Assembly and Regulation of the Yeast Vacuolar H+-ATPase

The yeast vacuolar H(+)-ATPase (V-ATPase) consists of a complex of peripheral subunits containing the ATP binding sites, termed the V(1) sector, attached to a complex of membrane subunits containing the proton pore, termed the V(o) sector. Interaction between the V(1) and V(o) sectors is essential for ATP-driven proton transport, and this interaction is manipulated in vivo as a means of regulating V-ATPase activity. When yeast (Saccharomyces cerevisiae) cells are deprived of glucose for as little as 5 min, up to 75% of the assembled V-ATPase complexes are disassembled into cytoplasmic V(1) sectors and membrane-bound V(o) sectors. Remarkably, this disassembly is completely reversible. Restoration of glucose to the growth medium results in quantitative reassembly of the disassembled complexes in as little as 5 min, even in the absence of any new protein synthesis. Cells also appear to regulate the extent of V(1)V(o) assembly on a long-term basis. Yeast cells grown for extended periods in a poor carbon source contain a high proportion of free V(1) and V(o) sectors, and these sectors remain poised for reassembly when growth conditions improve. Parallel experiments on the Manduca sexta V-ATPase suggest that reversible disassembly may be a general regulatory mechanism for V-ATPases. These results imply that V-ATPases are surprisingly dynamic structures, and their unique ‘regulated instability’ raises a number of interesting physiological and structural questions. How are extracellular conditions such as carbon source communicated to V-ATPase complexes present on intracellular membranes? How are such major structural changes in the V-ATPase generated and how are V(1) sectors ‘silenced’ in vivo to prevent unproductive hydrolysis of cytoplasmic ATP by the dissociated enzyme? We are addressing these questions using a combination of genetic and biochemical approaches.