Roles of calcium ions in the membrane binding of C2 domains

The C2 domain is a membrane-targeting domain found in many cellular proteins involved in signal transduction or membrane trafficking. The majority of C2 domains co-ordinate multiple Ca(2+) ions and bind the membrane in a Ca(2+)-dependent manner. To understand the mechanisms by which Ca(2+) mediates the membrane binding of C2 domains, we measured the membrane binding of the C2 domains of group IV cytosolic phospholipase A(2) (cPLA(2)) and protein kinase C-alpha (PKC-alpha) by surface plasmon resonance and lipid monolayer analyses. Ca(2+) ions mainly slow the membrane dissociation of cPLA(2)-C2, while modulating both membrane association and dissociation rates for PKC-alpha-C2. Further studies with selected mutants showed that for cPLA(2) a Ca(2+) ion bound to the C2 domain of cPLA(2) induces the intra-domain conformational change that leads to the membrane penetration of the C2 domain whereas the other Ca(2+) is not directly involved in membrane binding. For PKC-alpha, a Ca(2+) ion induces the inter-domain conformational changes of the protein and the membrane penetration of non-C2 residues. The other Ca(2+) ion of PKC-alpha-C2 is involved in more complex interactions with the membrane, including both non-specific and specific electrostatic interactions. Together, these studies of isolated C2 domains and their parent proteins allow for the determination of the distinct and specific roles of each Ca(2+) ion bound to different C2 domains.