Spider silk consists of protein molecules, long chains comprising thousands of amino-acid elements. X-ray structure analyses show that the finished fiber has areas in which several protein chains are interlinked via stable physical connections. These connections provide the high stability. Between these connections are unlinked areas that give the fibers their great elasticity.
The situation within the silk gland is, however, very different: The silk proteins are stored in high concentrations in an aqueous environment, awaiting deployment. The areas responsible for interlinking may not approach each other too closely; otherwise the proteins would clump up instantaneously. Hence, these molecules must have some kind of special storage configuration.
X-ray structure analysis, which is so successful in other domains, was of little help here, since it can only be used to analyze crystals. And up to the instant in which the solid silk fiber is formed, everything takes place in solution. The method of choice was therefore nuclear magnetic resonance spectroscopy (NMR). Using the equipment of the Bavarian NMR Center, Franz Hagn, a biochemist at the Institute for Advanced Study (TUM-IAS) at the TU Muenchen, managed to unravel the structure of a control element responsible for the formation of the solid fiber. Now the researchers could investigate this control element's mode of operation.
"Under storage conditions in the silk gland these control domains are connected pair-wise in such a way that the interlinking areas of both chains can not lie parallel to each other," co-author Thomas Scheibel explains. "Interlinking is thus effectively prevented." The protein chains are stored with the polar areas on the outside and the hydrophobic parts of the chain on the inside, ensuring good solubility in the aqueous environment.
When the protected proteins enter the spinning duct, they encounter an environment with an entirely different salt concentration and composition. This renders two salt bridges of the control domain unstable, and the chain can unfold. Furthermore, the flow in the narrow spinning duct results in strong shear forces. The long protein chains are aligned in parallel, thus placing the areas responsible for interlinking side by side. The stable spider silk fiber is formed.
"Our results have shown that the molecular switch we discovered at the C-terminal end of the protein chain is decisive, both for safe storage and for the fiber formation process," says lead author Franz Hagn.
Citation: Hagn et al., 'A conserved spider silk domain acts as a molecular switch that controls fibre assembly', Nature, 465, 239-242, May 2010; doi:10.1038/nature08936 Letter