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 (By similarity). V-ATPase is a heteromultimeric enzyme composed of a peripheral catalytic V1 complex (components A to H) attached to an integral membrane V0 proton pore complex (components: a, c, c'', d and e). The proteolipid components c and c'' are present as a hexameric ring that forms the proton-conducting pore (By similarity). Interacts with APD2 (PubMed:22897245). Belongs to the V-ATPase proteolipid subunit family.