Nuclear pore complexes (NPCs) gate the only conduits for nucleocytoplasmic transport in eukaryotes. Their gate is formed by nucleoporins containing large intrinsically disordered domains with multiple phenylalanine-glycine repeats (FG domains). In combination, these are hypothesized to form a structurally and chemically homogeneous network of random coils at the NPC center, which sorts macromolecules by size and hydrophobicity. Instead, we found that FG domains are structurally and chemically heterogeneous. They adopt distinct categories of intrinsically disordered structures in non-random distributions. Some adopt globular, collapsed coil configurations and are characterized by a low charge content. Others are highly charged and adopt more dynamic, extended coil conformations. Interestingly, several FG nucleoporins feature both types of structures in a bimodal distribution along their polypeptide chain. This distribution functionally correlates with the attractive or repulsive character of their interactions with collapsed coil FG domains displaying cohesion toward one another and extended coil FG domains displaying repulsion. Topologically, these bipartite FG domains may resemble sticky molten globules connected to the tip of relaxed or extended coils. Within the NPC, the crowding of FG nucleoporins and the segregation of their disordered structures based on their topology, dimensions, and cohesive character could force the FG domains to form a tubular gate structure or transporter at the NPC center featuring two

Controllable biaxial and uniaxial nanowrinkles (see figure) are fabricated by a simple two‐step approach — metal deposition and subsequent heating — based on shape memory polymer (prestressed polystyrene) sheets. The wavelengths of the wrinkles can be tuned by controlling the thickness of deposited metal. The ready integration of the nanowrinkles into microchannels and their effectiveness in surface enhanced sensing is demonstrated.

Summary Although bacterial cells are known to experience large forces from osmotic pressure differences and their local microenvironment, quantitative measurements of the mechanical properties of growing bacterial cells have been limited. We provide an experimental approach and theoretical framework for measuring the mechanical properties of live bacteria. We encapsulated bacteria in agarose with a user-defined stiffness, measured the growth rate of individual cells, and fit data to a thin-shell mechanical model to extract the effective longitudinal Young's modulus of the cell envelope of Escherichia coli (50–150 MPa), Bacillus subtilis (100–200 MPa), and Pseudomonas aeruginosa (100–200 MPa). Our data provide estimates of cell wall stiffness similar to values obtained via the more labor-intensive technique of atomic force microscopy. To address physiological perturbations that produce changes in cellular mechanical properties, we tested the effect of A22-induced MreB depolymerization on the stiffness of E. coli. The effective longitudinal Young's modulus was not significantly affected by A22 treatment at short time scales, supporting a model in which the interactions between MreB and the cell wall persist on the same time scale as growth. Our technique therefore enables the rapid determination of how changes in genotype and biochemistry affect the mechanical properties of the bacterial envelope.

A motile cell, when stimulated, shows a dramatic increase in the activity of its membrane, manifested by the appearance of dynamic membrane structures such as lamellipodia, filopodia, and membrane ruffles. The external stimulus turns on membrane bound activators, like Cdc42 and PIP2, which cause increased branching and polymerization of the actin cytoskeleton in their vicinity leading to a local protrusive force on the membrane. The emergence of the complex membrane structures is a result of the coupling between the dynamics of the membrane, the activators, and the protrusive forces. We present a simple model that treats the dynamics of a membrane under the action of actin polymerization forces that depend on the local density of freely diffusing activators on the membrane. We show that, depending on the spontaneous membrane curvature associated with the activators, the resulting membrane motion can be wavelike, corresponding to membrane ruffling and actin waves, or unstable, indicating the tendency of filopodia to form. Our model also quantitatively explains a variety of related experimental observations and makes several testable predictions.

We present a general method to study the nonequilibrium behavior of Casimir-type fluctuation-induced forces for classical free scalar field theories. In particular, we analyze the temporal evolution of the force toward its equilibrium value when the field dynamics is given by a general class of overdamped stochastic dynamics (including the model A and model B classes). The steady-state force is also analyzed for systems which have nonequilibrium steady states, for instance, where they are driven by colored noise. The key to the method is that the out of equilibrium force is computed by specifying an energy of interaction between the field and the surfaces in the problem. In general, we find that there is a mapping of the dynamical problem onto a corresponding static one and in the case where the latter can be solved, the full dynamical behavior of the force can be extracted. The method is used to compute the nonequilibrium Casimir force induced between two parallel plates by a fluctuating field, in the cases of Dirichlet, Neumann, and mixed boundary conditions. Various other examples, such as the fluctuation-induced force between inclusions in fluctuating media, are discussed.

