Biomedical subjects

Jay X Tang

Publications and source records attributed to Jay X Tang.

At least 19 recordsLinked to original sources

PubMed2006

Isotropic to nematic liquid crystalline phase transition of F-actin varies from continuous to first order.

We report that the properties of the isotropic to nematic liquid crystalline phase transition of F-actin depend critically on the average filament length. For average filament lengths longer than 2 microm, we confirm previous findings that the phase transition is continuous in both alignment and concentration. For average filament lengths shorter than 2 microm, we show for the first time a first order transition with a clear discontinuity in both alignment and concentration. Tactoidal droplets of coexisting isotropic and nematic phases, differing in concentration by approximately 30%, form over the course of hours and appear to settle into near equilibrium metastable states.

Actin Cytoskeleton↗

PubMed2006

Temperature effects on threshold counterion concentration to induce aggregation of fd virus.

We seek to elucidate the dominant mechanism of attractive interaction between like-charged biopolymers by measuring the temperature dependence of the critical divalent counterion concentration (Cc) for the aggregation of fd viruses. A decrease in either temperature or the dieletric constant alone causes a decrease in Cc, providing evidence for the Wigner crystal model. Surprisingly, the effects of these two parameters can be combined so that Cc is expressed as a function of a single parameter: the Bjerrum length. Cc decreases exponentially as the Bjerrum length increases, suggesting that an energetic balance between the entropic effect of counterions and the counterion mediated attractive interaction gives rise to the onset of bundle formation.

Bacteriophages↗

PubMed2006

Low flagellar motor torque and high swimming efficiency of Caulobacter crescentus swarmer cells.

We determined the torque of the flagellar motor of Caulobacter crescentus for different motor rotation rates by measuring the rotation rate and swimming speed of the cell body and found it to be remarkably different from that of other bacteria, such as Escherichia coli and Vibrio alginolyticus. The average stall torque of the Caulobacter flagellar motor was approximately 350 pN nm, much smaller than the values of the other bacteria measured. Furthermore, the torque of the motor remained constant in the range of rotation rates up to those of freely swimming cells. In contrast, the torque of a freely swimming cell for V. alginolyticus is typically approximately 20% of the stall torque. We derive from these results that the C. crescentus swarmer cells swim more efficiently than both E. coli and V. alginolyticus. Our findings suggest that C. crescentus is optimally adapted to low nutrient aquatic environments.

Biomechanical Phenomena↗

PubMed2006

Microtubule bundling and nested buckling drive stripe formation in polymerizing tubulin solutions.

Various mechanisms govern pattern formation in chemical and biological reaction systems, giving rise to structures with distinct morphologies and physical properties. The self-organization of polymerizing microtubules (MTs) is of particular interest because of its implications for biological function. We report a study of the microscopic structure and properties of the striped patterns that spontaneously form in polymerizing tubulin solutions and propose a mechanism driving this assembly. Microscopic observations reveal that the pattern comprises wave-like MT bundles. The retardance of the solution and the fluorescence intensity of labeled MTs vary periodically in space, suggesting a coincident periodic variation in MT alignment and density. This wave-like structure forms through the development and coordinated buckling of initially aligned MT bundles. Both static magnetic fields and convective flow can induce the initial alignment. The nesting of the buckled MT bundles gives rise to density variations that are in quantitative accord with the data. We further propose that the buckling wavelength is selected by a balance between the bending energy of the bundles and the elastic energy of the MT network surrounding them. These studies reveal a unique physical chemical mechanism by which mechanical buckling couples with protein polymerization to produce macroscopic patterns. Self-organization of this type may be important to the formation of certain biological structures.

Animals↗

PubMed2006

Orientational order parameter of the nematic liquid crystalline phase of F-actin.

We measured the orientational order parameter of F-actin traversing the isotropic-nematic phase transition using a combination of techniques, including fluorescence imaging, local birefringence measurements, and small-angle x-ray scattering. The order parameter approaches a saturated value of 0.75 for actin concentrations above the region of the isotropic-nematic phase transition. This result implies a significant extent of misalignment and consequently entanglement among long actin filaments, even in the nematic phase. We determine the specific birefringence of completely aligned F-actin to be Deltan(0)=2.3 x 10(-5) ml/mg. At concentrations slightly below the isotropic-nematic transition, nonzero values of the order parameter are detected for hours following an initial alignment, indicating extremely slow rotational kinetics of F-actin in the entangled networks.

