Lipid domains in the crystalline lipovitellin/phosvitin complex : a phosphorus-31 and deuterium nuclear magnetic resonance study

The crystalline lipovitellin/phosvitin complex has a molecular weight of 456 000 and contains nearly 100 molecules of bound phospholipid. Earlier work using electron microscopy and three-dimensional image reconstruction methods established the symmetrical dimeric nature of this lipoprotein, but the organization of the lipid was unknown. Under conditions where the lipoprotein is in solution, the high-resolution 31P nuclear magnetic resonance (NMR) spectra contain two well-resolved peaks which can be assigned to phosphoserine moieties in both lipovitellin and phosvitin and to the phospholipid microdomains. The spin-lattice relaxation times, T1, for the phosphoserines and the phospholipid head groups are distinctly different, with the serine phosphates having faster reorientation rates. 31P NMR spectra of crystalline lipoprotein contain a broad symmetric component with a chemical shielding anisotropy of about -50 ppm. By obtaining 31P MNR spectra from several modified forms of the lipoprotein as well as from the extracted lipids, it is shown that the chemical shielding anisotropy is characteristic of phospholipid in a bilayer arrangement. As a further check on 31P NMR results, samples of the lipovitellin/phosvitin complex containing deuterium-labeled 1,2-dielaidoyl-sn-glycero-3-phosphocholine were studied by 2H NMR methods. The resulting spectrum has characteristics similar to those obtained from model lipid systems in a lamellar state. The results of the 31P and 2H NMR experiments correlate with the low-resolution model of the crystalline lipovitellin complex obtained by diffraction studies. It is proposed that each subunit of lipovitellin contains a microdomain of phospholipid in a bilayer-like arrangement.