The vacuolar H(+)-ATPase (V-ATPase) along with ion channels and transporters 
maintains vacuolar pH. V-ATPase ATP hydrolysis is coupled with proton transport 
and establishes an electrochemical gradient between the cytosol and vacuolar 
lumen for coupled transport of metabolites. Btn1p, the yeast homolog to human 
CLN3 that is defective in Batten disease, localizes to the vacuole. We 
previously reported that Btn1p is required for vacuolar pH maintenance and 
ATP-dependent vacuolar arginine transport. We report that extracellular pH 
alters both V-ATPase activity and proton transport into the vacuole of wild-type 
Saccharomyces cerevisiae. V-ATPase activity is modulated through the assembly 
and disassembly of the V(0) and V(1) V-ATPase subunits located in the vacuolar 
membrane and on the cytosolic side of the vacuolar membrane, respectively. 
V-ATPase assembly is increased in yeast cells grown in high extracellular pH. In 
addition, at elevated extracellular pH, S. cerevisiae lacking BTN1 (btn1-Delta), 
have decreased V-ATPase activity while proton transport into the vacuole remains 
similar to that for wild type. Thus, coupling of V-ATPase activity and proton 
transport in btn1-Delta is altered. We show that down-regulation of V-ATPase 
activity compensates the vacuolar pH imbalance for btn1-Delta at early growth 
phases. We therefore propose that Btn1p is required for tight regulation of 
vacuolar pH to maintain the vacuolar luminal content and optimal activity of 
this organelle and that disruption in Btn1p function leads to a modulation of 
V-ATPase activity to maintain cellular pH homeostasis and vacuolar luminal 
content.