Brownian Dynamics Simulation of Polydisperse Hard Spheres

Standard algorithms for the numerical integration of the Langevin equation require that interactions are slowly varying during to the integration timestep. This in not the case for hard-body systems, where there is no clearcut between the correlation time of the noise and the timescale of the interactions. Starting from a short time approximation of the Smoluchowsky equation, we introduce an algorithm for the simulation of the overdamped Brownian dynamics of polydisperse hard-spheres in absence of hydrodynamics interactions and briefly discuss the extension to the case of external drifts

Phase equilibria and glass transition in colloidal systems with short-ranged attractive interactions. Application to protein crystallization

We have studied a model of a complex fluid consisting of particles interacting through a hard core and a short range attractive potential of both Yukawa and square-well form. Using a hybrid method, including a self-consistent and quite accurate approximation for the liquid integral equation in the case of the Yukawa fluid, perturbation theory to evaluate the crystal free energies, and mode-coupling theory of the glass transition, we determine both the equilibrium phase diagram of the system and the lines of equilibrium between the supercooled fluid and the glass phases. For these potentials, we study the phase diagrams for different values of the potential range, the ratio of the range of the interaction to the diameter of the repulsive core being the main control parameter. Our arguments are relevant to a variety of systems, from dense colloidal systems with depletion forces, through particle gels, nano-particle aggregation, and globular protein crystallization.Comment: 20 pages, 10 figure

The role of TG2 in regulating S100A4-mediated mammary tumour cell migration

The importance of S100A4, a Ca2+-binding protein, in mediating tumour cell migration, both intracellularly and extracellularly, is well documented. Tissue transglutaminase (TG2) a Ca2+-dependent protein crosslinking enzyme, has also been shown to enhance cell migration. Here by using the well characterised non-metastatic rat mammary R37 cells (transfected with empty vector) and highly metastatic KP1 cells (R37 cells transfected with S100A4), we demonstrate that inhibition of TG2 either by TG2 inhibitors or transfection of cells with TG2 shRNA block S100A4-accelerated cell migration in the KP1cells and in R37 cells treated with exogenous S100A4. Cell migration was also blocked by the treatment with the non-cell permeabilizing TG2 inhibitor R294, in the human breast cancer cell line MDA-MB-231 (Clone 16, which has a high level of TG2 expression). Inhibition was paralleled by a decrease in S100A4 polymer formation. co-immunoprecipitation and Far Western blotting assays and cross-linking assays showed not only the direct interaction between TG2 and S100A4, but also confirmed S100A4 as a substrate for TG2. Using specific functional blocking antibodies, a targeting peptide and a recombinant protein as a competitive treatment, we revealed the involvement of syndecan-4 and a5ß1 integrin co-signalling pathways linked by activation of PKCa in this TG2 and S100A4-mediated cell migration. We propose a mechanism for TG2-regulated S100A4-related mediated cell migration, which is dependent on TG2 crosslinking

Tropomyosin and troponin cooperativity on the thin filament

The regulation of muscle contraction by the thin filament proteins tropomyosin (Tm) and troponin (Tn) has remained an area of interest since the proteins were first discovered 40 years ago.1,2 Although we have learnt a great deal about the proteins themselves and the mechanism by which they regulate muscle contraction some aspects of the mechanism remain to be adequately explained. Our interest is in the cooperativity of the calcium regulatory process and this remains poorly understood and several different models have been proposed. At it simplest the essence of the problem can be simply outlined. In skeletal muscle the binding of calcium to the two regulatory sites of TnC is required for activation of muscle contraction. Isolated TnC binds the calcium cooperatively, as might be expected for a two-calcium-ion switch, with a hill coefficient (h) of between 1 & 2.3,4 In contrast the calcium activation of isometric force in a skinned muscle fibre occurs with a much larger hill coefficient5,6 leading to the idea that cooperativity extends beyond the single actin7TmTn structural unit of the thin filament. Some models of muscle activation suggest the whole filament switches as a single unit while studies of the purified proteins in solution tend to indicate more limited cooperativity extending to only the nearest neighbour actin7TmTn units. In this paper the reasons why the nature of the cooperativity remains a problem will be explored together with an overview of what our recent studies of the proteins in solution have revealed about thin filament cooperativity