3) were spread on ultrathin films of cellulose, forming continuous supported membranes. One fraction of the integrin receptors, which were facing their extracellular domain toward the aqueous phase, were mobile, exhibiting a diffusion constant of 0.6 micro m(2) s(-1). The functionality of receptors on bare glass and on cellulose cushions was compared by measuring adhesion strength to giant vesicles. The vesicles contained lipid-coupled cyclic hexapeptides that are specifically recognized by integrin alpha(IIb)beta(3). To mimic the steric repulsion forces of the cell glycocalix, lipids with polyethylene glycol headgroups were incorporated into the vesicles. The free adhesion energy per unit area deltag(ad) was determined by micro-interferometric analysis of the vesicle's contour near the membrane surface in terms of the equilibrium of the elastic forces. By accounting for the reduction of the adhesion strength by the repellers and from measuring the density of receptors one could estimate the specific receptor ligand binding energy. We estimate the receptor-ligand binding energy to be 10 k(B)T under bioanalogue conditions.  (+info)

Partitioning of individual flexible polymers into a nanoscopic protein pore. (13/499)

Polymer dynamics are of fundamental importance in materials science, biotechnology, and medicine. However, very little is known about the kinetics of partitioning of flexible polymer molecules into pores of nanometer dimensions. We employed electrical recording to probe the partitioning of single poly(ethylene glycol) (PEG) molecules, at concentrations near the dilute regime, into the transmembrane beta-barrel of individual protein pores formed from staphylococcal alpha-hemolysin (alphaHL). The interactions of the alpha-hemolysin pore with the PEGs (M(w) 940-6000 Da) fell into two classes: short-duration events (tau approximately 20 micro s), approximately 85% of the total, and long-duration events (tau approximately 100 micro s), approximately 15% of the total. The association rate constants (k(on)) for both classes of events were strongly dependent on polymer mass, and values of k(on) ranged over two orders of magnitude. By contrast, the dissociation rate constants (k(off)) exhibited a weak dependence on mass, suggesting that the polymer chains are largely compacted before they enter the pore, and do not decompact to a significant extent before they exit. The values of k(on) and k(off) were used to determine partition coefficients (Pi) for the PEGs between the bulk aqueous phase and the pore lumen. The low values of Pi are in keeping with a negligible interaction between the PEG chains and the interior surface of the pore, which is independent of ionic strength. For the long events, values of Pi decrease exponentially with polymer mass, according to the scaling law of Daoud and de Gennes. For PEG molecules larger than approximately 5 kDa, Pi reached a limiting value suggesting that these PEG chains cannot fit entirely into the beta-barrel.  (+info)

The conformation of myosin head domains in rigor muscle determined by X-ray interference. (14/499)

In the absence of adenosine triphosphate, the head domains of myosin cross-bridges in muscle bind to actin filaments in a rigor conformation that is expected to mimic that following the working stroke during active contraction. We used x-ray interference between the two head arrays in opposite halves of each myosin filament to determine the rigor head conformation in single fibers from frog skeletal muscle. During isometric contraction (force T(0)), the interference effect splits the M3 x-ray reflection from the axial repeat of the heads into two peaks with relative intensity (higher angle/lower angle peak) 0.76. In demembranated fibers in rigor at low force (<0.05 T(0)), the relative intensity was 4.0, showing that the center of mass of the heads had moved 4.5 nm closer to the midpoint of the myosin filament. When rigor fibers were stretched, increasing the force to 0.55 T(0), the heads' center of mass moved back by 1.1-1.6 nm. These motions can be explained by tilting of the light chain domain of the head so that the mean angle between the Cys(707)-Lys(843) vector and the filament axis increases by approximately 36 degrees between isometric contraction and low-force rigor, and decreases by 7-10 degrees when the rigor fiber is stretched to 0.55 T(0).  (+info)

Biomimetic organization: Octapeptide self-assembly into nanotubes of viral capsid-like dimension. (15/499)

The controlled self-assembly of complex molecules into well defined hierarchical structures is a promising route for fabricating nanostructures. These nanoscale structures can be realized by naturally occurring proteins such as tobacco mosaic virus, capsid proteins, tubulin, actin, etc. Here, we report a simple alternative method based on self-assembling nanotubes formed by a synthetic therapeutic octapeptide, Lanreotide in water. We used a multidisciplinary approach involving optical and electron microscopies, vibrational spectroscopies, and small and wide angle x-ray scattering to elucidate the hierarchy of structures exhibited by this system. The results revealed the hexagonal packing of nanotubes, and high degree of monodispersity in the tube diameter (244 A) and wall thickness (approximately equal to 18 A). Moreover, the diameter is tunable by suitable modifications in the molecular structure. The self-assembly of the nanotubes occurs through the association of beta-sheets driven by amphiphilicity and a systematic aromatic/aliphatic side chain segregation. This original and simple system is a unique example for the study of complex self-assembling processes generated by de novo molecules or amyloid peptides.  (+info)

Biomimetic systems for studying actin-based motility. (16/499)

Actin polymerization provides a major driving force for eukaryotic cell motility. Successive intercalation of monomeric actin subunits between the plasma membrane and the filamentous actin network results in protrusions of the membrane enabling the cell to move or to change shape. One of the challenges in understanding eukaryotic cell motility is to dissect the elementary biochemical and biophysical steps that link actin polymerization to mechanical force generation. Recently, significant progress was made using biomimetic, in vitro systems that are inspired by the actin-based motility of bacterial pathogens such as Listeria monocytogenes. Polystyrene microspheres and synthetic phospholipid vesicles coated with proteins that initiate actin polymerization display motile behavior similar to Listeria, mimicking the leading edge of lamellipodia and filopodia. A major advantage of these biomimetic systems is that both biochemical and physical parameters can be controlled precisely. These systems provide a test bed for validating theoretical models on force generation and polarity establishment resulting from actin polymerization. In this review, we discuss recent experimental progress using biomimetic systems propelled by actin polymerization and discuss these results in the light of recent theoretical models on actin-based motility.  (+info)