Proton-conducting pore forming subunit of the membrane integral V0 complex of vacuolar ATPase. V-ATPase is responsible for acidifying a variety of intracellular compartments in eukaryotic cells. Necessary for the crassulacean acid metabolism. V-ATPase is a heteromultimeric enzyme composed of a peripheral catalytic V1 complex (main components: subunits A, B, C, D, E, and F) attached to an integral membrane V0 proton pore complex (main component: the proteolipid protein; which is present as a hexamer that forms the proton-conducting pore). Tonoplast. Higher expression in leaves, followed by roots and weakly in flowers. Expression in leaves is light-dependent. Belongs to the V-ATPase proteolipid subunit family.