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J. Biol. Chem., Vol. 282, Issue 49, 35831-35841, December 7, 2007

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Structural Basis for the Recognition and Cross-linking of Amyloid Fibrils by Human Apolipoprotein E^*

From the Department of Biochemistry and Molecular Biology and Bio21 Molecular Science and Biotechnology Institute, the University of Melbourne, Parkville, Victoria 3010, Australia

Apolipoprotein (apo) E is a well characterized lipid-binding protein in plasma that also exists as a common nonfibrillar component of both cerebral and systemic amyloid deposits. A genetic link between a common isoform of apoE, apoE4, and the incidence of late onset Alzheimer disease has drawn considerable attention to the potential roles of apoE in amyloid-related disease. We examined the interactions of apoE with amyloid fibrils composed of apoC-II and the amyloid-β (Aβ) peptide. Aggregates of apoE with Aβ and apoC-II are found in Alzheimer and atherosclerotic plaques, respectively. Sedimentation velocity and fibril size distribution analysis showed that apoE3 and E4 isoforms bind and noncovalently cross-link apoC-II fibrils in a similar manner. This ability to cross-link apoC-II fibrils was abolished by the dissociation of the apoE tetramer to monomers or by thrombin cleavage to yield separate N- and C-terminal domains. Preparative ultracentrifuge binding studies indicated that apoE and the isolated N- and C-terminal domains of apoE bind with submicromolar affinities to both apoC-II and Aβ fibrils. Fluorescence quenching and resonance energy transfer experiments confirmed that both domains of apoE interact with apoC-II fibrils and demonstrated that the binding of the isolated N-terminal domain of apoE to apoC-II or Aβ fibrils is accompanied by a significant conformational change with helix three of the domain moving relative to helix one. We propose a model involving the interaction of apoE with patterns of aligned residues that could explain the general ability of apoE to bind to a diverse range of amyloid fibrils.

Received for publication, August 3, 2007 , and in revised form, September 28, 2007.

^* The costs of publication of this article were defrayed in part by the payment of page charges. This article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.

¹ To whom correspondence should be addressed: Dept. of Biochemistry and Molecular Biology and Bio21 Institute, the University of Melbourne, 30 Flemington Rd., Parkville, Victoria 3010, Australia. Tel.: 61-3-8344-2271; Fax: 61-3-9348-1421; E-mail: ghowlett{at}unimelb.edu.au.

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