MOORE,A.M.F.*; CHIU,H.; LAWRENCE,B.A.: Mechanical Characterization of Egg Case Silk in the Black Widow Spider (Latrodectus hesperus)

Silk from egg case of the black widow spider (Latrodectus hesperus) differs in amino acid composition from the animal’s dragline silk. Because the primary function of an egg case is non-load bearing, we expected that the molecular differences in the two silks might lead to differences in their material properties. To test this hypothesis, we stretched egg-case silk to breaking and compared the resulting stress-strain curve to the stress-strain curve of the animal’s dragline silk. The two curves are similar in that they both have an initial linear region, followed by a yield point and then a second, less stiff, linear region. The presence of a yield point suggests a conformational change in the protein that, in turn, changes the way the molecular interactions resist the load as the silk is stretched. The two silks are also similar in strength and in the stiffness of the initial linear region of the curve. However, they differ in that (1) egg case silk is twice as extensible as dragline silk, (2) its yield point occurs at about three times higher strain and (3) after the yield point, egg case silk is almost an order of magnitude less stiff than dragline silk. We used carbon-13 solid state NMR spectroscopy to look at the secondary structure of the silk proteins. The spectra show that the alanine in both dragline and egg case silk is in beta-sheet conformation. This is consistent with the presence of alanine-rich beta-sheet crystals commonly found in silks. The spectra also confirm the presence of a large amount of serine in the egg case silk that is not present in dragline silk. It is possible that serine affects the non-crystaline region of the molecule to cause the observed differences in bulk properties. (NSF DBI-9996072).