Mechanisms of native and artificial spinning of spider silk

Sammanfattning: Spider silk is tougher than all other known natural and man-made fibers, and represents an environmentally friendly material that could potentially be used for many different purposes, ranging from biomaterials to construction materials. However, for large-scale production of silk, methods to produce artificial silk fibers must be developed. In this thesis, the molecular mechanisms of silk spinning were studied with the aim of developing a biomimetic method for production of artificial spider silk fibers. Major ampullate glands of spiders were studied using light and transmission electron microscopy. Three different epithelial cell types were identified in the tail and sac, two of which produce spider silk proteins (spidroins) that make up the silk and one of which produces carbonic anhydrase that maintains a pH gradient along the gland. The pH gradient was determined to go from pH 7.6 in the tail to pH 5.7 halfway along the duct. Silkworm silk glands were also shown to contain several different epithelial cell types and it was determined that carbonic anhydrase maintains a pH gradient from 8.2 to 6.2 along the gland. Spidroins consist of a repetitive region with alternating poly-alanine blocks and glycine-rich repeats, flanked by highly conserved globular domains, the N-terminal (NT) and C-terminal (CT) domain. Recombinant versions of NT and CT were studied under the conditions found in the major ampullate gland, and were shown to regulate fiber formation by responding to pH in coordinated but opposite ways, following a lock and trigger mechanism. While NT gets more stable and dimerizes as pH is lowered, and thereby interconnects the spidroins into large networks (lock), CT is destabilized, unfolds and turns into β-sheet amyloid-like fibrils in response to low pH and high pCO2, which may nucleate further β-sheet formation of the repetitive region (trigger). Based on the knowledge generated on native silk spinning, a biomimetic method to spin artificial spider silk fibers was developed. A chimeric recombinant spidroin was designed by combining a highly soluble NT and CT with a short repetitive region. The extremely soluble spidroin could be concentrated to unprecedented levels, and formed tough, kilometer-long fibers upon spinning into a low pH aqueous buffer.

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