Amphiphilicity determines binding properties of three mitochondrial presequences to lipid surfaces

The interactions of three peptides, which correspond to presequences that direct mitochondrial protein import, with model membrane systems were characterized using NMR, fluorescence, and circular dichroism spectroscopies. The positively charged peptides adopted an ordered secondary structure only when the negatively charged phospholipid, cardiolipin, was present in small unilamellar vesicles. Conversely, the peptides adopted an ordered secondary structure in the presence of micelles formed from both formally neutral and negatively charged detergents. The peptides had the same relative affinity for micelles and small unilamellar vesicles containing 20% cardiolipin. Amide proton exchange rates showed that the region of the helical structure which had the greatest hydrophobic moment interacted most readily with micelles. Therefore, it appears that a major determinant of binding to lipid surfaces is the ability of the peptide to attain the correct orientation of hydrophobic and hydrophilic groups. For the three peptides studied, affinity also correlated with the length of the helix, but not with hydrophobic surface area. In each case, the interacting segment of the peptide was toward the C-terminal end of the helix. Previous work has allowed us to postulate that the N-terminus of the presequence is vital for import [Wang, Y., & Weiner, H. (1993) J. Biol. Chem. 268, 4759-4765] and the C-terminal end is essential for membrane interaction [Karslake, C., Piotto, M., Pak, Y. K., Weiner, H., & Gorenstein, D. G. (1990) Biochemistry 29, 9872-9878]. On the basis of the data that are now available, it appears that the interaction with membrane surfaces may depend on the location of an amphiphilic region of the sequence that is near but not necessarily at the C-terminus.