FIGURE 6.

Representative cross-link depicting an intra- versus intermolecular contact in the monomer versus dimer of apoA-IV. The dimeric crystal structure for apoA-IV^64–335 is shown on the left with molecule A in green and molecule B in teal. The α-carbons of Lys residues are shown as red spheres. The long, “swapped” helices that reciprocate across the dimer interface can be seen on the right side of the dimer. In our proposed model of the monomeric structure (right), the long helix in molecule A has a hinge introduced at its midpoint and folds back onto the molecule, making exactly the same interactions as the swapped helix from molecule B makes in the dimer. The insets show the cross-link between Lys¹⁹¹ and Lys¹⁰³ (pink line), which was determined to be intermolecular in the dimer but intramolecular in the monomer (the mass spectra for a representative ion for this cross-link is shown in Fig. 4b).