Molecular characterization of Rab11 interactions with members of the family of Rab11-interacting proteins

The Rab11 subfamily of GTPases plays an important role in vesicle trafficking from endosomes to the plasma membrane. At least six Rab11 effectors ( family of Rab11-interacting proteins (FIPs)) have been shown to interact with Rab11 and are hypothesized to regulate various membrane trafficking pathways such as transferrin recycling, cytokinesis, and epidermal growth factor trafficking. In this study, we characterized interactions of FIPs with the Rab11 GTPase using isothermal titration calorimetric studies and mutational analysis. Our data suggest that FIPs cannot differentiate between GTP-bound Rab11a and Rab11b in vitro ( 50 - 100 nM affinity) and in vivo. We also show that, although FIPs interact with the GDP-bound form of Rab11 in vitro, the binding affinity (> 1000 nM) is not sufficient for FIP and GDP-bound Rab11 interactions to occur in vivo. Mutational analysis revealed that both the conserved hydrophobic patch and Tyr(628) are important for the GTP-dependent binding of Rab11 to FIPs. The entropy and enthalpy analyses suggest that binding to Rab11a/b may induce conformational changes in FIPs.