Exploring the self-assembly of silk proteins through liquid-liquid phase separation

Silk fibers have been used by humans for millennia to create textiles and have recently gained the attention of scientists due to their unsurpassed mechanical properties. These properties arise from a sophisticated process by which the starting material, a liquid feedstock consisting of high-molecular-weight silk proteins, is rapidly converted within silk glands into solid fibers with a multi-scale hierarchical structure that is responsible for the material’s incredible robustness. Recently, liquid-liquid phase separation (LLPS) has emerged as a powerful framework for understanding the self-assembly behavior of silk proteins. Interestingly, LLPS-associated proteins typically exhibit disordered or dynamic conformations and have sequences rich in low-complexity multivalent repeats, reminiscent of silk protein sequences. In this review, we explore the evidence indicating that LLPS is a major aspect of silk fiber storage and assembly in both lepidopteran and spider systems. We discuss insights derived from comparative analyses of amino acid sequences, specific chemical triggers, and potential chemical interactions and contextualize the results from recent empirical investigations based on native and recombinant silk materials. We also discuss how LLPS mechanisms might be applied to the sustainable production of silk-like materials that replicate native hierarchical structures. Finally, we outline important areas for future investigations and speculate on how findings from the field of silk research may help illuminate the more general field of biomolecular condensates. The production of silk in spiders and silkworms involves the transformation of concentrated liquid protein feedstock into hierarchically organized solid fibers through a highly controlled mechanism facilitated by their respective glandular spinning apparatus. Recent insights suggest that liquid–liquid phase separation (LLPS) plays a central role in organizing the initially disordered silk protein chains into dense yet dynamic condensates, which is a key step towards rapid fiber formation. This hierarchical assembly process underlies the remarkable mechanical properties of silk fibers.