S. purpuratus caveolin displays various curvatures in 2D classes and 3DVA. (A–C) S. purpuratus caveolin 2D class averages that show a flat (A), concave (B), and convex (C) curvature of the complex. Scale bar, 100 Å. (D) Difference in curvature is highlighted with overlaid traces of the membrane-facing surface from 2D class averages. (E) Structures representing the negative and positive values along the reaction coordinate of a 3DVA component calculated with a resolution limit of 8 Å. The complex on the left (orange) shows a concave membrane surface curvature, while the complex on the right (light blue) shows a flat membrane surface curvature and “lifting” of the β-barrel above the rim of the complex. The proposed membrane-facing surface is shown (top), rotated 90° around the x axis to show a side view (middle), and rotated an additional 45° to show a view of the predicted cytoplasmic-facing surface (bottom).