a phosphate monoester + H2O = an alcohol + phosphate The soluble form arises by proteolytic processing of the membrane-bound form. The membrane-bound form is converted to the soluble form by sequential proteolytic processing. First, the C-terminal cytoplasmic tail is removed. Cleavage by a lysosomal protease releases the soluble form in the lysosome lumen (By similarity). Belongs to the histidine acid phosphatase family.