PP2A, PP4, and PP6 phosphoprotein phosphatases, metallophosphatase domain
PP2A-like family of phosphoprotein phosphatases (PPP's) including PP4 and PP6. PP2A (Protein phosphatase 2A) is a critical regulator of many cellular activities. PP2A comprises about 1% of total cellular proteins. PP2A, together with protein phosphatase 1 (PP1), accounts for more than 90% of all serine/threonine phosphatase activities in most cells and tissues. The PP2A subunit in addition to having a catalytic domain homologous to PP1, has a unique C-terminal tail, containing a motif that is conserved in the catalytic subunits of all PP2A-like phosphatases including PP4 and PP6, and has an important role in PP2A regulation. The PP2A-like family of phosphatases all share a similar heterotrimeric architecture, that includes: a 65kDa scaffolding subunit (A), a 36kDa catalytic subunit (C), and one of 18 regulatory subunits (B). The PPP (phosphoprotein phosphatase) family, to which PP2A belongs, is one of two known protein phosphatase families specific for serine and threonine. The PPP family also includes: PP1, PP2B (calcineurin), PP4, PP5, PP6, PP7, Bsu1, RdgC, PrpE, PrpA/PrpB, and ApA4 hydrolase. The PPP catalytic domain is defined by three conserved motifs (-GDXHG-, -GDXVDRG- and -GNHE-). The PPP enzyme family is ancient with members found in all eukaryotes, and in most bacterial and archeal genomes. Dephosphorylation of phosphoserines and phosphothreonines on target proteins plays a central role in the regulation of many cellular processes. PPPs belong 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: 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.
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