Influence of lipid on the structure and phosphorylation of protein kinase C alpha substrate peptides.

The structure and phosphorylation of two protein kinase C (PKC) alpha substrate peptides were investigated in varying lipid systems using enzyme activity assays and circular dichroism (CD) spectroscopy. The alpha-peptide, which exhibits the typical PKC alpha substrate motif and is based on the pseudosubstrate region of PKCalpha, was phosphorylated to a similar extent in bovine brain phosphatidylserine vesicles or diheptanoylphosphatidylcholine (PC7) micelles (both with 5 mol % 1,2-dioleoyl-sn-glycerol), whereas neuromodulin (NM)-peptide, which does not exhibit this motif by virtue of its primary structure, was phosphorylated to a much lesser extent in the PC7 micellar system. CD spectra of the peptides indicated that NM-peptide underwent a dramatic structural change in the presence of dimyristoylphosphatidylserine (DMPS) vesicles, whereas spectra acquired in PC7 micelles were similar to those acquired in buffer alone. No significant structural change was observed in the alpha-peptide in the presence of either lipid. PKC activity assays conducted with a series of NM-peptides successively substituted with nitroxide spin labels at each residue position suggested that several residues distal to the phosphorylation site are necessary for substrate recognition. The effect of these substitutions is not consistent with the binding of the NM-peptide to PKC in an extended structure, but is consistent with the binding of this peptide in a helical conformation. Furthermore, the docking of a helical NM-peptide to the substrate binding site of PKC suggests that the interaction is energetically feasible. These results suggest that PKC may recognize some non-linear substrate motifs and that lipid binding may convert a protein into a better PKC substrate.

Defining protein-protein interactions using site-directed spin-labeling: the binding of protein kinase C substrates to calmodulin.

EPR spectroscopy was used to examine protein-protein interactions between calmodulin and spin-labeled peptides based on the protein kinase C substrate domains of the myristoylated alanine rich C kinase substrate (MARCKS) and neuromodulin. When bound to calmodulin, the C- and N-terminal ends of a 25 residue MARCKS derived peptide exhibited large amplitude motion on the nanosecond time scale and were accessible to paramagnetic agents in aqueous solution. However, residues 5-23 were highly protected and in contact with side chains from calmodulin. These data are consistent with an alpha-helical configuration for this segment of MARCKS and with structures that have been obtained for other calmodulin-substrate complexes. For the 17 residue neuromodulin derived peptide, which is Ca2+ independent in its binding to calmodulin, oxygen collision rates demonstrate that one helical face of this peptide interacts strongly with calmodulin. The data are consistent with an interaction of this face specifically with the C-terminal lobe of calmodulin, where this lobe is either in an "open" or "semiopen" configuration. The EPR data also indicate that the N-terminal lobe of calmodulin is in contact with the peptide, but that this lobe is not as strongly associated with the peptide target. Overall, the binding pocket for neuromodulin appears to be less compact and more dynamic than that formed by MARCKS. This behavior has not previously been seen for calmodulin substrates, and it may play a role in the Ca2+ independent binding of this class of substrates. This work demonstrates the utility of EPR spectroscopy to define protein-protein interactions; in addition, oxygen collision frequencies obtained at buried sites appear to provide information on the conformational dynamics of proteins.

Solution and membrane bound structure of a peptide derived from the protein kinase C substrate domain of neuromodulin.

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.