title: Rationalising lysozyme amyloidosis: insights from the structure and solution dynamics of T70N lysozyme.
abstract: T70N human lysozyme is the only known naturally occurring destabilised lysozyme variant that has not been detected in amyloid deposits in human patients. Its study and a comparison of its properties with those of the amyloidogenic variants of lysozyme is therefore important for understanding the determinants of amyloid disease. We report here the X-ray crystal structure and the solution dynamics of T70N lysozyme, as monitored by hydrogen/deuterium exchange and NMR relaxation experiments. The X-ray crystal structure shows that a substantial structural rearrangement results from the amino acid substitution, involving residues 45-51 and 68-75 in particular, and gives rise to a concomitant separation of these two loops of up to 6.5A. A marked decrease in the magnitudes of the generalised order parameter (S2) values of the amide nitrogen atom, for residues 70-74, shows that the T70N substitution increases the flexibility of the peptide backbone around the site of mutation. Hydrogen/deuterium exchange protection factors measured by NMR spectroscopy were calculated for the T70N variant and the wild-type protein. The protection factors for many of backbone amide groups in the beta-domain of the T70N variant are decreased relative to those in the wild-type protein, whereas those in the alpha-domain display wild-type-like values. In pulse-labelled hydrogen/deuterium exchange experiments monitored by mass spectrometry, transient but locally cooperative unfolding of the beta-domain of the T70N variant and the wild-type protein was observed, but at higher temperatures than for the amyloidogenic variants I56T and D67H. These findings reveal that such partial unfolding is an intrinsic property of the human lysozyme structure, and suggest that the readiness with which it occurs is a critical feature determining whether or not amyloid deposition occurs in vivo.
authors.affiliation: Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
authors.forename: Russell J K
authors.initials: RJ
authors.lastname: Johnson
authors.forename: John
authors.initials: J
authors.lastname: Christodoulou
authors.forename: Mireille
authors.initials: M
authors.lastname: Dumoulin
authors.forename: Gemma L
authors.initials: GL
authors.lastname: Caddy
authors.forename: Marcos J C
authors.initials: MJ
authors.lastname: Alcocer
authors.forename: Gareth J
authors.initials: GJ
authors.lastname: Murtagh
authors.forename: Janet R
authors.initials: JR
authors.lastname: Kumita
authors.forename: Göran
authors.initials: G
authors.lastname: Larsson
authors.forename: Carol V
authors.initials: CV
authors.lastname: Robinson
authors.forename: David B
authors.initials: DB
authors.lastname: Archer
authors.forename: Ben
authors.initials: B
authors.lastname: Luisi
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Dobson
mesh_terms: D000686:Amyloidosis / Q000235:genetics / Q000378:metabolism; D018360:Crystallography, X-Ray; D006801:Humans; D008958:Models, Molecular; D009113:Muramidase / Q000737:chemistry* / Q000235:genetics* / Q000378:metabolism; D009154:Mutation; D019906:Nuclear Magnetic Resonance, Biomolecular; D011487:Protein Conformation
chemical_list: D009113:Muramidase