Silk Spinning in Silkworms and Spiders

Abstract

Spiders and silkworms spin silks that outcompete the toughness of all natural and manmade fibers. Herein, we compare and contrast the spinning of silk in silkworms and spiders, with the aim of identifying features that are important for fiber formation. Although spiders and silkworms are very distantly related, some features of spinning silk seem to be universal. Both spiders and silkworms produce large silk proteins that are highly repetitive and extremely soluble at high pH, likely due to the globular terminal domains that flank an intermediate repetitive region. The silk proteins are produced and stored at a very high concentration in glands, and then transported along a narrowing tube in which they change conformation in response primarily to a pH gradient generated by carbonic anhydrase and proton pumps, as well as to ions and shear forces. The silk proteins thereby convert from random coil and alpha helical soluble conformations to beta sheet fibers. We suggest that factors that need to be optimized for successful production of artificial silk proteins capable of forming tough fibers include protein solubility, pH sensitivity, and preservation of natively folded proteins throughout the purification and initial spinning processes.

Keywords: Bombyx mori; carbonic anhydrase; fibroin; major ampullate gland; pH gradient; protein conformation; spidroin.

Figure 1

Macroscopic appearance of a B. mori silk gland with anterior silk gland (ASG), Funnel, middle silk gland (MSG) and posterior silk gland (PSG) identified (a) and a Major ampullate gland with Duct, Funnel, Sac and Tail indicated (b); Schematic image of a B. mori silk gland (c) and major ampullate gland (d) with pH values indicated in different parts. In (c) and (d) the regions containing epithelial cells with CA activity are shaded in grey, and fibroin/spidroin secreting parts are purple. Adapted from [8] (a,c) and [9,10] (b,d).

Figure 2

Transmission electron micrograph of a cross-section of the third limb of the duct of an Euprosthenops australis major ampullate gland (a) and a hematoxylin-eosin stained light micrograph of a cross-section of the ASG from a B. mori silk gland (b). Mv: microvilli, Lu: lumen, CI: Cuticular intima. Scale bars (a) 2 µm (b) 15 µm. In (a) the microvilli appear detatched from the cuticular intima, likely due to processing of the section for transmission electron microscopy.

Figure 3

Stress vs. strain curves of forcibly silked native dragline silk fibers from A. trifasciata (in red) and silk fibers from B. mori (in blue). Original data from [88,89].

Figure 4

Fibrillation propensity profiles of the first 100 amino acid residues of B. mori fibroin heavy chain repetitive part (AF226688.1) (top) and three repetitive blocks from E. australis MaSp1 (GenBank AM490183) (bottom). The amino acid residues are presented in one-letter format above each plot. The energies are color-coded, blue and green representing higher energies while orange and red represents lower energies that are deemed to have high fibrillation propensity. Profiles generated by ZipperDB [93].