Characterization of the N- and C-terminal domain interface of the three main apoE isoforms: A combined quantitative cross-linking mass spectrometry and molecular modeling study.

Apolipoprotein E (apoE) polymorphism is associated with different pathologies such as atherosclerosis and Alzheimer's disease. Knowledge of the three-dimensional structure of apoE and isoform-specific structural differences are prerequisites for the rational design of small molecule structure modulators that correct the detrimental effects of pathological isoforms. In this study, cross-linking mass spectrometry (XL-MS) targeting Asp, Glu and Lys residues was used to explore the intramolecular interactions in the E2, E3 and E4 isoforms of apoE. The resulting quantitative XL-MS data combined with molecular modeling revealed isoform-specific characteristics of the N- and C-terminal domain interfaces as well as the isoform-dependent dynamic equilibrium of these interfaces. Finally, the data identified a network of salt bridges formed by R61-R112-E109 residues in the N-terminal helical bundle as a modulator of the interaction with the C-terminal domain making this network a potential drug target.