Abstract
Spider aciniform (wrapping) silk is a remarkable fibrillar biomaterial with outstanding mechanical properties. It is a modular protein consisting, in Argiope trifasciata, of a core repetitive domain of 200 amino acid units (W units). In solution, the W units comprise a globular folded core, with five α-helices, and disordered tails that are linked to form a ~63-residue intrinsically disordered linker in concatemers. Herein, we present nuclear magnetic resonance (NMR) spectroscopy-based 15N spin relaxation analysis, allowing characterization of backbone dynamics as a function of residue on the ps–ns timescale in the context of the single W unit (W1) and the two unit concatemer (W2). Unambiguous mapping of backbone dynamics throughout W2 was made possible by segmental NMR active isotope-enrichment through split intein-mediated trans-splicing. Spectral density mapping for W1 and W2 reveals a striking disparity in dynamics between the folded core and the disordered linker and tail regions. These data are also consistent with rotational diffusion behaviour where each globular domain tumbles almost independently of its neighbour. At a localized level, helix 5 exhibits elevated high frequency dynamics relative to the proximal helix 4, supporting a model of fibrillogenesis where this helix unfolds as part of the transition to a mixed α-helix/β-sheet fibre.
Highlights
Spider aciniform silk is the toughest type of silk and is a remarkable biomaterial with outstanding mechanical properties [1]
Mean values of T1 and T2 were determined for four subdivided regions of the W unit chosen based upon our previous structural and 19F-nuclear magnetic resonance (NMR) studies: the globular core, helix 5 within the core, tails (W1: residues 1–11 and 150–199; W2: residues 1–11 and 350–400), and the linker (Figure 2)
We recently demonstrated that the W unit is composed of a well-folded globular domain of ~138 residues connected to adjacent globular domains by intrinsically-disordered linkers ~62 residues in length [19]
Summary
Spider aciniform (or wrapping) silk is the toughest type of silk and is a remarkable biomaterial with outstanding mechanical properties [1]. Spider silk proteins (spidroins) and silkworm silk proteins (fibroins) share a general architecture of a relatively long repetitive domain, comprising a concatenated series of repetitive units or sequence motifs, flanked by much shorter non-repetitive N- and C-terminal domains [2,3]. In Argiope trifasciata, it is a modular protein containing at least 14 identical concatenated repeats of a 200 amino acid unit (termed here “W” units, from wrapping) [4]. In which discrete structured modules are connected together by linkers that range from rigid to highly flexible, is common in nature [5,6,7]. The orientation of domains relative to one another, their dynamics, and the relation between domains is crucial for expanding our understanding of their function [9,12,13]
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