Cellular mechanisms underlying the development of left-right asymmetry in tissues and embryos remain obscure. Here, the development of a chiral pattern of actomyosin was revealed by studying actin cytoskeleton self-organization in cells with isotropic circular shape. A radially symmetrical system of actin bundles consisting of α-actinin-enriched radial fibres (RFs) and myosin-IIA-enriched transverse fibres (TFs) evolved spontaneously into the chiral system as a result of the unidirectional tilting of all RFs, which was accompanied by a tangential shift in the retrograde movement of TFs. We showed that myosin-IIA-dependent contractile stresses within TFs drive their movement along RFs, which grow centripetally in a formin-dependent fashion. The handedness of the chiral pattern was shown to be regulated by α-actinin-1. Computational modelling demonstrated that the dynamics of the RF-TF system can explain the pattern transition from radial to chiral. Thus, actin cytoskeleton self-organization provides built-in machinery that potentially allows cells to develop left-right asymmetry.

DNA sequence design. DNA sequences for 3-, 4-, 5-, and 6-helix ribbon systems, and 4-, 5-, and 6-helix tube systems were designed and optimized using the SEQUIN software (S1) and the TileSoft software (S2) to minimize sequence symmetry (S1). The other systems were designed using an unpublished sequence design component of the NUPACK server (www.nupack.org) to maximize the affinity and specificity for the target structures (S3). Sometimes, manual optimization was further performed on selected regions. Sample preparation. DNA strands were synthesized by Integrated DNA Technology, Inc. (www.idtdna.com) and purified by denaturing polyacrylamide gel electrophoresis or HPLC. The concentrations of the DNA strands were determined by the measurement of ultraviolet absorption at 260 nm. To assemble the structures, DNA strands were mixed stoichiometrically to a final concentration of ∼1 μM for 20-helix ribbons and 20-helix tubes and ∼3 μM for other structures in 1× TAE/Mg ++ buffer (20 mM Tris, pH 7.6, 2 mM EDTA, 12.5 mM MgCl 2) and annealed in a water bath in a styrofoam box by cooling from 90 • C to 23 • C over a period of 24 to 72 hours. AFM imaging. AFM images were obtained using an MultiMode SPM with a Nanoscope IIIa controller (Veeco, Santa Barbara, CA) equipped with an Analog Q-control to optimize the sensitivity of the tapping mode (nanoAnalytics GmbH, Münster, Germany). A ∼40 μL drop of 1× TAE/Mg ++ followed by a ∼5 μL drop of annealed sample was applied onto the surface of a freshly cleaved mica and left for approximately 2 minutes. Sometimes, additional dilution of the sample was performed to achieve the desired sample density. On a few occasions, supplemental 1× TAE/8mM Ni ++ was added to increase the strength of DNA-mica binding (S4). Before placing the fluid cell on top of the mica puck, an additional ∼20 μL of 1× TAE/Mg ++ buffer was added to the cavity between the fluid cell and the AFM cantilever chip to avoid bubbles. The AFM tips used were either the short and thin cantilever in the DNP-S oxide sharpened silicon nitride cantilever chip (Veeco Probes, Camarillo, CA) or the short cantilever in the SiNi chip (BudgetSensors, Sofia, Bulgaria). Fluorescence imaging and length measurements. For fluorescence microscopy imaging, the 5-end of the U1 strand was labeled with a Cy3 fluorophore. A 4 μL drop of 10 nM SST sample was deposited onto an untreated coverslip. The light microscope is a home-built prism-based TIRF microscope. The samples were excited with 532 nm solid-state laser (CrystaLaser, Reno, NV). The Cy3 emission was detected by a 60×, 1.2 NA water immersion objective (Nikon), a Dual-View 2-channel filter cube (Photometrics, Pleasanton, CA), and a C9100-02 electron multiplier CCD camera (Hamamatsu). The images were analyzed using the imageJ image processing software (NIH) and MATLAB. A threshold was applied to each image to differentiate the nanotubes and the glass surface. The "skeletonize" command in imageJ is used to reduce a tube image to a single pixel wide skeleton, and the length of the skeleton ...

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DNA nanotechnology has emerged as a reliable and programmable way of controlling matter at the nanoscale through the specificity of Watson–Crick base pairing, allowing both complex self-assembled structures with nanometer precision and complex reaction networks implementing digital and analog behaviors. Here we show how two well-developed frameworks, DNA tile self-assembly and DNA strand-displacement circuits, can be systematically integrated to provide programmable kinetic control of self-assembly. We demonstrate the triggered and catalytic isothermal self-assembly of DNA nanotubes over 10 μm long from precursor DNA double-crossover tiles activated by an upstream DNA catalyst network. Integrating more sophisticated control circuits and tile systems could enable precise spatial and temporal organization of dynamic molecular structures.