ORIENTATION OF THE ALPHA-HELICES OF APOCYTOCHROME-C AND DERIVED FRAGMENTS AT MEMBRANE INTERFACES, AS STUDIED BY CIRCULAR-DICHROISM

The orientation of the different helical regions of the mitochondrial precursor protein apocytochrome c has been studied using circular dich roism on isolated fragments of this protein associated with oriented f ilms composed of various phospholipids [de Jongh, H. H. J., Goormaghti gh, E., and Killian, J. A. (1994) Biochemistry (preceding article in t his issue)]. Both the N and C terminus adopt helical structures in a m embrane environment. The middle region can also be helical, but only i n the presence of the N-terminal domain of the protein. In the presenc e of the unsaturated lipids dioleoylphosphatidylcholine and dioleoylph osphatidylglycerol, all three helices are found to have a preferred or ientation perpendicular to the membrane normal, whereas in the presenc e of the saturated lipids dimyristoylphosphatidylcholine and dimyristo ylphosphatidylglycerol, the terminal helices are preferentially orient ed parallel to the membrane normal. In films composed of dioleoylphosp hatidylserine, it is found that the N-terminal helix is oriented prefe rentially perpendicular, whereas the C-terminal helix is aligned more parallel to the membrane normal. The differences in preferred orientat ion between the terminal helices are demonstrated by molecular modelin g of the helices at a water-lipid interface. The results are discussed in light of the translocation of apocytochrome c over the outer mitoc hondrial membrane, an important step in the import process of this pro tein in mitochondria.