X-ray diffraction analysis of cytochrome b5 reconstituted in egg phosphatidylcholine vesicles

Cytochrome b5 was reconstituted asymmetrically into large unilamellar egg phosphatidylcholine vesicles. Asymmetry was preserved after sedimentation and partial dehydration to form oriented stacks of membranes. The periodicity of the centrosymmetric unit cell ranged between 145 and 175 A, depending upon the water content of the oriented multilayer. X-ray diffraction data were collected to a resolution of 12 A and phase factors were unambiguously assigned by a swelling analysis to a resolution of 15 A. The lower-resolution profile structures clearly showed a highly asymmetric single membrane containing the heme peptide segment of the cytochrome on one side of the membrane bilayer. The higher-resolution data were also analyzed and profile structures were compared with various models for the distribution of cytochrome b5 nonpolar peptide within the membrane bilayer region. The data favor an asymmetric distribution of protein mass within the membrane bilayer.