Methionine Redox Controlled Crystallization of Biosynthetic Silk Spidroin

The formation of intractable β-sheet crystallites is a major cause of insolubility in proteins that can form β-sheets. To study this phenomenon, recombinant DNA techniques were used to prepare a protein modeling the consensus sequence of Nephila clavipes spider dragline silk, incorporating redox “triggering” residues. X-ray diffraction, electron diffraction, transmission electron microscopy (TEM), and Fourier transform infrared spectroscopy (FTIR) were used to characterize the ability of the recombinant protein to form β-sheet crystals dependent on the redox trigger oxidation state. Changes in the crystallinity were observed when triggered (oxidized/soluble) and untriggered (reduced/insoluble) protein samples were compared. The β-sheet content was undetectable in the triggered state, while clear evidence of β-sheet crystallinity was observed in the untriggered state. TEM and electron diffraction data of thin films of the untriggered protein indicated that spontaneous local orientation of needlelike crystalline aggregates occurs over small regions, suggestive of morphologies analogous to native dragline silk. There was also evidence of a liquid crystalline or oriented amorphous phase in the untriggered protein, but the d-spacings observed for the liquid crystal did not match any structure reported for the natural spider silk. To elucidate the behavior of the amorphous phase, we synthesized a water-soluble 27-residue peptide model of the dragline silk consensus amorphous sequence. The interchain packing distance observed for crystals of this peptide matched the d-spacing observed for the amorphous phase in the untriggered recombinant dragline silk protein. The results suggest that methionine-modified silks assemble into a silk-like structure in the reduced state (untriggered), structure which is lost upon oxidation or activation of the triggers.