Figure 3

Microtubules regulate formation of contractile F-actin rings in calcium-depleted epithelial cells. (A) In confluent SK-CO-15 cells cultured in the normal-calcium medium, apical actin is organized in a perijunctional F-actin belt at the AJC level. In vehicle treated cells, calcium depletion for 1 h resulted in formation of apical contractile F-actin rings, whereas depolymerization of microtubules with nocodazole (30 μM) prevents assembly of F-actin rings. Bar, 10 μm. (B) In SK-CO-15 cells cultured at normal-calcium conditions, apical microtubules (red color) are organized in a loose horizontal meshwork, which does not significantly overlap with F-actin (green color). Calcium depletion results in reorganization of microtubules into dense plaques positioned inside contractile F-actin rings where microtubules partially colocalize with actin microfilaments (arrows). Bar, 5 μm.