Protein aggregation via polyglutamine stretches occurs in a number of severe neurodegenerative diseases such as Huntington's disease. We have investigated fibrillar aggregates of polyglutamine peptides below, at, and above the toxicity limit of around 37 glutamine residues using solid-state NMR and electron microscopy. Experimental data are consistent with a dry fibril core of at least 70-80 Å in width for all constructs. Solid-state NMR dipolar correlation experiments reveal a largely β-strand character of all samples and point to tight interdigitation of hydrogen-bonded glutamine side chains from different sheets. Two approximately equally frequent populations of glutamine residues with distinct sets of chemical shifts are found, consistent with local backbone dihedral angles compensating for β-strand twist or with two distinct sets of side-chain conformations. Peptides comprising 15 glutamine residues are present as single extended β-strands. Data obtained for longer constructs are most compatible with a superpleated arrangement with individual molecules contributing β-strands to more than one sheet and an antiparallel assembly of strands within β-sheets.

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Eukaryotic cell surface proteins are often modified by a glycosylphosphatidylinositol (GPI) anchor. More than 200 of these post-translationally altered proteins are presently known, a prominent example being the prion protein (PrP). Although the significance of the GPI anchor is well recognized, efforts to study its function are hampered due to its complex chemical nature, which combines hydrophilic glycosyl chains with hydrophobic lipid moieties. Here we describe a general method for the synthesis of a GPI-anchored peptide containing an N-terminal Cys. This module can be employed for the production of proteins containing a natural GPI anchor using expressed protein ligation.

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