Definition of Ω and its relationship with membrane velocity. (*A*) A schematic diagram illustrating how membrane progresses tangentially around an ED. **T̄** represents the average of tangential angles from θ = 0 to θ = π/2. Ω is the angle between **T̄** and membrane normal at the site of attachment, **N̄**. (*B*) Membrane velocity (distance traveled by the membrane divided by time to internalize, *n* ≥ 3; error bars represent SD) decreases with increasing Ω for a variety of shapes and sizes of particles. Nonopsonized particles are indicated by filled circles, and IgG-opsonized particles are indicated by open squares. Each data point represents a different shape, size, or aspect ratio particle. The internalization velocity is positive for Ω ≤ 45° (*P* < 0.001). Above a critical value of Ω, ≈45°, the internalization velocity is zero (*P* < 0.001) and there is only membrane spreading after particle attachment, not internalization. The arrows above the plot indicate the point of attachment for each shape that corresponds to the value of Ω on the *x* axis. Error in Ω is due to the difference in the actual point of contact in time-lapse microscopy from that used to calculate Ω. Only points of contact within 10° of that used to calculate Ω were selected. All data points at the critical point, Ω = 45°, except UFOs, do not have error associated with Ω because of their symmetry.

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