Characterization of the egg glue protein from two belostomatidae family aquatic water bugs, Appasus japonicus and Lethocerus indicus.

Abstract

Egg glue proteins are essential for the survival of aquatic insects, as they provide the means for securely attaching eggs to substrates and preventing them from being dislodged by water. In this study, we aimed to identify and characterize the egg glue proteins from two species of Belostomatidae family aquatic insects, Appasus japonicus and Lethocerus indicus, which are both commonly known as "toe biters" or "(giant) water bugs". Both species exhibit unique adhesive strategies to secure their eggs in their respective environments: A. japonicus lay eggs on the back of the male, often being submerged underwater, while L. indicus deposit eggs on vegetation above the waterline. Using proteomic approaches, including mass spectrometry and amino acid analysis, we identified the cysteine-rich glue protein from the egg mass of A. japonicus. Using this sequence, an orthologous protein in the L. indicus genome was found, and the two proteins recombinantly produced for further study. As expected, the A. japonicus egg glue protein showed greater water-resistance than that of L. indicus with similar adhesive strengths in both dry and wet conditions. In addition, circular dichroism experiment and infrared spectroscopy suggested that structural transition takes place during the curing process to result in a higher level of β-sheet in the solid form. STATEMENT OF SIGNIFICANCE: Aquatic insects keep their eggs securely attached near watery environments, but the glue they use has remained largely unstudied at the molecular level. This research identifies and analyzes the egg glue proteins of two "giant water bugs" with different egg-laying strategies: Appasus japonicus, which lays eggs on the backs of males, and Lethocerus indicus, which attaches eggs to plant stalks. We discovered that the cysteine-rich glue protein in each species is ∼13 kDa in size sharing ∼62% identity in their amino acid sequence. We also found that their adhesiveness relies on disulfide bonding between cysteine residues. This work expands our understanding of insect reproductive strategies and exemplifies another protein-based water-resistant adhesion, which may benefit in designing bioadhesives.