Networks of the cytoskeletal biopolymer actin cross-linked by the compliant protein filamin form soft gels that stiffen dramatically under shear stress. We demonstrate that the elasticity of these networks shows a strong dependence on the mean length of the actin polymers, unlike networks with small, rigid cross-links. This behavior is in agreement with a model of rigid filaments connected by multiple flexible linkers.

Networks of the biopolymer actin, cross-linked by the compliant protein filamin, form soft gels. They can, however, withstand large shear stresses due to their pronounced nonlinear elastic behavior. The nonlinear elasticity can be controlled by varying the number of cross-links per actin filament. We propose and test a model of rigid filaments decorated by multiple flexible linkers that is in quantitative agreement with experiment. This allows us to estimate loads on individual cross-links, which we find to be less than 10 pN. DOI: 10.1103/PhysRevE.79.041928 PACS number͑s͒: 87.16.Ka, 83.60.Df, 87.15.La Cells interact mechanically with their environment largely through their cytoskeleton, a mechanical framework consisting of filamentous protein polymers and associated proteins that regulate cytoskeletal microstructure and connectivity ͓1,2͔. The in vivo cytoskeleton is remarkably complex, making in vitro studies of purified cytoskeletal networks useful for elucidating basic physical principles governing cytoskeletal mechanics ͓3,4͔ and mechanosensing ͓5͔. Networks of filamentous actin ͑F-actin͒, a major component of the cytoskeleton, exhibit unusual material properties ͓6-14͔. Among the most striking properties is a strongly nonlinear response to shear ͓6-10͔; this depends sensitively on the cross-linking protein. For noncompliant cross-links, the network response arises from the compliance of the actin filaments themselves ͓7-10,15͔. By contrast, for compliant cross-links, such as those commonly found in cells, actin networks exhibit dramatically different elasticity ͓16-19͔. One such cross-link is human filamin, a large protein with a contour length of 150 nm ͓20,21͔, ͓Fig. 1͑b͔͒. For small forces, filamin behaves like a flexible wormlike chain; whereas for forces larger than 50-100 pN, unfolding of individual Ig-like domains occurs ͓22͔. Human filamin forms a weak gel with F-actin, with linear shear moduli of ϳ1 Pa. However, these gels exhibit a large mechanical stiffening under strain ͓16,17͔, and can withstand very large stresses, as high as 100 Pa, at shear strains of order of one. A theoretical understanding of the molecular origins of this unusual behavior will help elucidate the basic design principles of cytoskeletal mechanics.Here, we show that the nonlinear elastic behavior of filamin-F-actin gels is controlled by the number of crosslinks per actin filament, and we account for this unusual behavior by modeling the networks as rigid rods connected with multiple flexible linkers ͓23͔. The model quantitatively explains the dramatic nonlinear stiffening of filamin-F-actin networks, providing fundamental insight into its origins. It also provides an estimate of the maximum load experienced by individual cross-links, which is less than 10 pN, too small to result in significant unfolding of filamin Ig-like domains.We reconstitute networks of F-actin cross-linked with filamin A using purified monomeric actin, filamin A ͓24͔, and gelsolin ͓25͔. We control network microstructure by varying the actin concent...

No abstract

No abstract

The goal of this paper is to make modeling and quantitative testing accessible to behavioral decision researchers interested in substantive questions. We provide a novel, rigorous, yet very general, quantitative diagnostic framework for testing theories of binary choice. This permits the nontechnical scholar to proceed far beyond traditionally rather superficial methods of analysis, and it permits the quantitatively savvy scholar to triage theoretical proposals before investing effort into complex and specialized quantitative analyses. Our theoretical framework links static algebraic decision theory with observed variability in behavioral binary choice data. The paper is supplemented with a custom-designed public-domain statistical analysis package, the QTest software. We illustrate our approach with a quantitative analysis using published laboratory data, including tests of novel versions of “Random Cumulative Prospect Theory.” A major asset of the approach is the potential to distinguish decision makers who have a fixed preference and commit errors in observed choices from decision makers who waver in their preferences.

This article presents a robust approach to navigating at high speed across desert terrain. A central theme of this approach is the combination of simple ideas and components to build a capable and robust system. A pair of robots were developed, which completed a 212 km Grand Challenge desert race in approximately 7 h. A pathcentric navigation system uses a combination of LIDAR and RADAR based perception sensors to traverse trails and avoid obstacles at speeds up to 15 m/s. The onboard navigation system leverages a human-based preplanning system to improve reliability and robustness. The robots have been extensively tested, traversing over 3500 km of desert trails prior to completing the challenge. This article describes the mechanisms, algorithms, and testing methods used to achieve this performance.

No abstract

No abstract