Fig. 6. Live-cell imaging of GFP-actin reveals the presence of a centrifugal force that expands the contractile circumferential actin cables in an aPKC-dependent manner during epithelial-cell polarization. (A,B) Confluent monolayers of MTD1-A cells transfected with an adenovirus vector encoding GFP-actin together with a vector encoding β-galactosidase (LacZ; A) or aPKC kn (B) were depolarized in LC medium for 20 hours and then subjected to a Ca²⁺ switch (00:00). z-stack images taken at 1-µm intervals for 4 µm were acquired by time-lapse confocal microscopy (see supplementary material Movies 2, 3). The images shown here are still frames from the time-lapse data at the indicated times. Projected xy views are presented. (A) Note that, in control cells, the actin-ring structures observed in depolarized cells are connected to cell-cell-contact regions and expand to form perijunctional actin belts as the cells become polarized. (B) In aPKC-kn-overexpressing cells, the rings are tethered by radial actin fibers but fail to expand and instead show hyper-shrinkage. Scale bars: 10 µm.