Solution and Membrane Bound Structure of a Peptide Derived from the Protein Kinase C Substrate Domain of Neuromodulin

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Abstract
The solution, micelle, and membrane bound structure of a peptide based on the protein kinase C and calmodulin binding domain of neuromodulin was studied using a combination of NMR, EPR, and circular dichroism. NMR spectroscopy on this peptide indicates that there is little secondary structure in aqueous solution or detergent micelles, but that the peptide is helical in methanol. This finding is in agreement with EPR experiments utilizing double spin-labeled derivatives of the peptide as well as circular dichroism. The membrane bound structure of this peptide was investigated with EPR by synthesizing a series of spin-labeled peptides based on the protein kinase C and calmodulin binding domain of neuromodulin. These peptides exhibit no binding to neutral membranes containing phosphatidylcholine, but associate strongly with membranes containing negatively charged lipids such as phosphatidylserine. The depth of penetration of the spin label was estimated using continuous wave power-saturation EPR and demonstrates that labels at the ends of the peptide are localized slightly outside the membrane interface, but that spin labels in the central portion of the sequence are near or within the membrane interface. In addition, the peptide is in an extended structure when bound to membranes containing acidic lipid with its more hydrophobic side chains interacting with the membrane interior. The results demonstrate that the binding of these peptides to membranes is not driven by purely electrostatic interactions, but includes the interaction of hydrophobic side chains with the membrane interior.