The effect of proline on the network structure of major ampullate silks as inferred from their mechanical and optical properties

Abstract

SUMMARYThe silk that orb-weaving spiders produce for use as dragline and for the frame of the web is spun from the major ampullate (MA) glands, and it is renowned for its exceptional toughness. The fibroins that make up MA silk have previously been organized into two major groupings, spidroin-1 and spidroin-2, based largely on differences in amino acid sequence. The most apparent difference between spidroin-1 and spidroin-2 fibroins is the lack of proline in spidroin-1. The MA silk of Araneus diadematus comprises two spidroin-2 fibroins, and is therefore proline-rich, whereas spidroin-1 is preferentially expressed in Nephila clavipes MA silk, and so this silk is proline deficient. Together, these two silks provide a system for testing the consequences of prolinerich and proline-deficient fibroin networks. This study measures the mechanical and optical properties of dry and hydrated Araneus and Nephila MA silks. Since proline acts to disrupt secondary structure, it is hypothesized that the fibroin network of Araneus MA silk will contain less secondary structure than the network of Nephila MA silk. Mechanical and optical studies clearly support this hypothesis. Although the dry properties of these two silks are indistinguishable, there are large differences between the hydrated silks. Nephila silk does not swell upon hydration to the same degree as Araneus silk. In addition, upon hydration, Nephila MA silk retains more of its initial dry stiffness, and retains more molecular order, as indicated by birefringence measurements.

The critical role of water in spider silk and its consequence for protein mechanics

“…The outer core does not provide this mechanism and, rather than contributing to toughness, is more likely to play a dominant role in the stiffness of the fibre, and is responsible for the linearity of the stress-strain behaviour in the elastic region. We speculate that the inner core region will exhibit a 'J' shaped stress-strain curve under ambient conditions, similar to that observed in bulk studies of supercontracted fibres 14,34 .…”

Wet webs work better: Humidity, supercontraction and the performance of spider orb webs

“…Unrestrained MA silk fibers consequently shrink by up to half their length, while restrained fibers instead develop high tension (Bell et al, 2002;Boutry and Blackledge, 2010;Savage et al, 2004). Supercontracted silk is also up to 1000 times more compliant and several times more extensible than dry silk (Guinea et al, 2005;Pérez-Rigueiro et al, 2003;Savage and Gosline, 2008a;Shao et al, 1999). Such changes in material properties could substantially affect how spider webs interact with prey.…”

Evolution of supercontraction in spider silk: structure–function relationship from tarantulas to orb-weavers

“…There is no consensus as to what structures are formed by the glycine-prolineglycine motifs. They have been described as helical fractions (Vollrath and Porter, 2009), proline-rich network chains (Savage and Gosline, 2008a), -spirals (Hayashi and Lewis, 1998;Hayashi et al, 1999) and various types of -turns (Ohgo et al, 2006). In this paper, we will simply refer to these motifs as GPGXX motifs, where G represents glycine, P proline and X any one of a small subset of amino acids.…”