A series of elegant studies leading to structures of the AP-1 11, 12– 14 and AP-2 15– 18 core domains have made transformative advances but, despite these, the nature of the interaction of these key complexes with assembled clathrin is not fully understood. AP-1, a homologue of AP-2, engages with clathrin during intracellular trafficking.
The ability of clathrin to form diverse structures is inherently determined by its molecular structure but how, has remained unclear.Ĭlathrin’s role in endocytosis is mediated through engagement with adaptor proteins, most notably the heterotetrameric complex AP-2, which interacts with clathrin through a ‘clathrin box’ motif on an extended linker region within its β2-adaptin subunit and through a binding site on the β2-appendage domain 7, 8, 9, which interacts with clathrin’s ankle domain 10. The multiple shapes adopted by those assemblies observed in cells are also seen with purified clathrin, which forms cages with different architectures 6. Avinoam et al 4 provided evidence that these changes may be enabled by the rapid exchange of clathrin triskelia 5 with the membrane-bound clathrin coat. Clathrin-coated vesicles have been seen to emerge directly from flat clathrin lattices indicating that clathrin assemblies adapt to changes in membrane shape at endocytic sites 3. In the case of clathrin-mediated endocytosis (CME), three-legged clathrin structures called triskelia form a latticed scaffold around the outside of a vesicle derived from the plasma membrane and coordinate binding of a network of adaptor proteins, which together drive cargo selection, vesicle formation and detachment from the membrane. These data reveal a universal mode of clathrin assembly that allows variable cage architecture and adaptation of coated vesicle size and shape during clathrin-mediated vesicular trafficking or endocytosis.Įndocytosis enables material to be absorbed via specific ligand-receptor interactions through the assembly of specialised protein coats around vesicles formed from the plasma membrane 1, 2. Analysis of the protein-protein interfaces shows remarkable conservation of contact sites despite architectural variation. Fitting structural models to three of these maps shows that their assembly requires only a limited range of triskelion leg conformations, yet inherent flexibility is required to maintain contacts. Furthermore, we report cryo-EM maps for five different clathrin cage architectures. We present the cryo-EM structure and molecular model of assembled porcine clathrin, providing new insights into interactions that stabilise key elements of the clathrin lattice, namely, between adjacent heavy chains, at the light chain-heavy chain interface and within the trimerisation domain. Clathrin forms diverse lattice and cage structures that change size and shape rapidly in response to the needs of eukaryotic cells during clathrin-mediated endocytosis and intracellular trafficking.
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