Pb2+ as modulator of protein-membrane interactions.
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Lead is a potent environmental toxin that mimics the effects of divalent metal ions, such as zinc and calcium, in the context of specific molecular targets and signaling processes. The molecular mechanism of lead toxicity remains poorly understood. The objective of this work was to characterize the effect of Pb(2+) on the structure and membrane-binding properties of C2. C2 is a peripheral membrane-binding domain of Protein Kinase C (PKC), which is a well-documented molecular target of lead. Using NMR and isothermal titration calorimetry (ITC) techniques, we established that C2 binds Pb(2+) with higher affinity than its natural cofactor, Ca(2+). To gain insight into the coordination geometry of protein-bound Pb(2+), we determined the crystal structures of apo and Pb(2+)-bound C2 at 1.9 and 1.5 resolution, respectively. A comparison of these structures revealed that the metal-binding site is not preorganized and that rotation of the oxygen-donating side chains is required for the metal coordination to occur. Remarkably, we found that holodirected and hemidirected coordination geometries for the two Pb(2+) ions coexist within a single protein molecule. Using protein-to-membrane Frster resonance energy transfer (FRET) spectroscopy, we demonstrated that Pb(2+) displaces Ca(2+) from C2 in the presence of lipid membranes through the high-affinity interaction with the membrane-unbound C2. In addition, Pb(2+) associates with phosphatidylserine-containing membranes and thereby competes with C2 for the membrane-binding sites. This process can contribute to the inhibitory effect of Pb(2+) on the PKC activity.
