Structural characterization of ordered domains in a hydrophobic membrane protein

A delipidized proteolipid protein fraction was purified from organic solvent extracts of bovine cerebral cortex and studied by means of diffraction, electron microscopic, and ir techniques. Special use was made of an electron diffraction procedure which minimized the electron damage to the biological specimens. The ir spectroscopy of the apoprotein fraction indicated the presence of polypeptides in extended β‐conformation, possibly in the antiparallel mode of packing. Electron microscopy of the fraction, negatively stained in organic media, made apparent the presence of both ordered and amorphous material. Only the former, characterized by repeating units of about 40–45 Å in diameter and varying length, produced diffraction patterns in the selected area mode exhibiting a highly undistorted lattice. The two‐dimensional cell parameters of the protein fraction were a = 4.79 Å, b = 7.20 Å, and γ = 90°. The plane group symmetry, corresponding to the systematic absences, was p 2gg, consistent with the β‐pleated sheet structure of simple polypeptides.