Transition to a beta-sheet-rich structure in spidroin in vitro: the effects of pH and cations

Unlike man-made fibers, the silks of spiders are spun from aqueous solutions and at atmospheric pressure in a process still poorly understood. The molecular mechanism of this process involves the conversion of a highly concentrated, predominantly disordered silk protein (spidroin) into beta-sheet-rich structures. To help store and transport the spidroins in solution, as well as probably control their conversion, a liquid crystalline arrangement is established in the storage region in the ampulla and persists into the duct. Although it has been suggested that changes in the concentration of hydrogen and metal ions play a role in the formation of the solid thread, there is no reported evidence that these ions influence the secondary structure of native spidroin in solution. Here, we demonstrate that pH values between approximately 3.5 and 4.5 induce a slow change of conformation from the disordered to the beta-sheet-rich form. We also report that Al(3+), K(+), and Na(+) ions induce similar changes in structure, while Ca(2+) and Mg(2+) stabilize the predominantly disorder state of the protein. Cs(+) and Li(+) have no apparent effect. Direct volumetric and spectrophotometric titration showed a pI of 4.22 +/- 0.33 and apparent pK values of 6.74 +/- 0.71 and 9.21 +/- 0.27, suggesting a mechanism for the effect of low pH on the protein and a rationale for the observed reduction in pH in the duct. We discuss the importance of these findings for the spinning process and the active role played by the spider to alter the kinetics of the transition.