The nucleus regulates cell contractility and migration in response to substratum rigidity. (A) REF52 cell velocity on compliant substrata for intact cells (n ≥ 45/stiffness) and cytoplasts (n ≥ 69/stiffness). (B) Same data in A, showing overlay of cell velocity ± 95% CI on compliant substrata. (C) Contractile energy of REF52 intact cells (n = 98) and cytoplasts (n = 72). (D) Graph (left) showing mean traction stress ± SEM for REF52 intact cells and cytoplasts. Representative images (right) of traction stresses. Force vectors (arrows) and cell outlines (cyan) are shown. Bar, 20 µm. Scale is traction stress magnitude (in pascals). (E) Cell velocity ± 95% CI on compliant substrata for HUVEC intact cells (n ≥ 72/stiffness) and cytoplasts (n ≥ 112/stiffness). (F) Contractile energy graph (left) of HUVEC intact cells (n = 89) and cytoplasts (n = 93). Graph (right) showing mean traction stress ± SEM for HUVEC intact cells and cytoplasts. Dotted lines in B and E show 8 and 25 kPa. All data are from at least three independent experiments. Mann–Whitney U tests were performed for all traction force data. One-way ANOVA with Tukey’s post-hoc test was performed between stepwise increases in stiffness for A, B, and E. Boxplots in C and F show 10th–90th percentiles. ***, P < 0.001; **, P < 0.01; *, P < 0.05.