The Top Loops of the C₂ Domains from Synaptotagmin and Phospholipase A₂ Control Functional Specificity

The phospholipid-binding specificities of C₂ domains, widely distributed Ca²⁺-binding modules, differ greatly despite similar three-dimensional structures. To understand the molecular basis for this specificity, we have examined the synaptotagmin 1 C₂A domain, which interacts in a primarily electrostatic, Ca²⁺-dependent reaction with negatively charged phospholipids, and the cytosolic phospholipase A₂ (cPLA₂) C₂ domain, which interacts by a primarily hydrophobic Ca²⁺-dependent mechanism with neutral phospholipids. We show that grafting the short Ca²⁺-binding loops from the tip of the cPLA₂ C₂ domain onto the top of the synaptotagmin 1 I C₂A domain confers onto the synaptotagmin 1 C₂A domain the phospholipid binding specificity of the cPLA₂ C₂ domain, indicating that the functional specificity of C₂ domains is determined by their short top loops.