Explanation for the mechanical strength of amyloid

Authors: Takeshi Fukuma Anika S Mostaert Suzanne P Jarvis

Publish Date: 2006/07/06

Volume: 22, Issue: 3, Pages: 233-237

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Abstract

The presence of “proteinaceous βsheet rich fibrillar structures” and amyloidogenic material has been alluded to extensively in the literature in association with natural materials exhibiting superior mechanical strength per unit volume Here we provide a clear experimental demonstration and explanation for why individual amyloid quaternary structures themselves have beneficial mechanical characteristicsAmyloid fibrils are quaternary protein structures formed from the nonspecific folding or misfolding and subsequent aggregation of proteins into intermolecular βsheets of infinite propagation These supramolecular crossed βsheet structures are considered to be the fundamental motif of amyloid fibril architecture 1 Whilst many aspects of the actual selfassembly process and the internal structure of individual fibrils remain unknown evidence suggests that at least some of these aspects are aminoacid specific 1 Amyloid fibrils are commonly associated with neurodegenerative diseases although there is evidence that the amyloid structure is a generic form into which any polypeptide can fold particularly in vitro under slightly denaturing conditions 23 Recently there has been an interest in the mechanical properties of amyloid 4 which in part has been triggered by the observed amyloidogenic nature of spider silk 5 This amyloidogenic nature takes the form of a βsheet enriching structural conversion of the major spidersilk protein spidron inside the silk gland of the spider Similar conformational changes have been observed for the silkworm protein fibroin from random coil/helix to βsheet 6 This together with the suspected nucleationdependent aggregation mechanism during silk production 7 have lead to the suggestion that such conformational changes during the spinning process are as much or more important as than the protein sequence and native structure 6 To date direct measurements of the nanoscale surface mechanical properties of spider silk have not been able to provide additional insight into the validity of this suggestion or into the potential mechanical role of amyloid within the silk 8The amyloidogenic nature of natural adhesives and cements has also been described In particular Kamino et al 9 observed a number of similarities between a barnacle underwater adhesive protein and proteins involved in the formation of amyloid plaque This together with the insoluble behaviour of the adhesive protein led them to speculate that the molecular mechanisms for forming an insoluble proteinaceous multimer may be similar between amyloid plaque and barnacle cement 9 Recently unusually well ordered repetitive nanomechanical responses have been observed in the natural adhesive of terrestrial algae with additional histochemical evidence suggesting that the mechanical responses can be attributed to the presence of amyloid within the material 10Here we use an ultralow noise atomic force microscope AFM 11 to investigate the tensile response of model amyloid fibrils formed in vitro also known as amyloidlike fibrils 12 In recent years significant effort has been made to determine and understand the complex molecular structure of amyloid fibrils by investigating model amyloid fibrils such as these formed from short peptide segments 13AFM images of TTR fibrils on mica in air obtained in intermittentcontact mode cantilever spring constant k ≈ 40 N/m and resonance frequency f 0 ≈ 300 kHz Image sizes a 3 × 3 μm2 b 600 × 600 nm2 The length of the fibrils ranges from 100 nm to 2 μm while the width is mostly within the range of 8 ± 1 nm The height corrugation on top of the fibrils suggests that the fibrils consist of twisted protofilaments The periodicity of the height corrugation measured along a fibril was about 90 nmAll measurements were performed with a custom designed thermal noise limited AFM capable of routine true atomic resolution imaging on mica when operating in frequencymodulation detection mode 11 The AFM is interfaced to a commercially available AFM controller Asylum Research MFP3D Bipolar ControllerHigh resolution imaging of fibrils was performed in air using intermittentcontact mode For stability when imaging at high resolution stiff silicon cantilevers NCH Nanosensors with a spring constant of approx 40 N/m and resonance frequency of approx 300 kHz in air were used

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