Submitted to: Proceedings of the National Academy of Sciences
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/30/2011
Publication Date: 10/11/2011
Citation: Chaudhari, S.S., Arakane, Y., Specht, C.A., Moussian, B., Boyle, D., Park, Y., Kramer, K.J., Beeman, R.W., Muthukrishnan, S. 2011. Knickkopf protein protects and organizes chitin in the newly synthesized insect exoskeleton. Proceedings of the National Academy of Sciences. 108(41): 17028-17033. http://dx.doi.org/10.1073/pnas.1112288108. Interpretive Summary: The shedding and replacement of the exoskeleton (skin)is a unique and sensitive aspect of insect physiology that could be exploited by appropriately-targeted biopesticides. A vital aspect of this process is digestion and recycling of the old skin, and reuse of recycled components for synthesis of the new skin. A great mystery has been how the developing new skin is protected from digestion while the old skin is being degraded. It was previously thought that such selective degradation was accomplished by a physical barrier (an impermeable membrane) that separated the old from the new skin, protecting the latter. In this work, we showed that this is not true, and that exoskeleton-digesting enzymes are present in both the new and the old skins. We also showed that the new skin is protected from degradation by these enzymes by a specific, protective protein called “Knk”. Discovery of insect proteins needed for protection of the exoskeleton could lead to the development of new biopesticides that act by disruption of such proteins.
Technical Abstract: New cuticle synthesis and molting are complex developmental processes that all insects must undergo to allow for growth. However, little is known about how insects regulate the selective degradation of the old cuticle while leaving the new one intact. In this study we show that in the red flour beetle, Tribolium castaneum, the knickkopf gene (TcKnk) encodes a protein that binds to chitin and selectively co-localizes with chitin in the new cuticle, protecting this essential matrix polysaccharide from the activity of molting fluid chitinases, which we show are co-localized with both new and old cuticle. Down-regulation of TcKnk expression results in loss of chitin and severe molting defects that are ultimately lethal to all stages of Tribolium. The susceptibility of chitin in old cuticle to degradation by chitinases is correlated with the depletion of TcKnk. The conservation of Knk in several other insect species indicates a critical role for this protein in exoskeletal chitin accumulation and maintenance.