Molecular mechanisms of spider silk

Abstract

Spiders spin high-performance silks through the expression and assembly of tissue-restricted fibroin proteins. Spider silks are composite protein biopolymers that have complex microstructures. Retrieval of cDNAs and genomic DNAs encoding silk fibroins has revealed an association between the protein sequences and structure-property relationships. However, before spider silks can be subject to genetic engineering for commercial applications, the complete protein sequences and their functions, as well as the details of the spinning mechanism, will require additional progress and collaborative efforts in the areas of biochemistry, molecular biology and material science. Novel approaches to reveal additional molecular constituents embedded in the spider fibers, as well as cloning strategies to manipulate the genes for expression, will continue to be important aspects of spider biology research. Here we summarize the molecular characteristics of the different spider fibroins, the mechanical properties and assembly process of spidroins and the advances in protein expression systems used for recombinant silk production. We also highlight different technical approaches being used to elucidate the molecular constituents of silk fibers.

The elaborate structure of spider silk

“…[26][27][28] Several carboxyterminal nonrepetitive sequences of different silks and spiders have been identified, revealing a high sequence homology amongst these domains. 24,29,30 However, only one full length sequence of a Flag silk protein from Nephila clavipes covering both termini has been identified so far. Further, only recently the first full length sequence of MA silk from the black widow spider has been reported.…”

Spidroin N-terminal Domain Promotes a pH-dependent Association of Silk Proteins during Self-assembly

“…Flanking this repeat array are short nonrepetitive N-and C-terminal domains (NTDs 2 and CTDs) of ϳ155 and ϳ100 amino acids, respectively (12,15). The CTDs are conserved, not just among major ampullate spidroins but among all fiber-forming spidroins, as are the N-terminal domains for all spidroins for which 5Ј gene sequences are currently known (16). Because these domains have been conserved over hundreds of millions of years of spider evolution, they likely confer an important function on spidroin proteins.…”

Spider Silk Composites and Applications

“…With respect to energy absorption prior to breaking, spider silks are unmatched in the world of synthetic and natural fibers (Table 2). Different spider silk types produce distinct stress-strain curves, demonstrating that spiders spin a broad range of fibers with diverse mechanical properties (Gosline et al, 1986;Hu et al, 2006a). Similar to other protein polymers, there are numerous factors that influence the strength and the load-deformation response of silk fibers, which includes temperature, humidity, and extrusion rate.…”

“…The variations in the mechanical behavior in dragline silk fibers across different species suggest that these silks have been fine-tuned by each spider for their nuanced needs. Other spider silk fiber types have also been studied using tensile testing, including aciniform silks (Hayashi et al, 2004), tubuliform silks (Hu et al, 2006a), flagelliform silks (Lewis, 2006) and minor ampullate silks (Liivak et al, 1997 Gosline et al 1999;Hayashi et al, 2004;Livak et al, 1997;Lawrence et al, 2004;Hu et al 2006a. nd = no data Table 2.…”

“…Tubuliform silks have been shown to be highly extensible, but display lower tensile breaking strength relative to dragline silk fibers (Hu et al 2006a;Zhao et al, 2006). However, because tubuliform silks have increased extensibility relative to dragline silks, tubuliform fibers display similar toughness (Table 2).…”