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Group III chitinase with two catalytic domains is required for organization of the cuticular extracellular matrix of a beetle
Insect cuticle or exoskeleton is an extracellular matrix consisting of three major morphologically distinct layers, the water-proofing envelope, the protein-rich epicuticle and the chitin/protein-rich procuticle. To accommodate growth, insects must periodically replace their cuticles in a process called “molting or ecdysis”. During each molt cycle a new cuticle is deposited simultaneously with degradation of the inner part of the chitinous procuticle of the old one by molting fluid enzymes including epidermal chitinases.
In this study, we show a novel role for an epidermal endochitinase containing two catalytic domains, TcCHT7, from the red flour beetle, Tribolium castaneum, belonging to a subfamily (group III) of insect chitinases in organizing chitin in the newly forming cuticle rather than in degrading chitin present in the prior one. RNAi of TcCHT7 reveals that this enzyme is nonessential for any type of molt or degradation of the chitinous matrix in the old cuticle. In contrast, TcCHT7 is required for formation of properly oriented long chitin fibers inside pore canals that are vertically oriented columnar structures, which contribute to maintain the integrity and the mechanical strength of a light-weight, yet rigid, adult cuticle. Because group III chitinases are highly conserved among insect and other arthropod species, these enzymes have a critical role in the higher ordered organization of chitin fibers for development of the structural integrity of many invertebrate cuticular extracellular matrices. This work was supported by NRFs (NRF-2015R1A2A2A01006614 and NRF-2018R1A2B6005106)
