Lipid domains in the crystalline lipovitellin/phosvitin complex: a phosphorus-31 and proton NMR study

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Abstract
The crystalline lipovitellin/phosvitin complex has a MW of 456,000 and contains nearly 100 molecules of bound phospholipid. Earlier work using EM and 3-dimensional image reconstruction methods established the symmetrical dimeric nature of this lipoprotein, but the organization of the lipid was unknown. Under conditons where the lipoprotein is in solution, the high-resolution 31P NMR spectra contain 2 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. 31P NMR spectra from several modified forms of the lipoprotein as well as from the extracted lipids, show 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 2H-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. Each subunit of lipovitellin probably contains a microdomain of phospholipid in a bilayer-like arrangement.
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