Posttranslational modifications in spider silk influence conformation and dimerization dynamics

Abstract

Spider silk is an archetypal biopolymer material with extreme tensile properties arising from its complex hierarchical assembly. While recent advances in sequencing have yielded abundant insights, relatively little is known concerning post-translational modifications (PTMs) in spider silk. Here, we probe the PTM landscape of dragline silk from the Jorō spider (Trichonephila clavata) using a combination of mass spectroscopy and solid-state nuclear magnetic resonance (NMR). The results reveal a wide array of potential modifications, including hydroxyproline, phosphorylation, and dityrosine cross-links, encompassing the different spidroin constituents. Notably, the MaSp3 repetitive region displayed numerous PTMs, whereas MaSp1 and MaSp2 variants showed distinct phosphorylation patterns in its terminal domains. The N-terminal domain (NTD) phosphorylation sites were found predominantly at the dimer interface, suggesting a modulatory function with respect to its pH-driven dimerization function, a hypothesis supported by studies using phosphomimetic NTD mutants. Possible roles of phosphoserine in limiting β-sheet formation, and hydroxyproline in disrupting β-turns are also discussed.

Impact statement

Spider silk is an archetypal biomaterial that can outperform our most sophisticated artificial fibers. The secret to its mechanical properties lies in its complex hierarchical structure—encompassing the nano- to macroscales—that forms through a process of molecular self-assembly of the constituent spidroin proteins. While recent advances in "biomateriomics” have given us tremendous insights into the sequence–function relationships that determine spider silk behavior, the picture is still far from complete. One area that has received little attention is posttranslational modifications (PTMs). PTMs are ubiquitous biological phenomena that are crucial for providing dynamic control of the proteome, and effectively expand the structural and functional design space of proteins beyond that provided by the canonical amino acids. Here, we undertook a comprehensive analysis of PTMs from spider dragline silk fiber, which revealed numerous potential sites for a wide array of modifications. The results provide a fascinating window into additional layers of complexity underlying the mechanical behavior of spider silk, and suggest further avenues for creating novel, dynamically tunable, bioinspired materials.

Graphical abstract