Identification of a putative effector protein for rab11 that participates in transferrin recycling

We have identified and cloned the cDNA. for a 912-aa protein, rab11BP, that interacts with the GTP-containing active form of rab11, a GTP-binding prot ein that plays a critical role in receptor recycling. Although rab11BP is p rimarily cytosolic, a significant fraction colocalizes with rab11 in endoso mal membranes of both the sorting and recycling subcompartments. In vitro b inding of rab11 to native rab11BP requires partial denaturation of the latt er to expose an internal binding site located between residues 334 and 504 that is apparently masked by the C-terminal portion of the protein, which i ncludes sis repeats known as WD40 domains. Within the cell, rab11BP must un dergo a conformational change in which the rab11-binding site becomes expos ed, because when coexpressed with rab11 in transfected cells the two protei ns formed abundant complexes in association with membranes. Furthermore, al though overexpression of rab11BP did not affect transferrin recycling, over expression of a truncated form of the protein, rab11BP(1-504), that include s the rab11-binding site but lacks the WD40 domains inhibited recycling as strongly as does a dominant negative rab11 mutant protein that does not bin d GTP. Strikingly, the inhibition caused by the truncated rab11BP was preve nted completely when the cells also expressed a C-terminally deleted, nonpr enylatable form of rab11 that, by itself, has no effect on recycling, We pr opose that rab11BP is an effector for rab11, whose association with this GT P-binding protein is dependent on the action of another membrane-associated factor that promotes the unmasking of the rab11-binding site in rab11BP.