Intermolecular Contact between Globular N-terminal Fold and C-terminal Domain of ApoA-I Stabilizes Its Lipid-bound Conformation

The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. Recombinant HDL particles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-A diameter were treated with the lysine-specific cross-linker, dithiobis(succinimidylpropionate) at varying molar ratios from 2:1 to 200:1. At low molar ratios of dithiobis(succinimidylpropionate) to apoA-I, two products were obtained corresponding to approximately 53 and approximately 80 kDa. At high molar ratios, these two products merged, yielding a product of approximately 59 kDa, close to the theoretical molecular mass of dimeric apoA-I. To identify the intermolecular cross-links giving rise to the two different sized products, bands were excised from the gel, digested with trypsin, and then analyzed by liquid chromatography-electrospray-tandem mass spectrometry. In addition, tandem mass spectrometry of unique cross-links found in the 53- and 80-kDa products suggested that a distinct conformation exists for lipid-bound apoA-I on 96-A recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.