acid sphingomyelinase and related proteins, metallophosphatase domain. Acid sphingomyelinase (ASMase) is a ubiquitously expressed phosphodiesterase which hydrolyzes sphingomyelin in acid pH conditions to form ceramide, a bioactive second messenger, as part of the sphingomyelin signaling pathway. ASMase is localized at the noncytosolic leaflet of biomembranes (for example the luminal leaflet of endosomes, lysosomes and phagosomes, and the extracellular leaflet of plasma membranes). ASMase-deficient humans develop Niemann-Pick disease. This disease is characterized by lysosomal storage of sphingomyelin in all tissues. Although ASMase-deficient mice are resistant to stress-induced apoptosis, they have greater susceptibility to bacterial infection. The latter correlates with defective phagolysosomal fusion and antibacterial killing activity in ASMase-deficient macrophages. ASMase belongs to the metallophosphatase (MPP) superfamily. MPPs are functionally diverse, but all share a conserved domain with an active site consisting of two metal ions (usually manganese, iron, or zinc) coordinated with octahedral geometry by a cage of histidine, aspartate, and asparagine residues. The MPP superfamily includes: the phosphoprotein phosphatases (PPPs), Mre11/SbcD-like exonucleases, Dbr1-like RNA lariat debranching enzymes, YfcE-like phosphodiesterases, purple acid phosphatases (PAPs), YbbF-like UDP-2,3-diacylglucosamine hydrolases, and acid sphingomyelinases (ASMases). The conserved domain is a double beta-sheet sandwich with a di-metal active site made up of residues located at the C-terminal side of the sheets. This domain is thought to allow for productive metal coordination.
