Roles of calcium ions in the membrane binding of C2 domains

The C2 domain is a membrane-targeting domain found in many cellular protein s involved in signal transduction or membrane trafficking. The majority of C2 domains co-ordinate multiple Call ions and bind the membrane in a Ca2+-d ependent manner. To understand the mechanisms by which Ca2+ mediates the me mbrane binding of C2 domains, we measured the membrane binding of the C2 do mains of group IV cytosolic phospholipase A(2) (cPLA(2)) and protein kinase C-alpha (PKC-alpha) by surface plasmon resonance and lipid monolayer analy ses. Ca2+ ions mainly slow the membrane dissociation of cPLA(2)-C-2, while modulating both membrane association and dissociation rates for PKC-alpha - C2. Further studies with selected mutants showed that for cPLA2 a Ca2+ ion bound to the C2 domain of cPLA(2) induces the intra-domain conformational c hange that leads to the membrane penetration of the C2 domain whereas the o ther Ca2+ is not directly involved in membrane binding. For PKC-alpha, a Ca 2+ ion induces the inter-domain conformational changes of the protein and t he membrane penetration of non-C2 residues. The other Ca2+ 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 dete rmination of the distinct and specific roles of each Ca2+ ion bound to diff erent C2 domains.