Tandem NMR and Mass Spectrometry Analysis of Human Nuclear Membrane Lipids

The human nuclear membrane is composed of a double bilayer, the inner membrane being linked to the protein lamina network and the outer nuclear membrane continuous with the endoplasmic reticulum. Nuclear membranes can form large invaginations inside the nucleus; their specific roles still remain unknown. Although much of the protein identification has been determined, their lipid composition remains largely undetermined. In order to understand the mechanical and dynamic properties of nuclear membranes we investigated their lipid composition by two quantitative methods, namely, ³¹P and ¹H multidimensional NMR and mass spectrometry, using internal standards. We also developed a nondetergent nuclei extraction protocol allowing to produce milligram quantities of nuclear membrane lipids. We found that the nuclear membrane lipid extract is composed of a complex mixture of phospholipids with different phosphatidylcholine species present in large amounts. Negatively charged lipids, with elevated amounts of phosphatidylinositol (PI), were also present. Mass spectrometry confirmed the phospholipid composition and provided further information on acyl-chain length and unsaturation. Lipid chain lengths ranged between 30 and 38 carbon atoms (two chains summed up) with a high proportion of 34 carbon atom length for most species. PI lipids have high amounts of chain lengths with 36-38 carbons. Independent of the chain length unsaturations were highly elevated with one to two double bonds per lipid species.