Trans-complex formation by proteolipid channels in the terminal phase of membrane fusion

SNAREs (soluble N-ethylmaleimide-sensitive factor attachment protein recept ors) and Rab-GTPases, together with their cofactors, mediate the attachment step in the membrane fusion of vesicles. But how bilayer mixing-the subseq uent core process of fusion-is catalysed remains unclear. Ca2+/calmodulin c ontrols this terminal process in many intracellular fusion events. Here we identify V0, the membrane-integral sector of the vacuolar H+-ATPase, as a t arget of calmodulin on yeast vacuoles. Between docking and bilayer fusion, V0 sectors from opposing membranes form complexes. V0 trans-complex formati on occurs downstream from trans-SNARE pairing, and depends on both the Rab- GTPase Ypt7 and calmodulin. The maintenance of existing complexes and compl etion of fusion are independent of trans-SNARE pairs. Reconstituted proteol ipids form sealed channels, which can expand to form aqueous pores in a Ca2 +/calmodulin-dependent fashion. V0 trans-complexes may therefore form a con tinuous, proteolipid-lined channel at the fusion site. We propose that radi al expansion of such a protein pore may be a mechanism for intracellular me mbrane fusion.