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Walker, Andrew. A.
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document
A new class of animal collagen masquerading as an insect silk
Abstract
Collagen is ubiquitous in the animal kingdom, where it comprises some 28 types that form the extracellular matrix within organisms.More than 50 years ago, Rudall proposed an extracorporeal collagen in the cocoon of certain hymenopteran insects.He showed that the X-ray fibre diffraction pattern of cocoon silk from the gooseberry sawfly had a collagen-like structure.However, amino acid analysis showed that it did not contain any hydroxyproline, which was a characteristic feature of animal collagens.In the present study we have examined the cocoon silk from a related sawfly, the willow sawfly, N. oligospilus.X-ray diffraction of the willow sawfly fibres also showed the reflection at 0.286 nm that is characteristic of mammalian collagen axial spacing.Mechanical testing of dry cocoon silk fibres showed that they were stronger than dry collagen fibres from mammalian tissues.Protein and mRNA were isolated from excised silk glands including the secretory cells.The protein extract showed 3 predominant protein bands.The cDNA prepared from the silk gland extract led to the identification of 3 different collagen chains.Each chain had a signal peptide, plus both N- and C-terminal non-collagenous terminal domains.Whereas the collagen domains were of similar length for each sequence, the terminal domains were quite variable in length.The cocoon proteins did not contain any hydroxyproline.MS of tryptic peptides of cocoon proteins were shown to be identical to those in the sequence data, including the non-collagenous domains, showing that these are present in the mature protein.Each protein was also produced as a recombinant product.These individual proteins folded as triple-helices as the collagen domains were resistant to pepsin and gave CD spectra with 220nm maxima of the triple-helix.Although there are 3 distinct chains it is not yet known if these form a heterotrimer or 3 separate homotrimer molecules.