We present a model for the packing of clathrin molecules into the characteristic hexagons and pentagons covering coated pits and vesicles. The assembly unit is a symmetrical trimer with three extended legs. Polymerization of these units occurs in seconds under suitable conditions, giving empty polyhedral cages resembling the structures around coated vesicles. Images of small, negatively stained fragments of cages, assembled directly on electron microscope grids, reveal details of the structure, which correlate well with the predicted features of the model. There is one clathrin trimer at each polyhedral vertex, and each leg of the trimer extends along two neighboring polyhedral edges. Quasi-equivalent packing in pentagons and hexagons in polyhedra of different sizes requires a variable joint at the vertex of the molecule and a hinge in each leg. The construction of clathrin coats is remarkable for the extended fibrous contacts that each molecule makes with many others. Such contacts may confer mechanical strength combined with flexibility needed when a vesicle is pinched off from the membrane.

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