Differential polymerization of the two main protein components of dragline silk during fibre spinning

Spider silks are some of the strongest materials found in nature(1,2). Achieving the high tensile strength and elasticity of the dragline of orb- weaving spiders, such as Nephila clavipes(3 - 5), is a principal goal in biomimetics research. The dragline has a composite nature and is predominantly made up by two proteins, the major ampullate spidroins 1 and 2 ( refs 3,6,7), which can be considered natural block copolymers(8). On the basis of their molecular structures both spidroins are thought to contribute, in different ways, to the mechanical properties of dragline silk(9). The spinning process itself is also considered important for determining the observed features by shaping the hierarchical structure of the fibre(10,11). Here we study the heterogeneous distribution of proteins along the radial axis of the fibre. This heterogeneity is generated during the conversion of the liquid spinning dope into solid fibre. Whereas spidroin 1 is distributed almost uniformly within the fibre core, spidroin 2 is missing in the periphery and is tightly packed in certain core areas. Our findings suggest that the role of spidroin 2 in the spinning process could be to facilitate the formation of fibrils and contribute directly to the elasticity of the silk.