THE STRUCTURAL BASIS OF LIPID INTERACTIONS IN LIPOVITELLIN, A SOLUBLELIPOPROTEIN

Background: The conformation and assembly of lipoproteins, proteins co ntaining large amounts of noncovalently bound lipid, is poorly underst ood. Lipoproteins present an unusual challenge as they often contain v arying loads of lipid and are not readily crystallized. Lipovitellin i s a large crystallizable oocyte protein of approximately 1300 residues that contains about 16% w/w lipid. Lipovitellin contains two large do mains that appear to be conserved in both microsomal triglyceride tran sfer protein and apolipoprotein B-100. To gain insight into the confor mation of a lipoprotein and the potential modes of binding of both neu tral and phospholipid, the crystal structure of lamprey lipovitellin h as been determined. Results: We report here the refined crystal struct ure of lipovitellin at 2.8 Angstrom resolution. The structure contains 1129 amino acid residues located on five peptide chains, one 40-atom phosphatidylcholine, and one 13-atom hydrocarbon chain. The protein co ntains a funnel-shaped cavity formed primarily by two beta sheets and lined predominantly by hydrophobic residues. Conclusions: Using the cr ystal structure as a template, a model for the bound lipid is proposed . The lipid-binding cavity is formed primarily by a single-thickness b eta-sheet structure which is stabilized by bound lipid. This cavity ap pears to be flexible, allowing lipid to be loaded or unloaded.