title: The other prey-capture silk: Fibres made by glow-worms (Diptera: Keroplatidae) comprise cross-β-sheet crystallites in an abundant amorphous fraction.
abstract: Glow-worms (larvae of dipteran genus Arachnocampa) are restricted to moist habitats where they capture flying prey using snares composed of highly extensible silk fibres and sticky mucus droplets. Little is known about the composition or structure of glow-worm snares, or the extent of possible convergence between glow-worm and arachnid capture silks. We characterised Arachnocampa richardsae silk and mucus using X-ray scattering, Fourier transform infrared spectroscopy and amino acid analysis. Silk but not mucus contained crystallites of the cross-β-sheet type, which occur in unrelated insect silks but have not been reported previously in fibres used for prey capture. Mucus proteins were rich in Gly (28.5%) and existed in predominantly a random coil structure, typical of many adhesive proteins. In contrast, the silk fibres were unusually rich in charged and polar residues, particularly Lys (18.1%), which we propose is related to their use in a highly hydrated state. Comparison of X-ray scattering, infrared spectroscopy and amino acid analysis data suggests that silk fibres contain a high fraction of disordered protein. We suggest that in the native hydrated state, silk fibres are capable of extension via deformation of both disordered regions and cross-β-sheet crystallites, and that high extensibility is an adaptation promoting successful prey capture. This study illustrates the rich variety of protein motifs that are available for recruitment into biopolymers, and how convergently evolved materials can nevertheless be based on fundamentally different protein structures.
authors.affiliation: Evolution, Ecology and Genetics, Research School of Biology, Australian National University, Canberra, 0200, Australia; Ecosystem Sciences, Commonwealth Scientific and Industrial Research Organisation, Black Mountain Laboratories, Canberra, 2601, Australia.
authors.forename: Andrew A
authors.initials: AA
authors.lastname: Walker
authors.affiliation: Ecosystem Sciences, Commonwealth Scientific and Industrial Research Organisation, Black Mountain Laboratories, Canberra, 2601, Australia.
authors.forename: Sarah
authors.initials: S
authors.lastname: Weisman
authors.affiliation: Ecosystem Sciences, Commonwealth Scientific and Industrial Research Organisation, Black Mountain Laboratories, Canberra, 2601, Australia.
authors.forename: Holly E
authors.initials: HE
authors.lastname: Trueman
authors.affiliation: School of Biological Sciences, The University of Queensland, Brisbane, Queensland, '4072', Australia.
authors.forename: David J
authors.initials: DJ
authors.lastname: Merritt
authors.affiliation: Ecosystem Sciences, Commonwealth Scientific and Industrial Research Organisation, Black Mountain Laboratories, Canberra, 2601, Australia. Electronic address: Tara.Sutherland@csiro.au.
authors.forename: Tara D
authors.initials: TD
authors.lastname: Sutherland
mesh_terms: D000818:Animals; D001522:Behavior, Animal; D004175:Diptera / Q000235:genetics; D057927:Hydrophobic and Hydrophilic Interactions; D019476:Insect Proteins / Q000737:chemistry* / Q000378:metabolism; D011480:Protease Inhibitors / Q000737:chemistry; D017433:Protein Structure, Secondary; D047011:Silk / Q000737:chemistry* / Q000378:metabolism; D059467:Transcriptome
chemical_list: D019476:Insect Proteins; D011480:Protease Inhibitors; D047011:Silk
keywords: Arachnocampa; Cross-β-sheet; Flagelliform; Glow-worm; Silk