T-cell immune regulator (TCIRG1) codes for the alpha 3 subunit of the osteoclast vacuolar proton pump (VPP). This protein is an integral component of the vacuolar proton-translocating adenosine triphosphatase (V-ATPase) complex which mediates the acidification of the extracellular compartment between the ruffled border membrane of the osteoclast and the bone surface, where the low pH is necessary both for dissolving inorganic bone material and for the optimal function of proteases which degrade the organic bone matrix.