Nano-channels in the spider fang for the transport of Zn ions to cross-link His-rich proteins pre-deposited in the cuticle matrix

Yael Politi,Eckhard Pippel,Ana C.J Licuco-Massouh,Luca Bertinetti,Horst Blumtritt,Friedrich G Barth,Peter Fratzl

doi:10.1016/j.asd.2016.06.001

Abstract

We identify the presence of multiple vascular channels within the spider fang. These channels seem to serve the transport of zinc to the tip of the fang to cross-link the protein matrix by binding to histidine residues. According to amino acid and elemental analysis of fangs extracted shortly after ecdysis, His-rich proteins are deposited before Zn is incorporated into the cuticle. Microscopic and spectroscopic investigations in the electron microscope and synchrotron radiation experiments suggest that Zn ions are transported through these channels in a liable (yet unidentified) form, and then form stable complexes upon His binding. The resulting cross-linking through the Zn–His complexes is conferring hardness to the fang. Our observations of nano-channels serving the Zn-transport within the His-rich protein matrix of the fibre reinforced spider fang may also support recent bio-inspired attempts to design artificial polymeric vascular materials for self-healing and in-situ curing.

Highlights

Spider fangs are mainly used as injection needles to puncture the prey's cuticle and to inject venom
In TEM images of unstained ethanol-stored slices taken from similar positions, i.e. the tip of the fang of adult spiders, objects with similar shape and size are observed, but here they show high electron density (Fig. 1E) and their content is identified as zinc oxide (ZnO) by selected-area electron diffraction (SAED) (Fig. S-1)
According to element specific (Zn, N) spectroscopic (XAS and energy loss spectroscopy (EELS)) analyses Zn is bound to the protein matrix at least in part via histidine residues which are abundant in the protein matrix of the fang tip

Summary

Introduction

Spider fangs are mainly used as injection needles to puncture the prey's cuticle and to inject venom. As shown for the wandering spider Cupiennius salei (Ctenidae), they are made of a specialised cuticle characterized by macro- and micro-structural gradients as well as compositional gradients along their length and within their tips (Bar-On et al, 2014; Erko et al, 2013; Politi et al, 2012). The material's local mechanical properties, its hardness and stiffness, as measured by nano-indentation on dry samples, were reported to increase in correlation with increased Zn concentration (Politi et al, 2012). Similar ZneHis interactions, with similar effect on the mechanical properties were reported earlier for annelid jaws (Nereis virens) with evidence from extended x-ray absorption fine structure (EXAFS) analysis for the occurrence of a Zn(His)3Cl complex (Lichtenegger et al, 2003)

Methods

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Discussion

Conclusion