Par1b inhibits cell spreading and stress fiber formation and promotes cell compaction. (A) Phase-contrast images of cell islands cultured on tissue culture plastic for 24 h (left) or brightfield images of a time lapse (Video 6) after cell plating on collagen I (right). Translucent holes in the 24-h Par1b (−dox) culture representing lateral lumina. (B) Spreading on collagen I. *, P < 0.001 for differences between ±dox. Error bars indicate SEM. (C, D, and F) Still images from time-lapse videos 30 (C and F) or 45 min (D) after plating on collagen. (C) Brightfield and mRFP fluorescence (Videos 1 and 2) are shown. Arrows point to stress fibers. (D) Note the dox-dependent size difference in Par1b cells and Paxillin-mCherry (Video 3). Arrowheads point to FAs. (F) GFP or Par1b-GFP and mRFP actin (Video 5) are shown. Note both the difference in stress fibers (arrows) and that Par1b-GFP but not GFP localizes to the tips of actin bundles in lamella (arrowheads). (E) Polarized Par1b cultures 24 h after Ca²⁺ switch. Phalloidin labels stress fibers at the basal domain (bottom) and microvilli-rich lateral lumina and apical domains in the maximum (max) projections (top; Video 6). (G) Confocal sections through collagen cysts of Par1b-MDCK cells (±dox; see Video 4 for full stacks). Arrowheads point to gaps in laminin. Also note that Par1b cysts are smaller than WT cysts (mean: 23 vs. 30 cells/cyst; n = 13/17). Bars: (A–F) 10 µm; (G) 25 µm.