Membrane topology of microsomal cytochrome P-450: Saturation transfer EPR and freeze-fracture electron microscopy studies

The rotation of cytochrome P-450 LM2 (CYPIIB4) incorporated into large microsomal-like lipid vesicles was investigated by saturation transfer EPR using 15N- and 2H-substituted spin labels. In combination with rotational diffusion, the distribution and size of protein particles in the bilayer were studied by freeze-fracture electron microscopy. The data from both methods suggest an oligomeric and membrane-spanning aggregate for the topology of microsomal cytochrome P-450.