Biomedical subjects

Raghuveer Parthasarathy

Publications and source records attributed to Raghuveer Parthasarathy.

7 recordsLinked to original sources

PubMed2006

Curvature-modulated phase separation in lipid bilayer membranes.

Cellular membranes exhibit a variety of controlled curvatures, with filopodia, microvilli, and mitotic cleavage furrows being only a few of many examples. Coupling between local curvature and chemical composition in membranes could provide a means of mechanically controlling the spatial organization of membrane components. Although this concept has surfaced repeatedly over the years, controlled experimental investigations have proven elusive. Here, we introduce an experimental platform, in which microfabricated surfaces impose specific curvature patterns onto lipid bilayers, that allows quantification of mechanochemical couplings in membranes. We find that, beyond a critical curvature value, membrane geometry governs the spatial ordering of phase-separated domain structures in membranes composed of cholesterol and phospholipids. The curvature-controlled ordering, a consequence of the distinct mechanical properties of the lipid phases, makes possible a determination of the bending rigidity difference between cholesterol-rich and cholesterol-poor lipid domains. These observations point to a strong coupling between mechanical bending and chemical organization that should have wide-reaching consequences for biological membranes. Curvature-mediated patterning may also be useful in controlling complex fluids other than biomembranes.

Lipid Bilayers↗

PubMed2006

Coupled membrane fluctuations and protein mobility in supported intermembrane junctions.

Junctions between lipid membranes make possible cell-free explorations of physical mechanisms that can contribute to protein and lipid organization at a variety of biophysical interfaces. Recent studies of mobile antibodies sandwiched between lipid bilayer membranes have shown that strong intermembrane adhesion and protein mobility alone are sufficient to drive inert proteins into micron-scale patterns of dense and sparse zones. Though the length scale of these patterns was suspected to be related to membrane rigidity, a quantitative understanding has so far been unavailable. We introduce data showing radially structured protein patterns that also demonstrate micron-scale organization. We then provide a simple model that relates the spectrum of membrane fluctuations to the observed protein distributions; in brief, only membrane modes that are slow enough to couple to the protein mobility drive intermembrane protein patterns.

Algorithms↗

PubMed2005

Electrostatically driven spatial patterns in lipid membrane composition.

To explore the physical mechanisms that can guide spatial organization at biological membranes, we have constructed simple, cell-free intermembrane junctions. We find that the mechanically driven patterning of proteins uncovered in our earlier work can electrostatically generate spatial patterns in the distribution of charged membrane lipids. Tuning the magnitude of the interaction as a function of composition and ionic strength, and analyzing the interplay between thermodynamics and electrostatics via a Poisson-Boltzmann approach, we are able to determine the charge density and surface potential of the junction components. Surprisingly, the electrostatic potential of the proteins is a minor factor in the lipid reorganization; the protein size and its modulation of the junction topography play the dominant role in driving the electrostatic patterns.

Lipid Bilayers↗

PubMed2004

Protein patterns at lipid bilayer junctions.

We introduce a simple intermembrane junction system in which to explore pattern and structure formation by membrane-bound proteins. The junction consists of a planar lipid bilayer to which one species of protein (an IgG antibody) is bound, forming a 2D, compressible fluid. Upon the adhesion of a second lipid bilayer, the formerly uniformly distributed proteins rapidly reorganize into patterns of dense and sparse zones. Using a combination of complementary imaging techniques (fluorescence microscopy, fluorescence interference contrast microscopy, and fluorescence resonance energy transfer), we reconstruct the 3D structure of these intermembrane patterns with nanometer-scale topographic resolution, revealing the orientation of the proteins. The patterns form as the rapid bilayer-bilayer adhesion, often radiating outward from an initial, circular contact site, pushes aside the antibodies, sweeping them into areas of high density and clearing low-density regions. Coarsening of these local features is energetically costly and therefore kinetically trapped; the patterns do not change over tens of minutes. These studies demonstrate that membrane mechanical forces alone, i.e., in the absence of specific biochemical interactions, can drive microm-scale organization of membrane proteins.

Fluorescence Resonance Energy Transfer↗

PubMed2004

Percolating through networks of random thresholds: finite temperature electron tunneling in metal nanocrystal arrays.

We investigate how temperature affects transport through large networks of nonlinear conductances with distributed thresholds. In monolayers of weakly coupled gold nanocrystals, quenched charge disorder produces a range of local thresholds for the onset of electron tunneling. Our measurements delineate two regimes separated by a crossover temperature T*. Up to T* the nonlinear zero-temperature shape of the current-voltage curves survives, but with a threshold voltage for conduction that decreases linearly with temperature. Above T* the threshold vanishes and the low-bias conductance increases rapidly with temperature. We develop a model that accounts for these findings and predicts T*.

Journal Article↗

PubMed2004

Optical techniques for imaging membrane topography.

In recent years three powerful optical imaging techniques have emerged that provide nanometer-scale information about the topography of membrane surfaces, whether cellular or artificial: intermembrane fluorescence resonance energy transfer (FRET), fluorescence interference contrast microscopy (FLIC), and reflection interference contrast microscopy (RICM). In intermembrane FRET, the sharp distance dependence of resonant energy transfer between fluorophores allows topographic measurements in the Angstrom to few-nanometer range. In FLIC and RICM, interference between light from a membrane (either from fluorescent probes, or reflected illumination) and light reflected by a planar substrate provide spatial sensitivity in the few to hundreds of nanometer range, with few-nanometer resolution. All of these techniques are fairly easy to implement. We discuss the physics and optics behind each of these tools, as well as practical concerns regarding their uses. We also provide examples of their application in imaging molecular-scale structures at intermembrane junctions.

Animals↗

PubMed2003

Conducting nanowires built by controlled self-assembly of amyloid fibers and selective metal deposition.

Recent research in the field of nanometer-scale electronics has focused on the operating principles of small-scale devices and schemes to realize useful circuits. In contrast to established "top-down" fabrication techniques, molecular self-assembly is emerging as a "bottom-up" approach for fabricating nanostructured materials. Biological macromolecules, especially proteins, provide many valuable properties, but poor physical stability and poor electrical characteristics have prevented their direct use in electrical circuits. Here we describe the use of self-assembling amyloid protein fibers to construct nanowire elements. Self-assembly of a prion determinant from Saccharomyces cerevisiae, the N-terminal and middle region (NM) of Sup35p, produced 10-nm-wide protein fibers that were stable under a wide variety of harsh physical conditions. Their lengths could be roughly controlled by assembly conditions in the range of 60 nm to several hundred micrometers. A genetically modified NM variant that presents reactive, surface-accessible cysteine residues was used to covalently link NM fibers to colloidal gold particles. These fibers were placed across gold electrodes, and additional metal was deposited by highly specific chemical enhancement of the colloidal gold by reductive deposition of metallic silver and gold from salts. The resulting silver and gold wires were approximately 100 nm wide. These biotemplated metal wires demonstrated the conductive properties of a solid metal wire, such as low resistance and ohmic behavior. With such materials it should be possible to harness the extraordinary diversity and specificity of protein functions to nanoscale electrical circuitry.

Amyloid↗