Solution Conditions Affect the Ability of the K30D Mutation To Prevent Amyloid Fibril Formation by Apolipoprotein C-II: Insights from Experiments and Theoretical Simulations

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• 作者：Yu Mao ; Nevena Todorova ; Courtney O. Zlatic ; Paul R. Gooley ; Michael D. W. Griffin ; Geoffrey J. Howlett ; Irene Yarovsky
• 刊名：Biochemistry
• 出版年：2016
• 出版时间：July 12, 2016
• 年：2016
• 卷：55
• 期：27
• 页码：3815-3824
• 全文大小：570K
• 年卷期：0
• ISSN：1520-4995

文摘

Apolipoproteins form amphipathic helical structures that bind lipid surfaces. Paradoxically, lipid-free apolipoproteins display a strong propensity to form cross-β structure and self-associate into disease-related amyloid fibrils. Studies of apolipoprotein C-II (apoC-II) amyloid fibrils suggest that a K30-D69 ion pair accounts for the dual abilities to form helix and cross-β structure. Consistent with this is the observation that a K30D mutation prevents fibril formation under standard fibril forming conditions. However, we found that fibril formation by K30D apoC-II proceeded readily at low pH and a higher salt or protein concentration. Structural analysis demonstrated that K30D apoC-II fibrils at pH 7 have a structure similar to that of the wild-type fibrils but are less stable. Molecular dynamics simulations of the wild-type apoC-II fibril model at pH 7 and 3 showed that the loss of charge on D69 at pH 3 leads to greater separation between residues K30 and D69 within the fibril with a corresponding reduction in β-strand content around residue 30. In contrast, in simulations of the K30D mutant model at pH 7 and 3, residues D30 and D69 moved closer at pH 3, accompanied by an increase in β-strand content around residue 30. The simulations also demonstrated a strong dominance of inter- over intramolecular contacts between ionic residues of apoC-II and suggested a cooperative mechanism for forming favorable interactions between the individual strands under different conditions. These observations demonstrate the important role of the buried K30-D69 ion pair in the stability and solution properties of apoC-II amyloid fibrils.