More than one way to spin a crystallite: multiple trajectories through liquid crystallinity to solid silk

Abstract

Arthropods face several key challenges in processing concentrated feedstocks of proteins (silk dope) into solid, semi-crystalline silk fibres. Strikingly, independently evolved lineages of silk-producing organisms have converged on the use of liquid crystal intermediates (mesophases) to reduce the viscosity of silk dope and assist the formation of supramolecular structure. However, the exact nature of the liquid-crystal-forming-units (mesogens) in silk dope, and the relationship between liquid crystallinity, protein structure and silk processing is yet to be fully elucidated. In this review, we focus on emerging differences in this area between the canonical silks containing extended-β-sheets made by silkworms and spiders, and 'non-canonical' silks made by other insect taxa in which the final crystallites are coiled-coils, collagen helices or cross-β-sheets. We compared the amino acid sequences and processing of natural, regenerated and recombinant silk proteins, finding that canonical and non-canonical silk proteins show marked differences in length, architecture, amino acid content and protein folding. Canonical silk proteins are long, flexible in solution and amphipathic; these features allow them both to form large, micelle-like mesogens in solution, and to transition to a crystallite-containing form due to mechanical deformation near the liquid-solid transition. By contrast, non-canonical silk proteins are short and have rod or lath-like structures that are well suited to act both as mesogens and as crystallites without a major intervening phase transition. Given many non-canonical silk proteins can be produced at high yield in E. coli, and that mesophase formation is a versatile way to direct numerous kinds of supramolecular structure, further elucidation of the natural processing of non-canonical silk proteins may to lead to new developments in the production of advanced protein materials.

Keywords: coiled coil; collagen; cross-β-sheet; liquid crystal; mesophase; silk.

Figure 1.

Canonical and non-canonical silks. (a) Silkworm (B. mori) cocoon fibres, a canonical silk. (b–d) Non-canonical silks produced by other insect species. (b) Lacewing (M. signata) egg-stalk silk, photograph by Holly Trueman. (c) Sawfly (Nematus oligospilus) cocoon silk. (d) Honeybee (Apis mellifera) silk and wax on cell caps of a hive, photograph by Alex Wild.

Figure 2.

Comparison of amino acid sequences of proteins that form canonical and non-canonical silks. (a) Amino acid sequences of non-canonical silk proteins from aculeates, sawflies and lacewings showing convergence to short sequences with high-repeat regularity. (b) Canonical silk proteins such as silkworm H-fibroin (pictured) are typically very long with ‘spacer’ regions.

Figure 3.

Comparison of liquid crystalline processing of canonical and non-canonical silk protein. Canonical silk proteins from silkworms and spiders (left) assemble into large, micelle-like mesogens by virtue of their flexibility and amphiphilicity, while non-canonical silk proteins (right) fold into comparatively small rod or lath-like mesogens with well-defined secondary structure. Liquid crystalline intermediates reduce flow viscosity, assist solubility and participate in the formation of supramolecular structure. Solidification occurs concurrently with a structural transition to the final extended-β-sheet structure for canonical silk proteins, whereas non-canonical silk proteins do not change structure markedly during solidification.