Fig. 2: Natural fiber assembly processes of selected protein families.
From: Bioinspired and biomimetic protein-based fibers and their applications

a Images of natural silk spinning glands. Upper panel: left: Spinning gland of B. mori divided into anterior (ASG), middle (MSG) and posterior (PSG) silk gland. Right: Spinning gland of spiders, divided into duct, sac and tail. Adapted and modified with permission from ref. 47, licensed under CC BY 4.0. Lower panel: In spiders, the silk proteins are secreted in the tail and undergo a liquid-liquid phase separation yielding micelle-like droplets within the ampulla. In the duct, chaotropic salts (NaCl) are exchanged by kosmotropic ones (K2PO3), and a pH shift occurs (indicated in blue and orange). Upon elongational flow, the fiber is formed and pulled out of the spinning duct. b α-and β-keratins form dimers, which assemble into protofilaments, further assembling into intermediate filaments. c The assembly of collagens starts with procollagen. The N- and C-terminal domains are degraded by proteases, initiating self-assembly. Lysyloxidase is cross-linking the tropocollagens resulting in collagen fibrils, which assemble into fibers. d The assembly process of elastins is similar to that of collagens, starting with the precursor tropoelastin, which is degraded enzymatically. Fibulin interacts with the precursor initiating the self-assembly to microfibrils. Lysyloxidase is cross-linking the microfibrils resulting in elastic elastin-fibers.