Secondary spectrum access is emerging as a promising approach for mitigating the spectrum scarcity in wireless networks. Coordinated spectrum access for secondary users can be achieved using periodic spectrum auctions. Recent studies on such auction design mostly neglect the repeating nature of such auctions, and focus on greedily maximizing social welfare. Such auctions can cause subsets of users to experience starvation in the long run, reducing their incentive to continue participating in the auction. It is desirable to increase the diversity of users allocated spectrum in each auction round, so that a trade-off between social welfare and fairness is maintained. We study truthful mechanisms towards this objective, for both local and global fairness criteria. For local fairness, we introduce randomization into the auction design, such that each user is guaranteed a minimum probability of being assigned spectrum. Computing an optimal, interference-free spectrum allocation is NP-Hard; we present an approximate solution, and tailor a payment scheme to guarantee truthful bidding is a dominant strategy for all secondary users. For global fairness, we adopt the classic maxmin fairness criterion. We tailor another auction by applying linear programming techniques for striking the balance between social welfare and max-min fairness, and for finding feasible channel allocations. In particular, a pair of primal and dual linear programs are utilized to guide the probabilistic selection of feasible allocations towards a desired tradeoff in expectation.

A biomimetic substrate for cell‐culture is fabricated by plasma treatment of a prestressed thermoplastic shrink film to create tunable multiscaled alignment “wrinkles”. Using this substrate, the functional alignment of human embryonic stem cell derived cardiomyocytes is demonstrated.

Classical particles driven through periodically modulated potential energy landscapes are predicted to follow a Devil's staircase hierarchy of commensurate trajectories depending on the orientation of the driving force. Recent experiments on colloidal spheres flowing through arrays of optical traps do indeed reveal such a hierarchy, but not with the predicted structure. The microscopic trajectories, moreover, appear to be random, with commensurability emerging only in a statistical sense. We introduce an idealized model for periodically modulated transport in the presence of randomness that captures both the structure and statistics of such statistically locked-in states.Objects driven through periodic potential energy surfaces face a myriad of choices: either they follow the driving force or they become entrained along any of the commensurate directions through the landscape. Variants of this problem appear in areas as diverse as driven charge density waves [1], electronic energy states in twodimensional electron gases [2], atom migration on crystal surfaces [3], chemical kinetics, and flux flow in type-II superconductors [4]. Quite recently, this problem was investigated [5] using a monolayer of colloidal spheres in flowing water as a model system and a square array of holographic optical tweezers [6] to provide the periodic potential energy surface. Depending on the array's orientation with respect to the driving force, the spheres were observed to trace out a Devil's staircase hierarchy of commensurate directions through the array, with particular directions being preferentially selected over certain ranges of orientations [5]. Trajectories deflected by a preference for commensurability are said to be kinetically locked in to the lattice. This anticipated result [7] was accompanied by two surprises. In the first place, not all kinetically lockedin states were centered on simple commensurate directions. Still more surprisingly, particles' microscopic trajectories in high-order locked-in states did not consist of sequences of commensurate jumps, but rather consisted of seemingly random lower-order hops whose combination, nonetheless, were commensurate. The appearance of statistical rather than deterministic commensurability suggests an unexpected role for thermal randomness in structuring transport through periodic potentials, and has been dubbed statistical lock-in [5].This Letter presents a highly simplified model for statistically locked-in transport through mesoscopic potential energy landscapes that nonetheless accounts for the emergence of combination jumps and their statistical commensurability. In particular this model reveals how the potential energy landscape's structure and extent establish the discrete spectrum of travel directions selected by biased random walkers. While our discussion is directed toward the purely classical behavior of flowing colloids, similar results should emerge for biased quantum mechanical hopping through arrays of potential An applied force drives objects through the array...