Clathrin-coated pits mature by continuous bending and remodeling of the coat
Clathrin mediated endocytosis (CME) is a fundamental eukaryotic process playing essential roles in nutrient uptake, membrane recycling, synaptic transmission and viral infection. Clathrin coated pits were among the first cellular structures described by electron microscopy over 5 decades ago, and have been intensively studied ever since. Nevertheless, researchers remain divided between two contradictory models for how clathrin coated vesicles are formed. Clathrin coated vesicles (CCVs) may form either through bending of a pre-assembled flat coat, or through coat assembly around the emerging vesicle directly. To elucidate the mechanism of CCV formation, I have applied two imaging approaches – high precision correlative light and electron tomography, and targeted fluorescence recovery after photobleaching (FRAP). I found that clathrin assembles into a defined flat lattice early in endocytosis, predetermining the size of the vesicle, and then rearranges through dynamic exchange with the cytosolic pool to wrap around the forming vesicle. This finding resolves the long-standing conflict about the mechanism of CME, leading perhaps to a surprising understanding of endocytosis where clathrin’s role in defining the endocytic site is independent of its characteristic assembly into curved polygonal shells, and is mediated by dynamic instability rather than stable polymerization. Furthermore, this study provides the first detailed quantitative description of membrane shape changes during endocytosis, data which will be essential for biophysical modeling.