Cell surface projections emerge from the cleavage furrow in CYK4 GAP-defective cells. (A) HeLa cells expressing EGFP-Rac1 and inducible mCherry-CYK4 or GAP activity–defective CYK4^R385A were transfected with control or CYK4 3′-UTR siRNA duplexes. CYK4 expression was induced for 60 h, and then, the cells were TCA fixed and stained with antibodies to mCherry to detect CYK4, RhoA, or EGFP to detect Rac1. Arrows mark the cell surface projections seen emerging from the cleavage furrow region in CYK4^R385A cells. (B and C) Alternatively, these CYK4 (B) or CYK4^R385A (C) cells were used for time-lapse imaging. A bright-field (BF) image is shown at the time points indicated together with the bottom section where the cells touch the glass growth surface, a middle section through the cell equator, a maximum intensity projection along the z axis of all sections, or a side projection along the y axis. The dotted line indicates where abscission has occurred. The 4-min time point, bottom section, includes a quantitation of the area of the cell in contact with the surface (n = 20). Arrows mark Rac1 accumulation at the cell poles and associated retraction fibers. WT, wild type. Bars, 10 µm.