Poly-(L-Alanine) Expansions Form Core -Sheets that Nucleate Amyloid Assembly

Expansion toa total of 11–17 sequential alanine residues from the normal number of 10 in the polyadenine-binding protein nuclear-1 (PABPN1) results in formation of intranuclear, fibrillar inclusions in skeletal muscle and hypothalamic neurons in adult-onset, dominantly inherited oculopharyngeal muscular dystrophy (OPMD). To understand the role that homopolymeric length may play in the protein misfolding that leads to the inclusions, we analyzed the self-assembly of synthetic poly-(L-alanine) peptides having 3–20 residues. We found that the conformational transition and structure of polyalanine (polyAla) assemblies in solution are not only lengthdependent but also are determined by concentration, temperature, and incubation time. No -sheet complexwas detected for those peptides characterized by n< 8,wheren is number of alanine residues. A second group of peptides with 7 < n < 15 showed varying levels of complex formation, while for those peptides having n > 15, the interconversion process from the monomeric to the -sheet complex was complete under any of the tested experimental conditions. Unlike the typical tinctorial properties of amyloid fibrils, polyalanine fibrils did not show fluorescence with thioflavin T or apple-green birefringence with Congo red; however, like amyloid, X-ray diffraction showed that the peptide chains in these fibrils were oriented normal to the fibril axis (i.e., in the crossarrangement). Neighboring -sheets are quarter-staggered in the hydrogen-bonding direction such that the alanine side-chains were closely packed in the intersheet space. Strong van der Waals contacts between sidechains in this arrangement likely account for the high stability of the macromolecular fibrillar complex in solution over a wide range of temperature (5–85°C), and pH (2–10.5), and its resistance to denaturant (< 8 M urea) and to proteases (protease K, trypsin). We postulate that a similar stabilization of an expanded polyalanine stretch could form a core -sheet structure that mediates the intermolecular association of mutant proteins into fibrillar inclusions in human pathologies. Proteins 2005;61:579–589. © 2005Wiley-Liss, Inc.