An effector of Ypt6p binds the SNARE Tlg1p and mediates selective fusion of vesicles with late Golgi membranes

Membrane traffic requires vesicles to fuse with a specific target, and SNAR E proteins and Rab/Ypt GTPases contribute to this specificity. In the yeast Saccharomyces cerevisae, the Rab/Ypt GTPase Ypt6p is required for fusion o f endosome-derived vesicles with the late Golgi. We have shown previously t hat activation of Ypt6p depends on its exchange factor, Ric1p-Rgp1p, a peri pheral membrane protein complex restricted to the Golgi. We show here that a conserved trimeric protein complex, VFT (Vps52/53/54), binds directly to Ypt6p:GTP. Localization of VFT to the Golgi requires Ypt6p, but is unaffect ed in gos1 and tlg1mutants, in which late Golgi integral membrane proteins, including SNAREs, are mislocalized. The VFT complex also binds directly to the N-terminal domain of the SNARE Tlg1p, both in vitro and in vivo, in a Ypt6p-independent manner. We suggest that the VFT complex links vesicles co ntaining Tlg1p to their target, which is defined by the local activation of Ypt6p.