Exciton scattering in light-harvesting systems of purple bacteria

Using the reduced density matrix formalism the exciton scattering in light-harvesting systems of purple bacteria is calculated. The static disorder (fluctuations of the site energies) as well as the dynamic disorder (dissipation) is taken into account in this work. Circular aggregates with 18 pigments are studied to model the B850 ring of bacteriochlorophylls with LH2 complexes. It can be shown that the influence of dissipation may not be neglected in the simulation of the time-dependent anisotropy of fluorescence. Also an elliptical deformation of the ring could be essential

Influence of Static and Dynamic Disorder on the Anisotropy of Emission in the Ring Antenna Subunits of Purple Bacteria Photosynthetic Systems

Using the reduced density matrix formalism the time dependence of the exciton scattering in light-harvesting ring systems of purple bacteria is calculated. In contrast to the work of Kumble and Hochstrasser (J. Chem. Phys. 109 (1998) 855) static disorder (fluctuations of the site energies) as well as dynamic disorder (dissipation) is taken into account. For the description of dissipation we use Redfield theory in exciton eigenstates without secular approximation. This is shown to be equivalent to the Markovian limit of Capek's theory in local states. Circular aggregates with 18 pigments are studied to model the B850 ring of bacteriochlorophyls within LH2 complexes. It can be demonstrated that the dissipation is important for the time-dependent anisotropy of the fluorescence. Smaller values of static disorder are sufficient to produce the same decay rates in the anisotropy in comparison with the results by Kumble and Hochstrasser

Programmed Bending Reveals Dynamic Mechanochemical Coupling in Supported Lipid Bilayers

In living cells, mechanochemical coupling represents a dynamic means by which membrane components are spatially organized. An extra-ordinary example of such coupling involves curvature-dependent polar localization of chemically-distinct lipid domains at bacterial poles, which also undergo dramatic reequilibration upon subtle changes in their interfacial environment such as during sporulation. Here, we demonstrate that such interfacially-triggered mechanochemical coupling can be recapitulated in vitro by simultaneous, real-time introduction of mechanically-generated periodic curvatures and attendant strain-induced lateral forces in lipid bilayers supported on elastomeric substrates. In particular, we show that real-time wrinkling of the elastomeric substrate prompts a dynamic domain reorganization within the adhering bilayer, producing large, oriented liquid-ordered domains in regions of low curvature. Our results suggest a mechanism in which interfacial forces generated during surface wrinkling and the topographical deformation of the bilayer combine to facilitate dynamic reequilibration prompting the observed domain reorganization. We anticipate this curvature-generating model system will prove to be a simple and versatile tool for a broad range of studies of curvature-dependent dynamic reorganizations in membranes that are constrained by the interfacial elastic and dynamic frameworks such as the cell wall, glycocalyx, and cytoskeleton

Hemorheology and Microvascular Disorders

The present review presents basic concepts of blood rheology related to vascular diseases. Blood flow in large arteries is dominated by inertial forces exhibited at high flow velocities, while viscous forces (i.e., blood rheology) play an almost negligible role. When high flow velocity is compromised by sudden deceleration as at a bifurcation, endothelial cell dysfunction can occur along the outer wall of the bifurcation, initiating inflammatory gene expression and, through mechanotransduction, the cascade of events associated with atherosclerosis. In sharp contrast, the flow of blood in microvessels is dominated by viscous shear forces since the inertial forces are negligible due to low flow velocities. Shear stress is a critical parameter in microvascular flow, and a force-balance approach is proposed for determining microvascular shear stress, accounting for the low Reynolds numbers and the dominance of viscous forces over inertial forces. Accordingly, when the attractive forces between erythrocytes (represented by the yield stress of blood) are greater than the shear force produced by microvascular flow, tissue perfusion itself cannot be sustained, leading to capillary loss. The yield stress parameter is presented as a diagnostic candidate for future clinical research, specifically, as a fluid dynamic biomarker for microvascular disorders. The relation between the yield stress and diastolic blood viscosity (DBV) is described using the Casson model for viscosity, from which one may be able determine thresholds of DBV where the risk of microvascular disorders is high

Real-time visualization of heterotrimeric G protein Gq activation in living cells

Contains fulltext : 97296.pdf (publisher's version ) (Open Access)BACKGROUND: Gq is a heterotrimeric G protein that plays an important role in numerous physiological processes. To delineate the molecular mechanisms and kinetics of signalling through this protein, its activation should be measurable in single living cells. Recently, fluorescence resonance energy transfer (FRET) sensors have been developed for this purpose. RESULTS: In this paper, we describe the development of an improved FRET-based Gq activity sensor that consists of a yellow fluorescent protein (YFP)-tagged Ggamma2 subunit and a Galphaq subunit with an inserted monomeric Turquoise (mTurquoise), the best cyan fluorescent protein variant currently available. This sensor enabled us to determine, for the first time, the kon (2/s) of Gq activation. In addition, we found that the guanine nucleotide exchange factor p63RhoGEF has a profound effect on the number of Gq proteins that become active upon stimulation of endogenous histamine H1 receptors. The sensor was also used to measure ligand-independent activation of the histamine H1 receptor (H1R) upon addition of a hypotonic stimulus. CONCLUSIONS: Our observations reveal that the application of a truncated mTurquoise as donor and a YFP-tagged Ggamma2 as acceptor in FRET-based Gq activity sensors substantially improves their dynamic range. This optimization enables the real-time single cell quantification of Gq signalling dynamics, the influence of accessory proteins and allows future drug screening applications by virtue of its sensitivity

