PIP5K-α−/− BMM are defective in actin polymerization during ingestion. BMM incubated with IgG-opsonized beads at 4°C for 10 min were warmed to 37°C for 1 min. (A) Quantitation of polymerized actin in situ. (left) Fluorescence images. Green, phalloidin; red, external beads. (middle) Line scans of phalloidin fluorescence intensity spanning the bottom of a nascent phagocytic cup (x) and its contralateral PM (y). Data shown were taken from representative cells. (right) Ratio of phalloidin intensity (x/y; n = 30 cups). Error bars indicate SEM. (B) Red, active WASP; green, phalloidin; blue, external beads. (inset) Total WASP (red) in another cell. (C) p34-Arc (red). Bars, 10 µm.