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These results are in agreement with the notion that TGN-derived vesicles are acidic and that the V-ATPase plays an active role in this process. Vesicle acidification induces the recruitment of coat proteins and small GTPases of the ADP-ribosylation factor (ARF) family, a process that is important for the appropriate targeting and recycling of various membrane proteins, including GLUT4 and AQP2. It was particularly surprising to find subunit dl in the apical membrane of principal cells. So far, no other V-ATPase subunits have been described in the apical membrane of principal cells, consistent with the absence of bafilomycin-sensitive proton secretion that has been previously reported in these cells. In yeast, subunit d remains tightly associated with the V0 domain following dissociation of the V1 domain. Because this subunit has no predicted transmembrane domain, its presence in the apical membrane of principal cells, where no other subunits of the V0 domain have been described, is puzzling. Further studies will be required to determine whether subunit d1 may associate with other integral membrane proteins and may participate in a function other than the proton transport activity of the V-ATPase.