Ruchuan Liu

Stretching Single Talin Rod Molecules Activates Vinculin Binding

The molecular mechanism by which a mechanical stimulus is translated into a chemical response in biological systems is still unclear. We show that mechanical stretching of single cytoplasmic proteins can activate binding of other molecules. We used magnetic tweezers, total internal reflection fluorescence, and atomic force microscopy to investigate the effect of force on the interaction between talin, a protein that links liganded membrane integrins to the cytoskeleton, and vinculin, a focal adhesion protein that is activated by talin binding, leading to reorganization of the cytoskeleton. Application of physiologically relevant forces caused stretching of single talin rods that exposed cryptic binding sites for vinculin. Thus in the talin-vinculin system, molecular mechanotransduction can occur by protein binding after exposure of buried binding sites in the talin-vinculin system. Such protein stretching may be a more general mechanism for force transduction.

Force-dependent conformational switch of α-catenin controls vinculin binding

Force sensing at cadherin-mediated adhesions is critical for their proper function. α-Catenin, which links cadherins to actomyosin, has a crucial role in this mechanosensing process. It has been hypothesized that force promotes vinculin binding, although this has never been demonstrated. X-ray structure further suggests that α-catenin adopts a stable auto-inhibitory conformation that makes the vinculin-binding site inaccessible. Here, by stretching single α-catenin molecules using magnetic tweezers, we show that the subdomains MI vinculin-binding domain (VBD) to MIII unfold in three characteristic steps: a reversible step at ~5u2009pN and two non-equilibrium steps at 10-15u2009pN. 5u2009pN unfolding forces trigger vinculin binding to the MI domain in a 1:1 ratio with nanomolar affinity, preventing MI domain refolding after force is released. Our findings demonstrate that physiologically relevant forces reversibly unfurl α-catenin, activating vinculin binding, which then stabilizes α-catenin in its open conformation, transforming force into a sustainable biochemical signal.

Hybrid Organic/Inorganic Nanocomposites for Photovoltaic Cells

Inorganic/organic hybrid solar cells have attracted a lot of interest due to their potential in combining the advantages of both components. To understand the key issues in association with photoinduced charge separation/transportation processes and to improve overall power conversion efficiency, various combinations with nanostructures of hybrid systems have been investigated. Here, we briefly review the structures of hybrid nanocomposites studied so far, and attempt to associate the power conversion efficiency with these nanostructures. Subsequently, we are then able to summarize the factors for optimizing the performance of inorganic/organic hybrid solar cells.

Mechanical characterization of Protein L in the low-force regime by electromagnetic tweezers/evanescent nanometry

Mechanical manipulation at the single molecule level of proteins exhibiting mechanical stability poses a technical challenge that has been almost exclusively approached by atomic force microscopy (AFM) techniques. However, due to mechanical drift limitations, AFM techniques are restricted to experimental recordings that last less than a minute in the high-force regime. Here we demonstrate a novel combination of electromagnetic tweezers and evanescent nanometry that readily captures the forced unfolding trajectories of protein L at pulling forces as low as 10-15 pN. Using this approach, we monitor unfolding and refolding cycles of the same polyprotein for a period of time longer than 30 min. From such long-lasting recordings, we obtain ensemble averages of unfolding step sizes and rates that are consistent with single-molecule AFM data obtained at higher stretching forces. The unfolding kinetics of protein L at low stretching forces confirms and extends the observations that the mechanical unfolding rate is exponentially dependent on the pulling force within a wide range of stretching forces spanning from 13 pN up to 120 pN. Our experiments demonstrate a novel approach for the mechanical manipulation of single proteins for extended periods of time in the low-force regime.

Bipedal Nanowalker by Pure Physical Mechanisms

Artificial nanowalkers are inspired by biomolecular counterparts from living cells, but remain far from comparable to the latter in design principles. The walkers reported to date mostly rely on chemical mechanisms to gain a direction; they all produce chemical wastes. Here we report a light-powered DNA bipedal walker based on a design principle derived from cellular walkers. The walker has two identical feet and the track has equal binding sites; yet the walker gains a direction by pure physical mechanisms that autonomously amplify an intrasite asymmetry into a ratchet effect. The nanowalker is free of any chemical waste. It has a distinct thermodynamic feature that it possesses the same equilibrium before and after operation, but generates a truly nonequilibrium distribution during operation. The demonstrated design principle exploits mechanical effects and is adaptable for use in other nanomachines.

