Mosquitoes Possess Specialized Cuticular Proteins That Are Evolutionarily Related to the Elastic Protein Resilin

Simple Summary Insects possess elastic cuticles in movable body parts such as the wing hinges and the basement of the hind legs of jumping insects. These cuticles contain a rubber-like protein called resilin. Resilin is a member of a cuticular protein family with a specific binding domain to chitin, which is another component of the cuticle. Resilin, or resilin-like proteins, have been reported in many insects. In a malaria vector mosquito, a curious cuticular protein has been found. It has a chitin-binding domain that is highly homologous to resilin, but it lacks repetitive sequences that are common in resilin and are essential for its elastic properties. It was unknown whether this protein is conserved in mosquitoes, whether this protein is the mosquito resilin, or whether mosquitoes possess another canonical resilin. In this paper, we found that this protein (termed resilin-related) is conserved in mosquitoes and seems to be evolutionarily derived from the resilin. Resilin-related, however, has very different structural features from resilin. Therefore, it probably plays special roles distinct from those of resilin in mosquitoes. Another resilin-like protein was found to exist commonly in holometabolous insects. Although this protein is likewise not a canonical resilin, it might confer elastic properties to cuticles in mosquitoes.Abstract Resilin is an elastic protein that is vital to insects' vigorous movement. Canonical resilin proteins possess the R&R Consensus, a chitin-binding domain conserved in a family of cuticular proteins, and highly repetitive sequences conferring elastic properties. In the malaria vector mosquito, Anopheles gambiae, however, a cuticular protein has been found that has an R&R Consensus resembling that of resilin but lacks the repetitive sequences (here, we call it resilin-related or resilin-r). The relationship between resilin-r and resilin was unclear. It was also unknown whether resilin-r is conserved in mosquitoes. In this paper, phylogenetic and structural analyses were performed to reveal the relationship of resilin homologous proteins from holometabolous insects. Their chitin-binding abilities were also assessed. A resilin-r was found in each mosquito species, and these proteins constitute a clade with resilin from other insects based on the R&R Consensus sequences, indicating an evolutionary relationship between resilin-r and resilin. The resilin-r showed chitin-binding activity as same as resilin, but had distinct structural features from resilin, suggesting that it plays specialized roles in the mosquito cuticle. Another resilin-like protein was found to exist in each holometabolous insect that possesses resilin-like repetitive sequences but lacks the R&R Consensus. These results suggest that similar evolutionary events occurred to create resilin-r and resilin-like proteins.