CLCb phosphorylation regulates CCP invagination. (a and b) HeLa cells expressing either CLCb^WT-GFP (a) or CLCb^SallA-GFP (b) were prepared for standard resin-embedded thin-section transmission EM, and individual CCPs were traced by hand using Illustrator and binned into shallow, U-shaped, or constricted. Bar, 200 nm. n = 67–88. (c and d) Survey view of the cytoplasmic surface of the plasma membrane in unroofed HeLa cells expressing either CLCb^WT-GFP or CLCb^SallA-GFP. Clathrin lattices are pseudocolored in purple. Insets corresponding with boxed regions in c and d show high-magnification views. Bars: 1 µm (main images); 200 nm (insets). (e) Percent membrane area covered by flat lattices. n = 35–41. ***, P < 0.001. (f) Area of all individual flat lattices from 35–41 different membranes. n = 278–461. (g) Circularity of all individual flat lattices as defined by Circularity = $4π×( Area Perimeter 2 )$⁠, with a value of 1.0 indicating a perfect circle. n = 278–461. ****, P < 0.0001. Values represent mean ± SD.