Ca2+ bridges the C2 membrane-binding domain of protein kinase Cα directly to phosphatidylserine

The CZ domain acts as a membrane-targeting module in a diverse group of proteins including classical protein kinase Cs (PKCs), where it plays an essential role in activation via calcium-dependent interactions with phosphatidylserine. The three-dimensional structures of the Ca2+-bound forms of the PKC alpha-C2 domain both in the absence and presence of 1,2-dicaproyl-sn-phosphatidyl-L-serine have now been determined by X-ray crystallography at 2.4 and 2.6 Angstrom resolution, respectively. In the structure of the C2 ternary complex, the glycerophosphoserine moiety of the phospholipid adopts a quasi-cyclic conformation, with the phosphoryl group directly coordinated to one of the Ca2+ ions. Specific recognition of the phosphatidylserine is reinforced by additional hydrogen bonds and hydrophobic interactions with protein residues in the vicinity of the Ca2+ binding region. The central feature of the PKC alpha-C2 domain structure is an eight-stranded, antiparallel beta-barrel with a molecular topology and organization of the Ca2+ binding region closely related to that found in PKC beta-C2, although only two Ca2+ ions have been located bound to the PKC alpha-C2 domain. The structural information provided by these results suggests a membrane binding mechanism of the PKC alpha-C2 domain in which calcium ions directly mediate the phosphatidylserine recognition while the calcium binding region 3 might penetrate into the phospholipid bilayer.