Transition to a β-Sheet-Rich Structure in Spidroin in Vitro:  The Effects of pH and Cations

Abstract

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.

The elaborate structure of spider silk

“…43 Several mechanisms are thought to be necessary to gain and maintain the high silk protein concentrations (up to 50% w/v) 44 in the gland, including lyotropic liquid crystallinity, glycosylation of the outer surface of the folded silk proteins and phase separation induced by a polyol or by a phospholipid surfactant. 45 Starting with an almost entire random coil structure in the gland, silk proteins rapidly assemble upon passage through the spinning duct and the silk structure becomes water-insoluble (Fig.…”

Tailored Spider Silk: The Future Solution to Biomaterials

“…Dicko et al [12] have shown that assembly process cannot initiate with globular folded protein monomers. It can start essentially with unfolded proteins at very high concentrations [12].…”

“…It can start essentially with unfolded proteins at very high concentrations [12]. In order to keep and maintain high concentrations of protein, Hijirida et al [13] have reported that several mechanisms should be necessary to keep these high concentrations as high as up to fifty percent w/v [13] inside the insect.…”

Parasitoid suppression and life-history modifications in a wolf spider following infection by larvae of an acrocerid fly

“…Solidification of the fiber occurs when the structure of these micelles is disrupted such that the termini can dimerize, and the crystal forming motifs in the central repetitive regions of the proteins are no longer isolated so that their hydrophobic nature instead leads to the formation of b-sheets that stack together and interlock individual fibroins Askarieh et al 2010;Hagn et al 2010). This process is mediated by a combination of water resorption, ion exchange, drop in pH, and shear flow as the dope passes through an elongated ''S''-shaped duct (Dicko et al 2004;Lefevre et al 2008;Askarieh et al 2010). A final draw-down of now solid, but still wet fiber occurs at the narrow distal end of the duct, which is mediated in part by a muscled valve in orb spiders Ortlepp & Gosline 2004;Pérez-Rigueiro et al 2005).…”