Clathrin Adaptor GGA1 Polymerizes Clathrin into Tubules

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GGAs, a class of monomeric clathrin adaptors, are involved in the sorting of cargo at the trans-Golgi network of eukaryotic cells. They are modular structures consisting of the VHS, the GAT, hinge, and GAE domains, which have been shown to interact directly with cargo, ARF, clathrin, and accessory proteins, respectively. Previous studies have shown that GGAs interact with clathrin both in solution and in the cell, but it has yet been shown whether they assemble clathrin. We find that GGA1 promoted assembly of clathrin with complete assembly achieved when one GGA1 molecule is bound per heavy chain. In the presence of excess GGA1, we obtained the unusual stoichiometry of five GGA1s per heavy chain, and even at this stoichiometry the binding was not saturated. The assembled structures were mostly baskets, but approximately 10% of the structures were tubular with an average length of 180 +/- 40 nm and width of approximately 50 nm. The truncated GGA1 fragment consisting of the hinge+GAE domains bound to clathrin with similar affinity as the full-length molecule and polymerized clathrin into baskets. Unlike the full-length molecule, this fragment saturated the lattices at one molecule per heavy chain and assembled clathrin only into baskets. The separated hinge and GAE domains bound much weaker to clathrin than the intact molecule, and these domains do not significantly polymerize clathrin into baskets. We conclude that clathrin adaptor GGA1 is a clathrin assembly protein, but it is unique in its ability to polymerize clathrin into tubules.