Department of Chemistry, Northwestern University, Evanston, IL 60208.
Department of Materials Science and Engineering, Northwestern University, Evanston, IL 60208.
Proc Natl Acad Sci U S A. 2018 Nov 6;115(45):11507-11512. doi: 10.1073/pnas.1810203115. Epub 2018 Oct 22.
Many natural silks produced by spiders and insects are unique materials in their exceptional toughness and tensile strength, while being lightweight and biodegradable-properties that are currently unparalleled in synthetic materials. Myriad approaches have been attempted to prepare artificial silks from recombinant spider silk spidroins but have each failed to achieve the advantageous properties of the natural material. This is because of an incomplete understanding of the in vivo spidroin-to-fiber spinning process and, particularly, because of a lack of knowledge of the true morphological nature of spidroin nanostructures in the precursor dope solution and the mechanisms by which these nanostructures transform into micrometer-scale silk fibers. Herein we determine the physical form of the natural spidroin precursor nanostructures stored within spider glands that seed the formation of their silks and reveal the fundamental structural transformations that occur during the initial stages of extrusion en route to fiber formation. Using a combination of solution phase diffusion NMR and cryogenic transmission electron microscopy (cryo-TEM), we reveal direct evidence that the concentrated spidroin proteins are stored in the silk glands of black widow spiders as complex, hierarchical nanoassemblies (∼300 nm diameter) that are composed of micellar subdomains, substructures that themselves are engaged in the initial nanoscale transformations that occur in response to shear. We find that the established micelle theory of silk fiber precursor storage is incomplete and that the first steps toward liquid crystalline organization during silk spinning involve the fibrillization of nanoscale hierarchical micelle subdomains.
许多由蜘蛛和昆虫产生的天然丝是具有独特性能的材料，它们具有异常的韧性和拉伸强度，同时重量轻且可生物降解——这些性能是目前合成材料无法比拟的。人们尝试了多种方法来用重组蜘蛛丝丝蛋白制备人工丝，但都未能达到天然材料的有利性能。这是因为对体内丝蛋白到纤维纺丝过程的不完全了解，特别是对前纺丝液中丝蛋白纳米结构的真实形态以及这些纳米结构如何转化为微米级丝纤维的机制缺乏认识。本文中，我们确定了天然丝蛋白前体纳米结构在蜘蛛腺体内的物理形式，这些纳米结构是蜘蛛丝形成的种子，并揭示了在挤出过程中形成纤维的初始阶段发生的基本结构转变。我们结合溶液相扩散 NMR 和低温透射电子显微镜（cryo-TEM），直接证明了浓缩的丝蛋白以复杂的、分级纳米组装体（∼300nm 直径）的形式储存在黑寡妇蜘蛛的丝腺中，这些组装体由胶束亚结构组成，而这些亚结构本身参与了应对剪切时发生的初始纳米尺度转变。我们发现，先前的丝纤维前体储存的胶束理论并不完整，而且在纺丝过程中向液晶组织转变的第一步涉及到纳米级分级胶束亚结构的原纤化。
