Solution Conditions Affect the Ability of the K30D Mutation To Prevent Amyloid Fibril Formation by Apolipoprotein C-II: Insights from Experiments and Theoretical Simulations.
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.
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne , Parkville, Victoria 3010, Australia.
Yu
Y
Mao
School of Engineering, RMIT University , GPO Box 2476V, Melbourne, Victoria 3001, Australia.
Nevena
N
Todorova
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne , Parkville, Victoria 3010, Australia.
Courtney O
CO
Zlatic
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne , Parkville, Victoria 3010, Australia.
Paul R
PR
Gooley
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne , Parkville, Victoria 3010, Australia.
Michael D W
MD
Griffin
Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, University of Melbourne , Parkville, Victoria 3010, Australia.
Geoffrey J
GJ
Howlett
School of Engineering, RMIT University , GPO Box 2476V, Melbourne, Victoria 3001, Australia.
Irene
I
Yarovsky
D000682:Amyloid / Q000737:chemistry*; D053304:Apolipoprotein C-II / Q000737:chemistry* / Q000235:genetics; D006801:Humans; D007700:Kinetics; D008962:Models, Theoretical; D056004:Molecular Dynamics Simulation; D016297:Mutagenesis, Site-Directed; D009154:Mutation / Q000235:genetics; D017433:Protein Structure, Secondary
D000682:Amyloid; D053304:Apolipoprotein C-II