Intermolecular contact between globular N-terminal fold and C-terminal domain of ApoA-I stabilizes its lipid-bound conformation - Studies employing chemical cross-linking and mass spectrometry

The structure of apoA-I on discoidal high density lipoprotein (HDL) was studied using a combination of chemical cross-linking and mass spectrometry. RecombinantHDLparticles containing 145 molecules of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and two molecules of apoA-I with a 96-angstrom 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 similar to 53 and similar to 80 kDa. At high molar ratios, these two products merged, yielding a product of similar to 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-angstrom recombinant HDL, emphasizing the inherent flexibility and malleability of the N termini and its interaction with its C-terminal domain.