Structural basis for Rab11-dependent membrane recruitment of a family of Rab11-interacting protein 3 (FIP3)/Arfophilin-1.

Family of Rab11-interacting protein (FIP)3/Arfophlin-1 and FIP4/Arfophilin-2 are dual effectors for Rab11 and ADP ribosylation factor (ARF)5/ARF6, which are involved in membrane delivery from recycling endosomes to the plasma membrane during cytokinesis. Here, we define the distinct C-terminal binding regions of FIP3 and FIP4 for Rab11 and ARF5/ARF6. Furthermore, we determined the crystal structure of Rab11 in complex with the Rab11-binding domain (RBD) of FIP3. The long amphiphilic alpha-helix of FIP3-RBD forms a parallel coiled-coil homodimer, with two symmetric interfaces with two Rab11 molecules. The hydrophobic side of the RBD helix is involved in homodimerization and mediates the interaction with the Rab11 switch 1 region, whereas the opposite hydrophilic side interacts with the Rab11 switch 2 and is the major factor contributing to the binding specificity. The bivalent interaction of FIP3 with Rab11 at the C terminus allows FIP3 to coordinately function with other binding partners, including ARFs.