Actin Cytoskeleton↗

PubMed2006

Adhesion of single bacterial cells in the micronewton range.

The adhesion of bacteria to surfaces plays critical roles in the environment, disease, and industry. In aquatic environments, Caulobacter crescentus is one of the first colonizers of submerged surfaces. Using a micromanipulation technique, we measured the adhesion force of single C. crescentus cells attached to borosilicate substrates through their adhesive holdfast. The detachment forces measured for 14 cells ranged over 0.11 to 2.26 microN, averaging 0.59 +/- 0.62 microN. Based on the calculation of stress distribution with the finite element analysis method (dividing an object into small grids and calculating relevant parameters for all of the elements), the adhesion strength between the holdfast and the substrate is >68 N/mm(2) in the central region of contact. To our knowledge, this strength of adhesion is the strongest ever measured for biological adhesives.

Bacterial Adhesion↗

PubMed2005

Fluctuation analysis of Caulobacter crescentus adhesion.

The aquatic bacterium Caulobacter crescentus divides asymmetrically to a flagellated swarmer cell and a cell with a stalk. At the end of the stalk is an adhesive organelle known as the holdfast, which the stalked cell uses to attach to a solid surface. Often there are two or more cells with their stalks attached to the same holdfast. By analyzing the fluctuations in the stalk angle for a pair of cells attached to a single holdfast, we determine the elastic stiffness of the holdfast. We model the holdfast as three torsional springs in series and find that the effective torsional spring constant for the holdfast is of the order of (10(-17)-10(-18)) Nm, with unequal spring constants. The asymmetry suggests the sequence in which the cells attach to each other, and in some cases suggests that strong crosslinks form between the stalks as they make a shared holdfast.

Bacterial Adhesion↗

PubMed2005

Anionic poly(amino acid)s dissolve F-actin and DNA bundles, enhance DNase activity, and reduce the viscosity of cystic fibrosis sputum.

Bundles of F-actin and DNA present in the sputum of cystic fibrosis (CF) patients but absent from normal airway fluid contribute to the altered viscoelastic properties of sputum that inhibit clearance of infected airway fluid and exacerbate the pathology of CF. Previous strategies to remove these filamentous aggregates have focused on DNase to enzymatically depolymerize DNA to constituent monomers and gelsolin to sever F-actin to small fragments. The high densities of negative surface charge on DNA and F-actin suggest that the bundles of these filaments, which alone exhibit a strong electrostatic repulsion, may be stabilized by multivalent cations such as histones, antimicrobial peptides, and other positively charged molecules prevalent in airway fluid. This study reports that bundles of DNA or F-actin formed after addition of histone H1 or lysozyme are efficiently dissolved by soluble multivalent anions such as polymeric aspartate or glutamate. Addition of poly-aspartate or poly-glutamate also disperses DNA and actin-containing bundles in CF sputum and lowers the elastic moduli of these samples to levels comparable to those obtained after treatment with DNase I or gelsolin. Addition of poly-aspartic acid also increased DNase activity when added to samples containing DNA bundles formed with histone H1. When added to CF sputum, poly-aspartic acid significantly reduced the growth of bacteria, suggesting activation of endogenous antibacterial factors. These findings suggest that soluble multivalent anions have potential alone or in combination with other mucolytic agents to selectively dissociate the large bundles of charged biopolymers that form in CF sputum.

Actin Cytoskeleton↗

PubMed2005

Hydrodynamic stability of helical growth at low Reynolds number.

A cylindrical object growing at a low Reynolds number can spontaneously develop a helical shape. We have studied this phenomenon numerically, and our results may shed some light on the spontaneous formation of helical tails of a dense protein network observed in experiments on actin based motility. We also identify an unstable critical pitch angle which separates helices that straighten into rods from helices that flatten into planar curves as they grow. At the critical angle the pitch angle remains constant, whereas both helical diameter and pitch increase with the helical contour length.

Actins↗

PubMed2005

Intriguing self-assembly of large granules of F-actin facilitated by gelsolin and alpha-actinin.

