Crystal structure of human GGA1 GAT domain complexed with the GAT-binding domain of Rabaptin5

GGA proteins coordinate the intracellular trafficking of clathrin-coated vesicles through their interaction with several other proteins. The GAT domain of GGA proteins interacts with ARF, ubiquitin, and Rabaptin5. The GGA Rabaptin5 interaction is believed to function in the fusion of trans-Golgi-derived vesicles to endosomes. We determined the crystal structure of a human GGA1 GAT domain fragment in complex with the Rabaptin5 GAT-binding domain. In this structure, the Rabaptin5 domain is a 90-residue-long helix. At the N-terminal end, it forms a parallel coiled-coil homodimer, which binds one GAT domain of GGA1. In the C-terminal region, it further assembles into a four-helix bundle tetramer. The Rabaptin5-binding motif of the GGA1 GAT domain consists of a three-helix bundle. Thus, the binding between Rabaptin5 and GGA1 GAT domain is based on a helix bundle-helix bundle interaction. The current structural observation is consistent with previously reported mutagenesis data, and its biological relevance is further confirmed by new mutagenesis studies and affinity analysis. The four-helix bundle structure of Rabaptin5 suggests a functional role in tethering organelles.