X-RAY-DIFFRACTION AND FAR-UV CD STUDIES OF FILAMENTS FORMED BY A LEUCINE-RICH REPEAT PEPTIDE - STRUCTURAL SIMILARITY TO THE AMYLOID FIBRILSOF PRIONS AND ALZHEIMERS-DISEASE BETA-PROTEIN

The development of neuro-degenerative diseases often involves amyloido sis, that is the formation of polymeric fibrillar structures from norm al cellular proteins or peptides, For example, in Alzheimer's disease, a 42 amino acid peptide processed from the amyloid precursor protein forms filaments with a beta-sheet structure, Because of this, the stru cture and dynamics of polymeric peptide filaments is of considerable i nterest, We showed previously that a 23 amino acid peptide constitutin g a single leucine-rich repeat (LRRN) polymerises spontaneously in sol ution to form long filaments of a beta-sheet structure, a property sim ilar to that of Alzheimer's beta-amyloid and prion peptides, Here we r eport that a variant of LRRN in which a highly conserved asparagine re sidue is replaced by aspartic acid does not form either filaments or b eta structure, By contrast, a variant which replaces this asparagine r esidue with glutamine forms filaments ultrastructurally indistinguisha ble from those of LRRN. Electron micrographs of LRRN filaments show th at many consist of two interleaved strands which appear to have a ribb on-like morphology. X-ray diffraction patterns from oriented LRRN fibr es reveal that they are composed of long beta-sheet arrays, with the i nterstrand hydrogen bonding parallel to the filament axis, This 'cross -beta' structure is similar to that adopted by beta-amyloid and prion derived fibres, Taken together, these results indicate that the LRR fi laments are stabilised by inter- or intra-strand hydrogen bonded inter actions comparable to the asparagine ladders of beta-helix proteins or the 'glutamine zippers' of poly-glutamine peptides, We propose that s imilar stabilising interactions may underlie a number of characterised predispositions to neuro-degenerative diseases that are caused by mut ations to amide residues, Our finding that amyloid-like filaments can form from a peptide motif not at present correlated with degenerative disease suggests that a propensity for beta-filament formation is a co mmon feature of protein sub-domains. (C) 1997 Federation of European B iochemical Societies.