The top loops of the C(2) domains from synaptotagmin and phospholipase A(2) control functional specificity.

The phospholipid-binding specificities of C(2) domains, widely distributed Ca(2+)-binding modules, differ greatly despite similar three-dimensional structures. To understand the molecular basis for this specificity, we have examined the synaptotagmin 1 C(2)A domain, which interacts in a primarily electrostatic, Ca(2+)-dependent reaction with negatively charged phospholipids, and the cytosolic phospholipase A(2) (cPLA(2)) C(2) domain, which interacts by a primarily hydrophobic Ca(2+)-dependent mechanism with neutral phospholipids. We show that grafting the short Ca(2+)-binding loops from the tip of the cPLA(2) C(2) domain onto the top of the synaptotagmin 1 C(2)A domain confers onto the synaptotagmin 1 C(2)A domain the phospholipid binding specificity of the cPLA(2) C(2) domain, indicating that the functional specificity of C(2) domains is determined by their short top loops.