Molecular characterization and functional analysis of novel carboxyl/cholinesterases with GQSAG motif in the silkworm Bombyx mori

We have previously cloned and characterized BmJHE, a juvenile hormone (JH)-selective esterase (JHE) that is important for JH titer regulation in the silkworm Bombyx mori. Here, we sought to determine whether multiple genes might function as JH-specific esterase in this species. We searched for putative carboxyl/cholinesterase (CCE) genes having GQSAG, a highly conserved motif in JHE, by the use of silkworm genomic database. Five novel CCE genes (Bmcce-1–5) were identified and their cDNA sequences and intron–exon structures were determined. We investigated the developmental expression patterns of these CCE genes by real-time quantitative PCR analysis and found that their expression patterns varied among developmental stages and organs. Of the proteins produced by the five genes, only BmCCE-5 had the ability to degrade JH; however, this protein might not function as a JH-specific esterase in vivo as it had a high Km value for JH. On the other hand, BmCCE-5 degraded general esterase substrates efficiently. Since Bmcce-5 was strongly expressed in Malpighian tubules and the gut, it might function in digestion or xenobiotic metabolism. Our results suggest that of the CCEs containing a GQSAG motif only BmJHE can function as a JH-specific degradation enzyme in the silkworm.