Proteina Fibroina

The Journal of Experimental Biology 202, 3295–3303 (1999) Printed in Great Britain © The Company of Biologists Limited 1999 JEB2216

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THE MECHANICAL DESIGN OF SPIDER SILKS: FROM FIBROIN SEQUENCE TO MECHANICAL FUNCTION
J. M. GOSLINE*, P. A. GUERETTE, C. S. ORTLEPP AND K. N. SAVAGE Department of Zoology, University of British Columbia, Vancouver, British Columbia, Canada V6T 1Z4
*e-mail:gosline@zoology.ubc.ca

Accepted 3 May; published on WWW 16 November 1999 Summary Spiders produce a variety of silks, and the cloning of fibroins into a polymer network with great stiffness, genes for silk fibroins reveals a clear link between strength and toughness. As illustrated by a comparison of protein sequence and structure–property relationships. MA silks from Araneus diadematus andNephila clavipes, The fibroins produced in the spider’s major ampullate variation in fibroin sequence and properties between spider (MA) gland, which forms the dragline and web frame, species provides the opportunity to investigate the design contain multiple repeats of motifs that include an 8–10 of these remarkable biomaterials. residue long poly-alanine block and a 24–35 residue long glycine-richblock. When fibroins are spun into fibres, the Key words: spider, Araneus diadematus, Nephila clavipes, silk, fibroin, structure/property relationship. poly-alanine blocks form β-sheet crystals that crosslink the

Introduction Orb-web-spinning spiders produce a variety of highperformance structural fibres with mechanical properties unmatched in the natural world and comparable with the very bestsynthetic fibres produced by modern technology. As a result, there is considerable interest in the design of these materials as a guide to the commercial production of proteinbased structural polymers through genetic engineering. To understand the design of silks, we must understand the relationships between structure and function, and this will require information from molecular biology, polymer physics,materials science and spider biology. We now have good information on the amino acid sequence motifs present in spider fibroins, and our understanding of the molecular architecture of spider silks is growing daily. Unfortunately, in many cases, our understanding of silk design is flawed because we do not know which mechanical properties are crucial to its function. To quote Wainwright (1988),‘Identification of the properties that are important to a particular function requires all the intuitive insights and creative abilities of objective scientists.’ Only when we understand the true function of spider silks will be able determine whether a spider’s dragline silk offers an appropriate model for man-made materials produced through genetic engineering. The function of spider silks Theorb-web-weaving araneid spiders provide ideal model organisms for studying the functional design of protein-based structural materials. These spiders have seven different gland–spinneret complexes, each of which synthesizes a unique blend of structural polymers and produces a fibre with a unique set of functional properties (Gosline et al., 1986; Vollrath, 1992). Unfortunately, our knowledge of functionalrelationships for most of these materials is very limited, with the exception of the major ampullate (MA) gland fibres and to some extent the viscid silk fibres produced by the flagelliform (FL) gland. The organization of these fibres in the spider’s orbweb is illustrated in Fig. 1, where it can be seen that MA silk fibres form the web frame and the spider’s dragline. The viscid silk forms theglue-covered catching spiral. Mechanical properties of MA and viscid silks Fig. 1 shows typical tensile test data for MA and viscid silk from the spider Araneus diadematus plotted as stress–strain curves. The stress (σ) is the normalized force (F), defined as σ=F/A, where A is the initial cross-sectional area of the silk fibre. The strain (ε) is the normalized deformation, defined as, ε=∆L/L0, where L0 is...