Self-Organized Gels in DNA/F-Actin Mixtures without Crosslinkers: Networks of Induced Nematic Domains with Tunable Density

We examine mixtures of DNA and filamentous actin (F-actin) as a model system of like-charged rigid rods and flexible chains. Confocal microscopy reveals the formation of elongated nematic F-actin domains reticulated via defect-free vertices into a network embedded in a mesh of random DNA. Synchrotron x-ray scattering results indicate that the DNA mesh squeezes the F-actin domains into a nematic state with an interactin spacing that decreases with increasing DNA concentration as $d_{\mathrm{actin}}\propto {\rho }_{\mathrm{DNA}}^{-1/2}$. Interestingly, the system changes from a counterion-controlled regime to a depletion-controlled regime with added salt, with drastic consequences for the osmotic pressure induced phase behavior.

Figure 1

(a) Fluorescence confocal image of phase-separated domains of F-actin [light gray (green)] and DNA [dark gray (red)]. Colocalized same-area fluorescence showing (b) DNA [gray (red)], (c) F-actin [white (bright green)], and (d) PM image of the birefringent F-actin domains.

Figure 2

Morphology and director structures in a network of nematic domains. (a) A fluorescence texture of a bent F-actin domain [gray (green)]. (b) A colocalized polarizing microscopy texture. (c) Schematics of $\widehat{n}$ in the bent domain shown in (a),(b); the dark brushes show where $\widehat{n}$ is parallel or perpendicular to the crossed polarizers. (d),(e) Schematics of different domain nodes: (d) formed without bulk defects in $\widehat{n}$ and frequently observed, (e) containing defects and rarely observed. (f)–(h) Vertical confocal cross sections showing (f) the F-actin domains [gray (green)] and (g),(h) surrounding DNA [gray (red)]. In the colocalized images (f),(g), single phase-separated domains span across the sample. The multidomain distribution is shown in (h). As the DNA concentration increases from (i) $1\mathrm{mg}/\mathrm{ml}$ to (j) $7.5\mathrm{mg}/\mathrm{ml}$, the actin domains decrease in width.

Figure 3

Circularly averaged SAXS data for (a) 48 kbp DNA and (b) 10 kbp DNA show similar trends. The interactin spacing decreases as the concentration of DNA increases. (c) Log-log plot of the interactin spacing $d_{\mathrm{actin}}$ versus molar ratio of DNA bp to G-actin monomers for 48 kbp DNA (solid line) and 10 kbp DNA (dashed line); best linear fits provide slopes $-0.502\pm 0.055$ and $-0.498\pm 0.087$, respectively. (d),(e) 2D SAXS images of (d) F-actin DNA mixtures ($5\mathrm{mg}/\mathrm{ml}$ actin) and that of (e) pure F-actin ($50\mathrm{mg}/\mathrm{ml}$ actin). (f) Phase diagram of DNA F-actin mixture in KCl. The shaded area indicates induced nematic order. (g) SAXS data for DNA F-actin mixture ($2.5\mathrm{mg}/\mathrm{ml}$ DNA, $5\mathrm{mg}/\mathrm{ml}$ F-actin) show suppression of nematic order with KCl. Inset: Log-log plot of $d_{\mathrm{actin}}$ at 10 mM KCl with slope $-0.201\pm 0.051$, which is distinct from the no-salt behavior.

Figure 4

Fluorescence confocal image of DNA F-actin mixture at 5 mM KCl ($[\mathrm{DNA}]/[\mathrm{G}\mathrm{\text{−}}\mathrm{\text{actin}}]=13$). (a) DNA [gray (red)] infiltrates into the elongated (b) F-actin [gray (green)] domains and breaks them up into smaller F-actin subdomains surrounded by DNA.