Rab6-interacting Protein 1 Links Rab6 and Rab11 Function

Rab11 and Rab6 guanosine triphosphatases are associ-ated with membranes of the recycling endosomes (REs)and Golgi complex, respectively. Evidence indicates thatthey sequentially regulate a retrograde transport path-way between these two compartments, suggesting theexistence of proteins that must co-ordinate their func-tions. Here, we report the characterization of two iso-forms of a protein, Rab6-interacting protein 1 (R6IP1),originally identified as a Rab6-binding protein. R6IP1 alsobinds to Rab11A in its GTP-bound conformation. Ininterphase cells, R6IP1 is targeted to the Golgi inaRab6-dependentmannerbutcanassociatewithRab11-positive compartments when the level of Rab11Ais increased within the cells. Fluorescence resonanceenergytransferanalysisusingfluorescencelifetimeimag-ing shows that the overexpression of R6IP1 promotes aninteraction between Rab11A and Rab6 in living cells.Accordingly, the REs marked by Rab11 and transferrinreceptor are depleted from the cell periphery and accu-mulate in the pericentriolar area. However, endosomaland Golgi membranes do not appear to fuse with eachother. We also show that R6IP1 function is requiredduring metaphase and cytokinesis, two mitotic steps inwhich a role of Rab6 and Rab11 has been previouslydocumented. We propose that R6IP1 may couple Rab6and Rab11 function throughout the cell cycle.Key words: cytokinesis,FRET/FLIM,Golgicomplex,intracel-lular transport, mitosis, Rab GTPases, recycling endosomesReceived 16 February 2007, revised and accepted for pub-lication 11 June 2007, uncorrected manuscript publishedonline 15 June 2007, published online 24 August 2007