Proposed model of different CCP closure types. Clathrin molecules upon attachment to the membrane either start nucleating into a CCP or form a flat sheet. Once the CCP has gone through its maturation process, it can close with (red circles) or without (green circles) a cap. In some cases, Clc-EGFP fluorescence spots can be seen without associated topographically detected pits (white circles) that most likely represent flat sheets of clathrin. Bars, 100 µm in all TEM images.