Structural and functional properties of the membrane binding segment of cytochrome b5.

Derivatives of cytochrome b5 that had been selectively shortened at the COOH-terminal, membrane binding segment of this amphipathic protein were employed to examine the minimum structural requirements for binding to phospholipid vesicles and for catalytic interactions in the stearyl-CoA desaturase system. Three derivatives shortened by 6, 18, and 27 amino acid residues were produced by controlled proteolysis with carboxypeptidases. The two largest derivatives bound to synthetic lipid vesicles and interacted with cytochrome b5. The third derivative neither bound to vesicles nor reacted with the desaturase. Whole nonpolar peptide and the nonpllar peptides of the two largest derivates contain only 29 to 34% polar residues, whereas the nonpolar peptide of the third derivative contains 44% polar residues. The secondary structure of the membrane binding segment was studied by circular dichroism of whole nonpolar peptide and the corresponding peptides of the three derivatives. The data for whole nonpolar peptide are consistent with a structure containing approximately 50% helical and 25% beta sheet structure. The CD of the nonpolar peptides of the two largest derivatives are consistent with structures containing 56% helix and 19% beta sheet structure, and 40% helix and 20% beta sheet structure. These data support a predicted model for secondary structure, proposed previously, based upon the primary structure (Fleming, P. J., Dailey, H. A., Corcoran D., and Strittmatter, P. (1978) J. Biol. Chem. 253, 5369-5372).