Cd2+ as a Ca2+ surrogate in protein-membrane interactions: isostructural but not isofunctional.
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abstract
Due to its favorable spectroscopic properties, Cd(2+) is frequently used as a probe of Ca(2+) sites in proteins. We investigate the ability of Cd(2+) to act as a structural and functional surrogate of Ca(2+) in protein-membrane interactions. C2 domain from protein kinase C (C2) was chosen as a paradigm for the Ca(2+)-dependent phosphatidylserine-binding peripheral membrane domains. We identified the Cd(2+)-binding sites of C2 using NMR spectroscopy, determined the 1.6 crystal structure of Cd(2+)-bound C2, and characterized metal-ion-dependent interactions between C2 and phospholipid membranes using fluorescence spectroscopy and ultracentrifugation experiments. We show that Cd(2+) forms a tight complex with the membrane-binding loops of C2 but is unable to support its membrane-binding function. This is in sharp contrast with Pb(2+), which is almost as effective as Ca(2+) in driving the C2-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.
