Mode of binding of cytochrome b5 to phospholipid bilayers in lamellar structure.

X-ray diffraction studies were made on the multilamellar systems produced by incubation of phospholipid bilayers and the membrane protein, cytochrome b5, or non-membrane proteins (albumin, ovalbumin and beta-lactoglobulin A) at pH 8.1 in aqueous 5 mM CaCl2 solutions. Detergent-extracted cytochrome b5 (soluble aggregate) forms two types of lamellar phase with dipalmitoyl phosphatidylcholine bilayers, depending upon the incubation temperature. One type, which has a repeat distance of 114 A, is formed above 34 degrees C, where the binding of cytochrome b5 to the bilayers is hydrophobic. The other type, with a repeat distance of 153 A, is formed below 34 degrees C, where the binding is electrostatic. It is also suggested that cytochrome b5 is monomeric in the former phase but remains aggregated in the latter phase. When dimyristoyl phosphatidylcholine is used, the boundary temperature for the two types shifts to 12 degrees C. These boundary temperatures coincide with the thermal pretransition points of hydrated dipalmitoyl phosphatidylcholine and dimyristoyl phosphatidylcholine, respectively. Trypsin-treated cytochrome b5 (monomeric) and the three non-membrane proteins exhibit only binding of the electrostatic type to the bilayers, indipendently of the incubation temperature. The observed repeat distances suggest that in these cases two layers of protein molecules are incorporated between the bilayers.