A Structural Model for Apolipoprotein C-II Amyloid Fibrils: Experimental Characterization and Molecular Dynamics Simulations

The self-assembly of specific proteins to form insoluble amyloid fibrils is a characteristic feature of a number of age-related and debilitating diseases. Lipid-free human apolipoprotein C-II (apoC-II) forms characteristic amyloid fibrils and is one of several apolipoproteins that accumulate in amyloid deposits located within atherosclerotic plaques. X-ray diffraction analysis of aligned apoC-II fibrils indicated a simple cross-β-structure composed of two parallel β-sheets. Examination of apoC-II fibrils using transmission electron microscopy, scanning transmission electron microscopy, and atomic force microscopy indicated that the fibrils are flat ribbons composed of one apoC-II molecule per 4.7-Å rise of the cross-β-structure. Cross-linking results using single-cysteine substitution mutants are consistent with a parallel in-register structural model for apoC-II fibrils. Fluorescence resonance energy transfer analysis of apoC-II fibrils labeled with specific fluorophores provided distance constraints for selected donor-acceptor pairs located within the fibrils. These findings were used to develop a simple ‘letter-G-like’ β-strand-loop-β-strand model for apoC-II fibrils. Fully solvated all-atom molecular dynamics (MD) simulations showed that the model contained a stable cross-β-core with a flexible connecting loop devoid of persistent secondary structure. The time course of the MD simulations revealed that charge clusters in the fibril rearrange to minimize the effects of same-charge interactions inherent in parallel in-register models. Our structural model for apoC-II fibrils suggests that apoC-II monomers fold and self-assemble to form a stable cross-β-scaffold containing relatively unstructured connecting loops.