Structural Evidence For The Micellar Model Of Spider Silk Fibrillogenesis

Spider silks are outstanding biomaterials with strength and toughness that surpass today's synthetic materials. Despite their desirable properties, very little is currently known about their molecular-level structure and how it relates to mechanical properties. We present the structure of a recombinantly produced uniformly 13-C and 15-N labeled 199aa repeating unit of the Argiope trafisciata aciniform spidroin 1 determined using solution-state nuclear magnetic resonance (NMR) spectroscopy. This protein is a key constituent of egg case sacs, providing both flexibility and strength. The repeat unit has a core 6-helix bundle with an unstructured, ∼50 amino acid flexible C-terminal tail. In parallel, atomic force microscopy (AFM) is being used to characterize both mechanical properties and the mechanism of fibril formation of recombinant wrapping silk comprised of multiple repeat units. There are two competing theories for spider silk fibrillogenesis, one involving micellar intermediates, the other involving nucleation and growth by insoluble aggregate formation. Our AFM studies in direct correlation with the structural properties of the repeat unit are strongly suggestive of the micellar model for fibril formation.