We report microscopic observations and a structural determination of actin granules self-assembled in concentrated solutions of actin filaments (F-actin). Optical microscopy shows reproducible formation of numerous and stable granules of densely packed F-actin of variable sizes on the order of 10 microm. These granules coexist with a uniform network of F-actin of a lower concentration. The microscopic segregation of F-actin into two distinct states is assisted by an actin cross-linking protein, alpha-actinin. The rapid on and off rates and temperature sensitivity of the alpha-actinin/F-actin interaction facilitate the formation of multi-micrometer-sized granules of well-defined shapes. Additional physical factors such as the excluded volume effect and the minimization of surface energy act in concert with the specific molecular interactions to define the intriguing granular formation. Both the biochemical specificity of alpha-actinin and the thermodynamics of phase transitions are required for understanding such large scale self-assembly.

Actinin↗

PubMed2005

Single filament electrophoresis of F-actin and filamentous virus fd.

We have developed an electrophoretic cell suitable for single-molecule electrophoresis. The setup works for fluorescently labeled macromolecules by direct recording of their motion under an external electric field. The electrophoretic mobility of rodlike, polydisperse actin filaments (F-actin) were measured, as well as its dependence on the orientation of the filaments. A dipping effect is observed and quantitatively accounted for by the difference in hydrodynamic drag between motions along and perpendicular to the long axis of a filament. When averaged over all orientations, the mobility of F-actin in 50 mM KCl and 2 mM MgCl(2) is determined to be -(8.5+/-0.7) x 10(-5) cm(2)(V s). This method is also used to compare the mobility of F-actin and fd virus in a mixture of them. A reliable ratio of 1.26 is measured for fd virus to F-actin. The influence of the orientation dependent drag on electrophoretic mobility is discussed and a strategy for reliable measurement is proposed.

Actins↗

PubMed2005

The elastic properties of the caulobacter crescentus adhesive holdfast are dependent on oligomers of N-acetylglucosamine.

The aquatic bacterium Caulobacter crescentus attaches to solid surfaces through an adhesive holdfast located at the tip of its polar stalk, a thin cylindrical extension of the cell membrane. In this paper, the elastic properties of the C. crescentus stalk and holdfast assembly were studied by using video light microscopy. In particular, the contribution of oligomers of N-acetylglucosamine (GlcNAc) to the elasticity of holdfast was examined by lysozyme digestion. C. crescentus cells attached to a surface undergo Brownian motion while confined effectively in a harmonic potential. Mathematical analysis of such motion enabled us to determine the force constant of the stalk-holdfast assembly, which quantifies its elastic properties. The measured force constant exhibits no dependence on stalk length, consistent with the theoretical estimate showing that the stalk can be treated as a rigid rod with respect to fluctuations of the attached cells. Therefore, the force constant of the stalk-holdfast assembly can be attributed to the elasticity of the holdfast. Motions of cells in a rosette were found to be correlated, consistent with the elastic characteristics of the holdfast. Atomic force microscopy analysis indicates that the height of a dried (in air) holdfast is approximately one-third of that of a wet (in water) holdfast, consistent with the gel-like nature of the holdfast. Lysozyme, which cleaves oligomers of GlcNAc, reduced the force constant to less than 10% of its original value, consistent with the polysaccharide gel-like nature of the holdfast. These results also indicate that GlcNAc polymers play an important role in the strength of the holdfast.

Acetylglucosamine↗

PubMed2004

Absence of charge inversion on rodlike polyelectrolytes with excess divalent counterions.

Filamentous viruses such as fd and M13 are highly charged rodlike polyelectrolytes. In this study, we employ fd virus to test the recent prediction of charge inversion [Nguyen, Rouzina, and Shklovskii, J. Chem. Phys. 112, 2562 (2000)]. Light scattering measurements show bundle formation and resolubilization of fd viruses when MgCl(2) was added from 0 to 600 mM. The effective charge of fd was studied by measuring their electrophoretic mobility using a filament tracking method uniquely suited for the system. Monte Carlo simulations were performed under canonical ensemble to predict the charge distribution around the rodlike virus. Charge inversion, which has been suggested theoretically to accompany with bundle resolubilization, was not observed in either experiments or simulations. A modified analysis of force balance is called upon to account for these new findings.

Bacteriophage M13↗

PubMed2004

Self-assembled virus-membrane complexes.