Plasmon-driven surface catalysis in hybridized plasmonic gap modes

Plasmon-driven surface catalytic (PDSC) reaction in Ag/Au nanoparticle monomer or dimer-film gaps are experimentally and theoretically investigated, using surface enhanced Raman scattering (SERS) and finite element method. The variation of SERS spectra in different nano gaps of nanoparticle-film systems indicated the PDSC reaction was largely depended on the number of nanoparticles. The higher Raman intensity of p,p′-dimercaptoazobenzene (DMAB) in dimer-film nanogap was because effective coupling of induced image charge on metal film in hybridized plasmonic gap mode, which was confirmed by the electric field distribution. Furthermore, the influence of material and wavelength was also studied to obtain the optimal experimental condition for best surface catalysis in hybridized plasmonic gap mode. Our studies in this common configuration of plasmonic nanostructure are of great significance not only in the field of catalysis on metal surface but also in other surface plasmon fields such as senor, photon detection, water splitting, etc.

Oriented collagen fibers direct tumor cell intravasation

Significance Intravasation is an early stage of metastasis that involves metastatic cells moving from the tumor into the extracellular matrix (ECM), breakthrough of the basement membrane, and entry into blood vessels. We found that the oriented fibers greatly enhance and facilitate the metastatic cell intravasation process during metastasis. We suggest that a possible “tissue treatment” therapy could be considered, in which the ECM fiber structure orientation in the tumor microenvironment might be altered to minimize the intravasation rate of metastatic cells. In this work, we constructed a Collagen I–Matrigel composite extracellular matrix (ECM). The composite ECM was used to determine the influence of the local collagen fiber orientation on the collective intravasation ability of tumor cells. We found that the local fiber alignment enhanced cell–ECM interactions. Specifically, metastatic MDA-MB-231 breast cancer cells followed the local fiber alignment direction during the intravasation into rigid Matrigel (∼10 mg/mL protein concentration).

Uncovering mechanosensing mechanisms at the single protein level using magnetic tweezers.

Mechanosensing of the micro-environments has been shown to be essential for cell survival, growth, differentiation and migration. The mechanosensing pathways are mediated by a set of mechanosensitive proteins located at focal adhesion and cell-cell adherens junctions as well as in the cytoskeleton network. Here we review the applications of magnetic tweezers on elucidating the molecular mechanisms of the mechanosensing proteins. The scope of this review includes the principles of the magnetic tweezers technology, theoretical analysis of force-dependent stability and interaction of mechanosensing proteins, and recent findings obtained using magnetic tweezers.

Intense ultraviolet photoluminescence from amorphous Si:O:C films prepared by liquid–solution-phase technique

Abstract Intense ultraviolet (UV) photoluminescence (PL) was observed at room temperature from amorphous Si:O:C films synthesized at 200, 300, 400°C by the liquid–solution-phase (LSP) method. The intensity was as strong as that of visible emission band from porous silicon. In all the films, a band existed, centered at 340 nm, while a shoulder band centered at 380 nm was observed from the films deposited at 400°C. By analyzing the PL and photoluminescence excitation (PLE) spectra, Fourier-transform infrared (FTIR) absorption, X-ray photoelectronic spectroscopy (XPS) and X-ray diffraction (XRD), we suggest that the photoluminescence peak at 340 nm originates from the defects in silicon oxide network, while the peak at 380 nm may be related to Si–C and Si–Si bonds in the samples.

Biophysical properties of intrinsically disordered p130Cas substrate domain--implication in mechanosensing.

Mechanical stretch-induced tyrosine phosphorylation in the proline-rich 306-residue substrate domain (CasSD) of p130Cas (or BCAR1) has eluded an experimentally validated structural understanding. Cellular p130Cas tyrosine phosphorylation is shown to function in areas without internal actomyosin contractility, sensing force at the leading edge of cell migration. Circular dichroism shows CasSD is intrinsically disordered with dominant polyproline type II conformations. Strongly conserved in placental mammals, the proline-rich sequence exhibits a pseudo-repeat unit with variation hotspots 2–9 residues before substrate tyrosine residues. Atomic-force microscopy pulling experiments show CasSD requires minimal extension force and exhibits infrequent, random regions of weak stability. Proteolysis, light scattering and ultracentrifugation results show that a monomeric intrinsically disordered form persists for CasSD in solution with an expanded hydrodynamic radius. All-atom 3D conformer sampling with the TraDES package yields ensembles in agreement with experiment when coil-biased sampling is used, matching the experimental radius of gyration. Increasing β-sampling propensities increases the number of prolate conformers. Combining the results, we conclude that CasSD has no stable compact structure and is unlikely to efficiently autoinhibit phosphorylation. Taking into consideration the structural propensity of CasSD and the fact that it is known to bind to LIM domains, we propose a model of how CasSD and LIM domain family of transcription factor proteins may function together to regulate phosphorylation of CasSD and effect machanosensing.