We study the aggregation of a fragment of the neuronal protein Tau that contains part of the proline rich domain and of the microtubule binding repeats. When incubated at 37 degrees C with heparin, the fragment readily forms fibers as witnessed by Thioflavin T fluorescence. Electron microscopy and NMR spectroscopy show bundled ribbon like structures with most residues rigidly incorporated in the fibril. Without its cysteines, this fragment still forms fibers of a similar morphology, but with lesser Thioflavin T binding sites and more mobility for the C-terminal residues. (C) 2014 Elsevier Inc. All rights reserved.