Forced dimerization of αCat enhances filopodia on radiating protrusions at nascent contacts. (A) ΔNαCat dimerization by B/B promotes filopodia formation. Filopodia were counted every 1 s during a video of force dimerization (n = 6 FOVs from two BRs; data are mean ± SD). Bars, 10 µm. See also Video 1. (B) Actin ultrastructure of dimerized ΔNαCat (±B/B) by platinum replica electron microscopy. Bars, 500 nm. Arrows show αCat enrichment at protrusions. (C) Epifluorescence microscopy of radial protrusions (RPs; white arrows) induced by homodimerization. Bars, 20 µm. (D) Blinded quantification of RPs (n > 150 cells; FOV counts ratioed to total number of cells to account for variations in cell density; Materials and methods; data are mean ± SD). (E) Structured illumination microscopy (SIM) of RPs with filopodia. Bars, 5 µm. (F) Quantification of filopodia length (n > 13 FOVs; three BRs; data are mean ± SD). Significance in D and F by ANOVA. **, P < 0.01; ***, P < 0.001; ****, P < 0.0001. (G) Time-lapse analysis of B/B-treated ΔNαCat cells coinfected with GFP-LifeAct. Arrows indicate prolonged cell–cell contact upon homodimerization, which is reversed by washout ligand. Bars, 10 µm. See also Video 2.