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With its intricate meshwork of protein braids and alluring symmetry, clathrin is sure to seize your attention. It was named in the 1960s for its clathrate (lattice of bars) appearance in electron micrographs, and to this day, this beautiful molecule invokes intensive study. Like many proteins, clathrin represents a perfect case of form following function; it performs critical roles in shaping rounded vesicles for intracellular trafficking.
Clathrin cages are composed of symmetrical three-legged components called triskelions. The structure shown here, PDB entry 1xi4, is built of 36 triskelia, one of which is highlighted in green. When triskelia snap together in solution, they can interact with enough flexibility to form either 6-sided rings that yield a flatter surface, or 5-sided rings with higher curvature. In a cell, a triskelion floating in the cytoplasm binds to an adaptor protein, linking one of its three feet to the membrane at a time. This triskelion will bind to other membrane-attached triskelia to form a rounded lattice of hexagons and pentagons, reminiscent of the panels on a soccer ball, that pulls the membrane into a bud. By constructing different combinations of 5-sided and 6-sided rings, vesicles of different sizes may assemble. The structure shown here represents the second smallest possible cage structure, which is actually too small to contain a functional vesicle. It was created in the laboratory by reconstituting triskelions without a lipid vesicle. The smallest clathrin cage commonly photographed, called a mini-coat, has 12 pentagons and only two hexagons. Even smaller cages with zero hexagons probably don’t form from the native protein, because the feet of the triskelia are too bulky.
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