Novel Amino Acid Assembly in the Silk Tubes of Arid-Adapted Segestriid Spiders

We investigated in different sites inside or outside the Namib Desert the amino acids composition of the protein material forming the tube silk of Ariadna spiders. These spiders belong to the primitive Segestriidae family and spend their life inside vertical silk burrows dug within the sandy and gravelly soil of arid areas. The silks, previously purified by solubilization in hexafluoroisopropanol, were subjected to partial or total acid hydrolysis. Partial hydrolyzed samples, analyzed by mass spectrometry (matrix assisted laser desorption/ionization and electrospray), led to relevant information on the amino acid sequences in the proteins. The free amino acids formed by complete hydrolysis were derivatized with the Marfey's reagent and characterized by electrospray mass spectrometry. The reconstruction of the amino acids highlights a homogeneous plan in the chemical structure of all the analyzed silks. Eight amino acids constituting the primary structure of the proteins were identified. Alanine and glycine are the most abundant ones, with a prevalence of alanine, constituting together at least 61% of the chemical composition of the protein material, differently from what occurs in known spidroins. High percentages of proline, serine and threonine and low percentages of leucine complete the peculiarity of these proteins. The purified silks were also characterized by Fourier-transform Infrared Spectroscopy and their thermal properties were investigated by differential scanning calorimetry. The comparison of the silk tubes among the various Namibian populations, carried out through a multivariate statistical analysis, shows significant differences in their amino acid assembly possibly due to habitat features.