Abstract
The foundations of silk spinning, the structure, storage, and activation of silk proteins, remain highly debated. By combining solution small-angle neutron and X-ray scattering (SANS and SAXS) alongside circular dichroism (CD), we reveal a shape anisotropy of the four principal native spider silk feedstocks from Nephila edulis. We show that these proteins behave in solution like elongated semiflexible polymers with locally rigid sections. We demonstrated that minor ampullate and cylindriform proteins adopt a monomeric conformation, while major ampullate and flagelliform proteins have a preference for dimerization. From an evolutionary perspective, we propose that such dimerization arose to help the processing of disordered silk proteins. Collectively, our results provide insights into the molecular-scale processing of silk, uncovering a degree of evolutionary convergence in protein structures and chemistry that supports the macroscale micellar/pseudo liquid crystalline spinning mechanisms proposed by the community.
Highlights
Biological materials are typically grown while silks are spun.[1]
There has been considerable debate about whether the secret of a spider’s unique ability to tune silk fiber properties is the primary amino acid sequence[4,5] or the spinning process.[6,7]. It is a combination supported by two recent studies showing that major ampullate silk proteins are packed, and their reactivity modulated, in micellar to granular subunits.[8,9]
That the heterogeneity was not of the scattering entities, (ii) their shape, and (iii) the local reflected in the small-angle scattering (SAS) data
Summary
Biological materials are typically grown while silks are spun.[1]. The process of silk spinning has been shown to have a significant overlap with and can be taken as a model for industrial polymer processing whereby a liquid feedstock undergoes solidification into a fiber as a result of pultrusion.[1−3] Silk fibers start as aqueous protein melts[2,4,5] originating from bespoke glands, ducts, and spigots with each type of silk having a specific protein sequence.[6,7] Not surprisingly, there has been considerable debate about whether the secret of a spider’s unique ability to tune silk fiber properties is the primary amino acid sequence[4,5] or the spinning process.[6,7] Most likely, it is a combination supported by two recent studies showing that major ampullate silk proteins are packed, and their reactivity modulated, in micellar to granular subunits.[8,9] the amphipaticity of silk has, for long, been cited as the reason for molecular un- and re-folding.[10,11]One barrier to a better understanding of the full process has been the strong bias toward studying in spiders the major ampullate dragline silk and in insects the cocoon silk of the mulberry worm Bombyx mori. Two transformations can be applied to the SAS data presented in Figure 1: a Fourier transformation to obtain the pair distribution function p(r) and a Kratky transformation to estimate the folding and flexibility of the silks.[26] The pair distribution function p(r) for the silk proteins in aqueous solution (Figure 2) allows the estimation of the radius of gyration (Rg) and maximum size (Dmax).
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