V-1-situated stalk subunits of the yeast vacuolar proton-translocating ATPase

The proton-translocating ATPase of the yeast vacuole is an enzyme complex consisting of a large peripheral membrane sector (V-1) and an integral membrane sector (V-0), each composed of multiple subunits. The V-1 sector contains subunits that hydrolyze ATP, whereas the V-0 sector contains subunits that translocate protons across the membrane. Additional subunits in both sectors couple these activities. Here we have continued our examination of intermediate subunits primarily associated with the V-1 but also implicated in interactions with the V-0. Interactions between Vma7p (F) and Vma8p (D) and between Vma4p (E) and Vma10p (G) are described. Although Vma7p and Vma10p have been observed to interact with the V-0 sector, our results indicate that these subunits behave primarily as canonical V-1 sector subunits. We categorize these four subunits as ''stalk'' subunits to distinguish them from the known catalytic (A and B) and proton-translocating (c, c', and Vma16p) subunits and to highlight their intermediate nature. Furthermore, we show that the in vivo stability of Vma4p is dependent upon interaction with Vma10p. This may be important in the regulation of assembly, since these two subunits add to the V-1 during later stages of V-1 assembly. This is the first demonstration of interdependence between ATPase subunits for structural stability.