Lipid Binding to the Amphipathic Membrane Protein Cytochrome b5
Abstract
The lipid binding properties of the membrane protein cytochrome b 5 (detergent-extracted from calf liver microsomal preparations) were characterized by studying the interaction of spin-labeled lipids (5-, 12-, and 16-doxylstearic acid and 5- and 16-doxylphosphatidyl-choline, where doxyl refers to the nitroxide moiety) with cytochrome b 5, using electron spin resonance spectroscopy. The intact cytochrome b 5 molecule immobilizes all of the lipid spin labels, while the segment of cytochrome b 5 released by trypsin does not affect lipid mobility. The immobilization of lipid spin labels on the hydrophobic surface of intact cytochrome b 5 is not appreciably altered by associating the protein with liposomes. Differences in polarity of the lipid binding sites between cytochrome b 5 and phospholipid vesicles were also observed. The lipid binding sites on cytochrome b 5 are hydrophobic by conventional criteria, but are more polar than the interior of fluid phospholipid bilayers.
