A conformational switch in clathrin light chain regulates lattice structure and endocytosis at the plasma membrane of mammalian cells.

The conformations of endocytic proteins and their interactions are key regulators of clathrin-mediated endocytosis. Three clathrin light chains (CLC), along with three clathrin heavy chains, assemble to form single clathrin triskelia that link into a geometric lattice that curves to drive endocytosis. Conformational changes in CLC have been shown to regulate triskelia assembly in solution, yet the nature of these structural changes, and their effects on lattice growth, curvature, and endocytosis in cells are unclear. Here, we develop a correlative fluorescence resonance energy transfer (FRET) and platinum replica electron microscopy method, named FRET-CLEM. With FRET-CLEM, we measure conformational changes in proteins at thousands of individual morphologically distinct clathrin-coated structures across cell membranes. We find that the N-terminus of CLC moves away from the plasma membrane and triskelia vertex as lattices curve. Preventing this conformational switch with acute chemical tools inside cells increased clathrin structure sizes and inhibited endocytosis. Therefore, a specific conformational switch in CLC regulates lattice curvature and endocytosis in mammalian cells.