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Cd2+ as a Ca2+ Surrogate in Protein–Membrane Interactions: Isostructural but Not Isofunctional
journal contributionposted on 2013-09-04, 00:00 authored by Krystal A. Morales, Yuan Yang, Zheng Long, Pingwei Li, Alexander B. Taylor, P. John Hart, Tatyana I. Igumenova
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 Ca2+-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 Cd2+-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.