Mechanics rules cell biology

Cells in the musculoskeletal system are subjected to various mechanical forces in vivo. Years of research have shown that these mechanical forces, including tension and compression, greatly influence various cellular functions such as gene expression, cell proliferation and differentiation, and secretion of matrix proteins. Cells also use mechanotransduction mechanisms to convert mechanical signals into a cascade of cellular and molecular events. This mini-review provides an overview of cell mechanobiology to highlight the notion that mechanics, mainly in the form of mechanical forces, dictates cell behaviors in terms of both cellular mechanobiological responses and mechanotransduction

The alpha-kinase family: an exceptional branch on the protein kinase tree

The alpha-kinase family represents a class of atypical protein kinases that display little sequence similarity to conventional protein kinases. Early studies on myosin heavy chain kinases in Dictyostelium discoideum revealed their unusual propensity to phosphorylate serine and threonine residues in the context of an alpha-helix. Although recent studies show that some members of this family can also phosphorylate residues in non-helical regions, the name alpha-kinase has remained. During evolution, the alpha-kinase domains combined with many different functional subdomains such as von Willebrand factor-like motifs (vWKa) and even cation channels (TRPM6 and TRPM7). As a result, these kinases are implicated in a large variety of cellular processes such as protein translation, Mg2+ homeostasis, intracellular transport, cell migration, adhesion, and proliferation. Here, we review the current state of knowledge on different members of this kinase family and discuss the potential use of alpha-kinases as drug targets in diseases such as cancer

Protein kinase C (PKC)-mediated phosphorylation of PACSIN2 triggers the removal of caveolae from the plasma membrane

PACSIN2, a membrane-sculpting BAR domain protein, localizes to caveolae. Here, we found that protein kinase C (PKC) phosphorylates PACSIN2 at serine 313, thereby decreasing its membrane binding and tubulation capacities. Concomitantly, phosphorylation decreased the time span for which caveolae could be tracked at the plasma membrane (the ‘tracking duration’). Analyses of the phospho-mimetic S313E mutant suggested that PACSIN2 phosphorylation was sufficient to reduce caveolar-tracking durations. Both hypotonic treatment and isotonic drug-induced PKC activation increased PACSIN2 phosphorylation at serine 313 and shortened caveolar-tracking durations. Caveolar-tracking durations were also reduced upon the expression of other membrane-binding-deficient PACSIN2 mutants or upon RNA interference (RNAi)-mediated PACSIN2 depletion, pointing to a role for PACSIN2 levels in modulating the lifetime of caveolae. Interestingly, the decrease in membrane-bound PACSIN2 was inversely correlated with the recruitment and activity of dynamin 2, a GTPase that mediates membrane scission. Furthermore, expression of EHD2, which stabilizes caveolae and binds to PACSIN2, restored the tracking durations of cells with reduced PACSIN2 levels. These findings suggest that the PACSIN2 phosphorylation decreases its membrane-binding activity, thereby decreasing its stabilizing effect on caveolae and triggering dynamin-mediated removal of caveolae

Electronic and Vibrational Coherence in Photosynthetic and Model Systems

Ultrafast optical response and excitation energy dynamics in light harvesting pigment-protein complexes and in a few model systems have been studied both experimentally and theoretically. Femtosecond light pulses were employed to excite the system and monitor the subsequent time evolution of the transient absorption kinetics and anisotropy. A new form of pump-probe technique was developed for performing the spectrally resolved transient absorption measurements using femtosecond pulses from single colour laser source; an appropriate description using density operator formalism was given. It has been shown both experimentally and theoretically that the interaction of broad-band femtosecond light pulses with a system consisting of three quantum levels leads to the appearance of anisotropic coherent transients in pump-probe signal, which are due to creation of optical coherence via tunneling processes. The time-resolved measurements performed on the light harvesting antennae of purple bacteria indicated an extremely short time scale of excitation transfer in these complexes; the process of depolarization of pump-probe signal typically occurs on a ~100 fs time scale but it can be considerably slower on the long wavelength tail of the absorption band at low temperatures due to localization effects. The exciton theory was used to fit the transient absorption spectra of the complexes in order to find the exciton delocalization (or coherence) length. Furthermore the measurements indicated the presence of oscillatory modulation in the pump-probe kinetics due to the vibrational coherence created by the short excitation pulse, which leads to wave-packet motion in the excited state. Numerical simulations based on the density operator theory satisfactorily reproduced experimentally observed wave-length dependent phase shifts and amplitude relaxation of these oscillations. Finally the extensive spectroscopic study of a double-bond bridged porphyrin dimer was undertaken. The experiments showed that the dimer exists in solution in a few different conformations. The CNDO/S calculations and experimental data confirmed that one of these conformers has a particular geometry favouring a common conjugation between the p orbitals of the porphyrin rings and the ethylene bridge; this leads to dramatic changes in the absorption and emission spectra and to ultrafast, viscosity dependent excited state deactivation. These supermolecular properties makes this dimer system conceptually similar to the special pair in the photosynthetic reaction centre