Interpreting nature’s finest insect silks (Order Embioptera): hydropathy, interrupted repetitive motifs, and fiber-to-film transformation for two neotropical species

Abstract Silks produced by webspinners (Order Embioptera) interact with water by transforming from fiber to film, which then becomes slippery and capable of shedding water. We chose to explore this mechanism by analyzing and comparing the silk protein transcripts of two species with overlapping distributions in Trinidad but from different taxonomic families. The transcript of one, Antipaluria urichi (Clothodidae), was partially characterized in 2009 providing a control for our methods to characterize a second species: Pararhagadochir trinitatis (Scelembiidae), a family that adds to the taxon sampling for this little known order of insects. Previous reports showed that embiopteran silk protein (dubbed Efibroin) consists of a protein core of repetitive motifs largely composed of glycine (Gly), serine (Ser), and alanine (Ala) and a highly conserved C-terminal region. Based on mRNA extracted from silk glands, Next Generation sequencing, and de novo assembly, P. trinitatis silk can be characterized by repetitive motifs of Gly-Ser followed periodically by Gly-Asparagine (Asn—an unusual amino acid for Efibroins) and by a lack of Ala which is otherwise common in Efibroins. The putative N-terminal domain, composed mostly of polar, charged and bulky amino acids, is ten amino acids long with cysteine in the 10th position—a feature likely related to stabilization of the silk fibers. The 29 amino acids of the C-terminus for P. trinitatis silk closely resemble that of other Efibroin sequences, which show 74% shared identity on average. Examination of hydropathicity of Efibroins of both P. trinitatis and An. urichi revealed that these proteins are largely hydrophilic despite having a thin lipid coating on each nano-fiber. We deduced that the hydrophilic quality differs for the two species: due to Ser and Asn for P. trinitatis silk and to previously undetected spacers in An. urichi silk. Spacers are known from some spider and silkworm silks but this is the first report of such for Embioptera. Analysis of hydropathicity revealed the largely hydrophilic quality of these silks and this feature likely explains why water causes the transformation from fiber to film. We compared spun silk to the transcript and detected not insignificant differences between the two measurements implying that as yet undetermined post-translational modifications of their silk may occur. In addition, we found evidence for codon bias in the nucleotides of the putative silk transcript for P. trinitatis, a feature also known for other embiopteran silk genes.