Fig. 2.

Effect of cross-linker and filament mechanics on domain length. (A) Schematic of a 2-filament bundle transitioning from a tight fascin bundle to a wider-spaced $\alpha$-actinin bundle in an idealized geometry. The filament bends twice at an angle $\theta$, leaving a gap of length $l_g$, defined along the contour of actin rather than between cross-linker centers so that the bending energy in the lattice model and AFINES have the same functional form. (B) Lattice model example, in which there is initially a gap of length $6l_b$ indicated by the black dashed line and the indicated lattice site is empty. If that lattice site switches to a short cross-linker, the gap will reduce to $2l_b$ (red solid line), whereas if it switches to a long cross-linker, the gap will reduce to $4l_b$ (cyan solid line), yielding energy changes ($\mathrm{\Delta }{U}_{\mathrm{l}\mathrm{a}\mathrm{t}}$). (C) Domain lengths from AFINES and lattice model as a function of cross-linker length difference ($\mathrm{\Delta }{l}_{xl}$), with $L_p=17\hspace{0.17em}\mathrm{\mu }$m, $l_{\mathrm{s}\mathrm{h}\mathrm{o}\mathrm{r}\mathrm{t}}=200$ nm, and $l_{\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{g}}$ varying. (D) Same as C, but varying filament persistence length ($L_p$) while $l_{\mathrm{l}\mathrm{o}\mathrm{n}\mathrm{g}}=300$ nm. In C and D, domain lengths are averaged over the last 100 s of a 2,000-s simulation, and 40 simulations; error bars are SEM. In D, filament repulsion ($k_{\mathrm{e}\mathrm{x}\mathrm{v}}=0.08$ pN$\mathrm{\mu }{\mathrm{m}}^2$) is also used to prohibit filaments from crossing each other, which occurred at low $L_p$.