Fusion of docked membranes requires the armadillo repeat protein Vac8p

The discovery of molecules required for membrane fusion has revealed a rema rkably conserved mechanism that centers upon the formation of a complex of SNARE proteins. However, whether the SNARE proteins or other components cat alyze the final steps of membrane fusion in vivo remains unclear. Understan ding this last step depends on the identification of molecules that act lat e in the fusion process. Here we demonstrate that in Saccharomyces cerevisi ae, Vac8p, a myristoylated and palmitoylated armadillo repeat protein, is r equired for homotypic vacuole fusion. Vac8p is palmitoylated during the fus ion reaction, and the ability of Vac8p to be palmitoylated appears to be ne cessary for its function in fusion. Both in vivo and in vitro analyses show that Vac8p functions after both Rab-dependent vacuole docking and the form ation of trans-SNARE pairs. We propose that Vac8p may bind the fusion machi nery through its armadillo repeats and that palmitoylation brings this mach inery to a specialized lipid domain that facilitates bilayer mixing.