Investigation of Domain Interaction in the Apolipoprotein E Isoforms by HDX-MS

Involvement of apoE4 in the pathology of Alzheimer's disease (AD) is hypothesized to arise from its unique structural properties, most importantly the interactions between the N- and C-terminal domains. However, structural understanding of the domain interaction is still lacking. Here, we use Hydrogen-Deuterium Exchange Mass Spectrometry (HDX-MS) to study domain interactions by measuring the effect of the C-terminal domain (CTD) on the solvent accessibility of the N-terminal domain (NTD) in both apoE3 and apoE4. Our results indicate that the presence of CTD enhances the solvent accessibility of all the four helices in the NTD in apoE4, but only two helices, specifically Helix-1 and 4 in apoE3. Therefore, the allosteric changes in the conformational ensemble of the NTD induced by the CTD is more extensive in apoE4 than in apoE3. Moreover, strong pH dependence suggests role of the salt bridges in the interdomain interactions. Since the NTD harbors the receptor binding region, the destabilizing effect of CTD on it provides a structural basis for the role of interdomain interactions on the pathological functions of apoE4. Furthermore, we propose HDX-MS as a methodology for screening and assessing the efficacy of 'structure corrector' molecules targeting apoE4 to mitigate its pathological effects in AD.