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    • In insects structural cuticular proteins CPs play important

    2021-09-11

    In insects, structural cuticular proteins (CPs) play important roles in determining the diverse physical properties of the cuticle, depending on developmental stages, as well as different body regions, as a result of interactions with other CPs and the structural biopolymer chitin [4]. Many studies and the availability of fully sequenced and annotated genomes of several insect species such as Apis mellifera, Drosophila melanogaster, Tribolium castaneum, Anopheles gambiae, Bombyx mori, Nasonia vitripennis, and Mandula sexta, indicate that there are large numbers of genes encoding CPs or CP-like proteins in their genomes. Indeed, >200 putative CP genes have been identified in D. melanogaster [5], A. gambiae [6], and M. sexta [7], and these genes comprise about 2% of the predicted protein-coding genes in the latter species. CPs have been classified into thirteen distinct families, as defined by the unique amino omacetaxine sequence motifs that are characteristic of each family [6,8]. The largest CP family is the CPR family whose members contain the R&R consensus motif [9], which can further be split into three subfamilies, RR-1, RR-2, and RR-3, with some correlation to the type or region of the cuticle, and is generally rich in histidine and devoid of cysteine. The R&R motif contains a chitin-binding domain (ChtBD4) that apparently helps to coordinate the interactions between chitin fibers and the proteinaceous matrix [[10], [11], [12], [13]]. To date, many CPs with an R&R consensus have been identified in D. melanogaster (101 CPRs), A. mellifera (28 CPRs), B. mori (148 CPRs), M. sexta (207 CPRs), and Nilaparvata lugens (96 CPRs) [5,7,[14], [15], [16]]. As reported, RR-1 is generally found in soft cuticle, RR-2 in hard cuticle [8,17], and RR-3 group only comprises a few CPs [8,18]. The timing of expression defines the kind of CPs that construct the layer and determine the nature of each cuticular layer. In wing discs of B. mori, fifty-two CP genes were examined for their expression profiles. According to the peak stages of expression of these genes, RR-2 genes were expressed until the day of pupation and RR-1 genes were expressed before and after pupation and for longer than RR-2 genes, suggesting different construction of the exo- and endocuticular layers [19]. Thus, it is speculated that the exocuticle is constructed with RR-1, RR-2, and other types of CPs, while the endocuticle is formed with RR-1 and other CPs from the expression profiles of CP genes of B. mori [19,20]. In T. castaneum, two major structural RR-2 protein genes, TcCPR27 and TcCPR18, mainly expressed in pre-ecdysis, were observed to be essential to maintain the rigidity of elytra [21]. In addition, the transcript levels of TcCPR4 (an RR-1 protein gene) were dramatically increased in 3 d-old pupae when adult cuticle synthesis began, and its encoded protein was uniquely distributed in the entire exocuticle of the elytron, which is important to determine the morphology and ultrastructure of the rigid cuticle [22]. In B. mori, mutation of the BmorCPR2 gene, encoding an RR-1 protein, is responsible for the stony mutant, which causes an abnormal distribution of internodes and intersegmental folds, resulting in reductions in both the chitin content and tensile/stretch properties of the larval cuticle [23]. In N. lugens, RNA interference (RNAi) against 15 CPR genes led to lethal phenotypes with high mortality, indicating their essential roles in the integument structure and normal development [16]. For post-ecdysis proteins, BmorCPH24, a gene encoding a cuticular protein with a low complexity sequence, is expressed in the larval epidermis after ecdysis, which produces an abnormal body shape and may be involved in the synthesis of endocuticle of B. mori [24]. More recently, a gene encoding a novel ungrouped cuticular protein, NlCP21.92, was identified, which is expressed at high levels in the integument post-ecdysis and is essential for normal endocuticle formation in N. lugens [25]. However, for RR-1 proteins of the post-ecdysis stage, their function and mechanism in the formation of the endocuticle remain unclear.