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Functional analysis of C1 family cysteine peptidases in the larval gut of Тenebrio molitor and Tribolium castaneum

Martynov, Alexander G., Elpidina, Elena N., Perkin, Lindsey, Oppert, Brenda
BMC genomics 2015 v.16 no.75
RNA interference, Tenebrio molitor, Tribolium castaneum, active sites, binding capacity, cathepsin B, cathepsin F, cathepsin L, cathepsin O, cereal proteins, chromosomes, cysteine, digestion, gene expression, grain products, humans, insects, larvae, major genes, messenger RNA, midgut, models, phylogeny, pseudogenes, sequence analysis, storage pests, stored grain, substrate specificity
We study protein digestion the tenebrionids Tenebrio molitor and Tribolium castaneum, pests of stored grains and grain products, to identify potential targets for biopesticide development. Tenebrionid larvae have highly compartmentalized guts, with primarily cysteine peptidases in the acidic anterior midgut that contribute to the early stages of protein digestion. In this study, we have used high throughput sequencing to quantify and characterize the more highly expressed transcripts encoding digestive cysteine peptidases in these tenebrionid larvae. For T. castaneum, transcript and genomic data identified 24 genes and three pseudogenes encoding cysteine peptidases, including 12 cathepsin L or L-like, nine cathepsin B or B-like, and one each F, K, and O. Many of these genes are clustered on four different chromosomes. The majority of transcript expression is from two cathepsin L genes on chromosome 10 (LOC659441 and LOC659502), and two on chromosome 8 (LOC660368, 26-29-p); for cathepsin B, the major genes in the T. castaneum larval gut are on chromosome 3 (LOC663145 and LOC663117). Some cysteine peptidases were expressed at lower levels or not at all in the larval gut, including cathepsin F, K, and O. Without a sequenced genome for T. molitor our data is still speculative, but suggests that there are 25 cysteine peptidase genes, which include 13 cathepsin L, 10 cathepsin B, and one each cathepsin O and F in sequences from the larval gut. Of these, there were three each of relatively highly expressed cathepsin L and B transcripts. Orthologs were found for all except seven genes from T. castaneum and six transcripts from T. molitor; additional sequencing, including a sequenced genome for T. molitor, may resolve some of the differences. In general, the more highly expressed transcripts encoded peptidases that were predicted to be extracellular, whereas the remainder were likely lysosomal; all were phylogenetically related, and clusters of genes found in tandem on chromosomes (and presumably due to duplication events) were grouped in a cladogram. Sequence analysis indicated that nonfunctional (lacking conserved residues in the active site) orthologs were found in both insects, suggesting that changes in these residues occurred prior to e Gvolutionary divergence. Cathepsin L sequences from both insects have a high degree of variability in the substrate binding regions, consistent with the ability of these enzymes to degrade a variety of cereal proteins and inhibitors. Cathepsin B sequences included those with a typical occluding loop, as well as atypical cathepsin B-like peptidases with a shortened occluded loop lacking the active site in the middle; these atypical cathepsin B-like peptidases are apparently unique to tenebrionid insects. Docking studies with characteristic substrates for human cysteine cathepsin L indicated that, while some tenebrionid cathepsin B and L peptidases have similar binding affinities, others do not and have presumably different substrate specificity, including the atypical cathepsin B-like peptidases. These studies have refined our model of protein digestion in the larval gut of tenebrionid insects, and suggest genes that may be targeted by inhibitors or RNA interference for the control of cereal pests in storage areas.