Quantitative analysis of clathrin-coated structures in SK-MEL-2 cells. (A) Clathrin coat geometry is quantified using LocMoFit. The fitted spherical cap model is drawn as a circle with its outer radius (top row, xy), as cross-sectional profile (50 nm xz slices, middle row), and as surface with the corresponding closing angle θ (bottom row). Scale bar, 100 nm. (B) Distributions of closing angle θ (median = 108°), curvature H (median = 0.011 nm⁻¹), and surface area A (median = 54,000 nm²) of endocytic sites in SK-MEL-2 cells (n = 1,798 sites, N = 13 cells) as determined from the model fit. (C) Two previously proposed mechanistic models for clathrin coat assembly during endocytosis (for details, see text). In both scenarios, θ increases monotonically and is thus a proxy for endocytic progression. (D) Development of curvature, surface area, and projected area of the clathrin coat during endocytosis. Color indicates point density.