Anionic polyelectrolytes and cationic lipid membranes can self-assemble into lamellar structures ranging from alternating layers of membranes and polyelectrolytes to 'missing layer' superlattice structures. We show that these structural differences can be understood in terms of the surface-charge-density mismatch between the polyelectrolyte and membrane components by examining complexes between cationic membranes and highly charged M13 viruses, a system that allowed us to vary the polyelectrolyte diameter independently of the charge density. Such virus-membrane complexes have pore sizes that are about ten times larger in area than DNA-membrane complexes, and can be used to package and organize large functional molecules; correlated arrays of Ru(bpy)(3)(2+) macroionic dyes have been directly observed within the virus-membrane complexes using an electron-density reconstruction. These observations elucidate fundamental design rules for rational control of self-assembled polyelectrolyte-membrane structures, which have applications ranging from non-viral gene therapy to biomolecular templates for nanofabrication.

Bacteriophage M13↗

PubMed2004

Microrheology of solutions of semiflexible biopolymer filaments using laser tweezers interferometry.

Semiflexible polymers are of great biological importance in determining the mechanical properties of cells. Techniques collectively known as microrheology have recently been developed to measure the viscoelastic properties of solutions of submicroliter volumes. We employ one such technique, which uses a focused laser beam to trap a micron-sized silica bead and interferometric photodiode detection to measure passively the position fluctuations of the trapped bead with nanometer resolution and high bandwidth. The frequency-dependent complex shear modulus G*(f) can be extracted from the position fluctuations via the fluctuation-dissipation theorem and the generalized Stokes-Einstein relation. Using particle tracking microrheology, we report measurements of shear moduli of solutions of fd viruses, which are filamentous, semiflexible, and monodisperse bacteriophages, each 0.9 microm long, 7 nm in diameter, and having a persistence length of 2.2 microm. Recent theoretical treatments of semiflexible polymer dynamics provide quantitative predictions of the rheological properties of such a model system. The fd samples measured span the dilute, semidilute, and concentrated regimes. In the dilute regime G*(f) is dominated by (rigid rod) rotational relaxation, whereas the high-frequency regime reflects single-semiflexible filament dynamics consistent with the theoretical prediction. Due to the short length of fd viruses used in this study, the intermediate regime does not exhibit a well-developed plateau. A dynamic scaling analysis gives rise to a concentration scaling of c(1.36) (r=0.99) in the transition regime and a frequency scaling of f(0.63) (r=0.98) at high frequencies.

Journal Article↗

PubMed2004

Diffusion of actin filaments within a thin layer between two walls.

Diffusion of the protein filaments F -actin confined in a thin layer between two walls is studied using the methods of single filament fluorescence imaging and particle tracking. The translational and rotational diffusion coefficients are measured for F -actin of lengths in the range of 1.5-5 microm. The length dependence of the measured diffusion coefficients is consistent with the predicted two-dimensional projection of the diffusion of a cylinder in an unbounded fluid. Fits based on the formulas for diffusion in the bulk fluid yield higher apparent viscosity values than that of the buffer solution by a factor of 2 for a layer thickness between 0.7 and 1.6 microm. We show that the measured results can be accounted for by correction based on the hydrodynamic theory of a long cylinder between confining walls.

Actins↗

PubMed2003

Ion multivalence and like-charge polyelectrolyte attraction.

It is known empirically that multivalent ions generate attractions between like-charged polyelectrolytes, with different valence requirements for different systems. How multivalent must an ion be before it can condense a given polyelectrolyte? Using charge-tunable M13 virus rods and a family of artificial homologous "dumbbell" divalent ions of different sizes, we have constructed a multivalent ion-polyelectrolyte phase diagram, and find an experimentally motivated general criterion for like-charged attraction based on the ion valence, ion size, and the Gouy-Chapman length.

Anions↗

PubMed2003

Lamellar phase of stacked two-dimensional rafts of actin filaments.

We examined liquid crystalline phases of the cytoskeletal polyelectrolyte filamentous (F-)actin in the presence of multivalent counterions. As a function of increasing ion concentration, the F-actin rods in either an isotropic or a nematic phase will transform into a new and unexpected lamellar phase of cross-linked rafts (L(XR) phase), before condensing into a bundled phase of parallel, close-packed rods. This behavior is generic for alkali earth divalent ions Mg2+, Ca2+, Sr2+, and Ba2+, and the structural transitions are achieved without any architecture-specific actin-binding linker proteins.

Actins↗