Why a Membrane‐Anchored Chitinase (CHT7) in Tribolium?

Abstract

Insect chitinases (CHTs) and chitinase‐like proteins, which belong to family 18 glycosylhydrolases (GH‐18), have been detected in molting fluid and gut tissues and are predicted to mediate the digestion of chitin present in the exoskeleton and peritrophic matrix (PM) in the gut to chitooligosaccharides. Based on amino acid sequence similarity and phylogenetic analysis, insect chitinase family proteins have been classified into eight groups (group I to VIII). The chitinases belonging to different groups have distinctly different developmental patterns of expression and tissue specificity, suggestive of distinct biological functions.CHT7s belong to Group III chitinase contain two catalytic domains and one CBD. The catalytic domain 1 of this group of chitinases exhibits greater sequence similarity to one another than to the catalytic domain 2 in the same protein(s), suggesting distinct functions and/or evolutionary origins for each of these two catalytic domains. This group of chitinases, unlike most insect CHTs, possesses a predicted transmembrane segment at the N‐terminal region. The recombinant T. castaneum CHT7 that was expressed in Hi‐5 insect cells was bound to the cell membrane. Apparently, the catalytic domains of this CHT face the extracellular space as revealed by its ability to hydrolyze an artificial chitin substrate added to the medium.DsRNA mediated post‐transcription gene silencing (RNAi) for several chitinases and chitinase‐like genes in Tribolium castaneum and demonstrated that chitinases belong to groups I (TcCHT5), II (TcCHT10) and V (TcIDGF4) play roles of insect molting. In other hand, RNAi for TcCHT7 did not affect any types of molting such as larval‐larval, larval‐pupal and pupal‐adult. The resulting pupae (and adults), however, failed to wing‐expansion and abdominal contraction suggesting that TcCHT7 may function in tissue differentiation rather than in molting.