Unconventional Spidroin Assemblies in Aqueous Dope for Spinning into Tough Synthetic Fibers

Spider dragline silk is a remarkable fiber made by spiders from an aqueous solution of spidroins, and this feat is largely attributed to the tripartite domain architecture of the silk proteins leading to the hierarchical assembly at the nano- and microscales. Although individual amino- and carboxy-terminal domains have been proposed to relate to silk protein assembly, their tentative synergizing roles in recombinant spidroin storage and spinning into synthetic fibers remain elusive. Here, we show biosynthesis and self-assembly of a mimic spidroin composed of amino- and carboxy-terminal domains bracketing 16 consensus repeats of the core region from spider Trichonephila clavipes. The presence of both termini was found essential for self-assembly of the mimic spidroin termed N16C into fibril-like (rather than canonical micellar) nanostructures in concentrated aqueous dope and ordered alignment of these nanofibrils upon extrusion into an acidic coagulation bath. This ultimately led to continuous, macroscopic fibers with a tensile fracture toughness of 100.9 ± 13.2 MJ m^-3, which is comparable to that of their natural counterparts. We also found that the recombinant proteins lacking one or both termini were unable to similarly preassemble into fibrillar nanostructures in dopes and thus yielded inferior fiber properties. This work thereby highlights the synergizing role of terminal domains in the storage and processing of recombinant analogues into tough synthetic fibers.