Lipid bilayer structure in the membrane of Mycoplasma laidlawii

Abstract

X-ray diffraction patterns from intact, isolated Mycoplasma laidlawii membranes at temperatures above and below the thermal phase transition of the membrane lipids reveal important features of the molecular structure of the membrane. (1) The presence of the close hexagonal fatty acid chain packing giving rise to a sharp 4.15 Å diffraction line indicates that the membrane lipids are in a bilayer below the thermal phase transition. (2) The characteristics predicted for the diffraction from a lipid bilayer can be identified in the low-angle patterns from intact membranes both above and below the thermal phase transition, and the differences between the patterns are as expected from the changes in the wideangle pattern. (3) The interpretation of the low-angle patterns is confirmed by changing the average fatty acid chain length and observing the expected changes in the pattern produced by changes in the width of the bilayer. (4) The membrane profile diffraction can be largely accounted for by the lipid bilayer, so the majority of the protein is not located in uniform layers on either side of the bilayer regions of the membrane. (5) The area per lipid molecule is 40 to 45 Å 2 in the membrane bilayer below the transition and 60 to 70 Å 2 above it. Since the lipids are very closely packed, protein cannot penetrate the head group layers below the transition to make hydrophobic contact without disrupting the packing. Furthermore, the lipid-protein interaction does not greatly constrain the spacing of the lipid head groups.