Amyloid-Forming Properties of Human Apolipoproteins: Sequence Analyses and Structural Insights

Abstract

Apolipoproteins are protein constituents of lipoproteins that transport cholesterol and fat in circulation and are central to cardiovascular health and disease. Soluble apolipoproteins can transiently dissociate from the lipoprotein surface in a labile free form that can misfold, potentially leading to amyloid disease. Misfolding of apoA-I, apoA-II, and serum amyloid A (SAA) causes systemic amyloidoses, apoE4 is a critical risk factor in Alzheimer's disease, and apolipoprotein misfolding is also implicated in cardiovascular disease. To explain why apolipoproteins are overrepresented in amyloidoses, it was proposed that the amphipathic α-helices, which form the lipid surface-binding motif in this protein family, have high amyloid-forming propensity. Here, we use 12 sequence-based bioinformatics approaches to assess amyloid-forming potential of human apolipoproteins and to identify segments that are likely to initiate β-aggregation. Mapping such segments on the available atomic structures of apolipoproteins helps explain why some of them readily form amyloid while others do not. Our analysis shows that nearly all amyloidogenic segments: (i) are largely hydrophobic, (ii) are located in the lipid-binding amphipathic α-helices in the native structures of soluble apolipoproteins, (iii) are predicted in both native α-helices and β-sheets in the insoluble apoB, and (iv) are predicted to form parallel in-register β-sheet in amyloid. Most of these predictions have been verified experimentally for apoC-II, apoA-I, apoA-II and SAA. Surprisingly, the rank order of the amino acid sequence propensity to form amyloid (apoB > apoA-II > apoC-II ≥ apoA-I, apoC-III, SAA, apoC-I > apoA-IV, apoA-V, apoE) does not correlate with the proteins' involvement in amyloidosis. Rather, it correlates directly with the strength of the protein-lipid association, which increases with increasing protein hydrophobicity. Therefore, the lipid surface-binding function and the amyloid-forming propensity are both rooted in apolipoproteins' hydrophobicity, suggesting that functional constraints make it difficult to completely eliminate pathogenic apolipoprotein misfolding. We propose that apolipoproteins have evolved protective mechanisms against misfolding, such as the sequestration of the amyloidogenic segments via the native protein-lipid and protein-protein interactions involving amphipathic α-helices and, in case of apoB, β-sheets. KeywordsLipid transport; Systemic amyloidosis; Atherosclerosis and Alzheimer's disease; Atomic protein structure; Sequence-based prediction algorithms Correspondence to: Olga Gursky. HHS Public AccessAuthor manuscript Adv Exp Med Biol. Author manuscript; available in PMC 2017 September 24.Published in final edited form as:Adv Exp Med Biol. 2015 ; 855: 175-211. doi:10.1007/978-3-319-17344-3_8. Author Manuscript Author ManuscriptAuthor ManuscriptAuthor Manuscript Lipoproteins and Apolipoproteins in Lipid Transport and MetabolismLipids in plasma, lymph and cerebrospinal fluid are transported via lipoproteins that are...

Structural Stability and Local Dynamics in Disease-Causing Mutants of Human Apolipoprotein A-I: What Makes the Protein Amyloidogenic?

“…4B, C), which are predicted to form the major amyloid hot spot. 2,22–25 In the crystal structure, these residues form a well-ordered kinked helical segment in the middle of the helix bundle (Fig. 1B, blue), which is consistent with the slow deuteration of this segment observed in the WT (Fig.…”

“…1). Although the extended segment 44–55 is largely polar and is predicted not to have high amyloidogenic potential, 2 it was proposed to facilitate α-helix to β-sheet conversion, 17,21 and peptide fragments encompassing this segment can form amyloid fibrils in vitro . 17,23,25 Lack of mutational effects in the HDX of segment 44–55 (Fig.…”

“…Among more than 40 polypeptides that have been implicated in amyloidoses, the apolipoprotein (apo) family is overrepresented, probably because these dynamic proteins possess high hydrophobicity and low structural stability, which are necessary for their function but also promote β-aggregation. 1,2 Apolipoproteins are lipid-surface-binding proteins that solubilize lipids and transport them in aqueous environments such as plasma. Dissecting molecular pathways of apolipoprotein misfolding, from the native highly α-helical conformation to the cross-β-sheet in amyloid, is an unresolved fundamental problem whose solution may help guide the search for the much-needed therapies.…”

“…In this mechanism, protein misfolding was proposed to precede proteolysis in AApoAI, although alternative pathways in which misfolding and proteolysis occur in parallel could not be excluded. 2 …”

Serum amyloid A forms stable oligomers that disrupt vesicles at lysosomal pH and contribute to the pathogenesis of reactive amyloidosis

“…5B), greatly decreased SAA protection against proteolysis. This finding is an exception from the general trend: Lipid binding by apolipoproteins protects them from proteolysis (3,33,34).…”