Cd²⁺ as a Ca²⁺ surrogate in protein-membrane interactions: Isostructural but not isofunctional

Due to its favorable spectroscopic properties, Cd²⁺ is frequently used as a probe of Ca²⁺ sites in proteins. We investigate the ability of Cd²⁺ to act as a structural and functional surrogate of Ca²⁺ in protein-membrane interactions. C2 domain from protein kinase Cα (C2α) was chosen as a paradigm for the Ca ²⁺-dependent phosphatidylserine-binding peripheral membrane domains. We identified the Cd²⁺-binding sites of C2α using NMR spectroscopy, determined the 1.6 Å crystal structure of Cd ²⁺-bound C2α, and characterized metal-ion-dependent interactions between C2α and phospholipid membranes using fluorescence spectroscopy and ultracentrifugation experiments. We show that Cd²⁺ forms a tight complex with the membrane-binding loops of C2α but is unable to support its membrane-binding function. This is in sharp contrast with Pb²⁺, which is almost as effective as Ca²⁺ in driving the C2α-membrane association process. Our results provide the first direct evidence for the specific role of divalent metal ions in mediating protein-membrane interactions, have important implications for metal substitution studies in proteins, and illustrate the potential diversity of functional responses caused by toxic metal ions.