CRYSTAL-STRUCTURE OF A CALCIUM-PHOSPHOLIPID BINDING DOMAIN FROM CYTOSOLIC PHOSPHOLIPASE-A2

Cytosolic phospholipase A2 (cPLA2) is a calcium-sensitive 85-kDa enzym e that hydrolyzes arachidonic acid-containing membrane phospholipids t o initiate the bio synthesis of eicosanoids and platelet-activating fa ctor, potent inflammatory mediators, The calcium-dependent activation of the enzyme is mediated by an N-terminal C2 domain, which is respons ible for calcium-dependent translocation of the enzyme to membranes an d that enables the intact enzyme to hydrolyze membrane-resident substr ates. The 2.4-Angstrom x-ray crystal structure of this C2 domain was s olved by multiple isomorphous replacement and reveals a beta-sandwich with the same topology as the C2 domain from phosphoinositide-specific phospholipase C delta 1, Two clusters of exposed hydrophobic residues surround two adjacent calcium binding sites, This region, along with an adjoining strip of basic residues, appear to constitute the membran e binding motif. The structure provides a striking insight into the re lative importance of hydrophobic and electrostatic components of membr ane binding for cPLA2, Although hydrophobic interactions predominate f or cPLA2, for other C2 domains such as in ''conventional'' protein kin ase C and synaptotagmins, electrostatic forces prevail.