Midgut aminopeptidase N expression profile in castor semilooper (Achaea janata) during sublethal Cry toxin exposure

The midgut of lepidopteran larvae is a multifunctional tissue that performs roles in digestion, absorption, immunity, transmission of pathogens and interaction with ingested various molecules. The proteins localized at the inner apical brush border membrane are primarily digestive proteases, but some of them, like aminopeptidase N, alkaline phosphatase, cadherins, ABC transporter C2, etc., interact with Crystal (Cry) toxins produced by Bacillus thuringiensis (Bt). In the present study, aminopeptidase N (APN) was characterized as Cry-toxin-interacting protein in the larval midgut of castor semilooper, Achaea janata. Transcriptomic and proteomic analyses revealed the presence of multiple isoforms of APNs (APN1, 2, 4, 6 and 9) which have less than 40% sequence similarity but show the presence of characteristic 'GAMENEG' and zinc-binding motifs. Feeding a sublethal dose of Cry toxin caused differential expression of various APN isoform. Further, 6thgeneration Cry-toxin-exposed larvae showed reduced expression of APN2. This report suggests that A. janata larvae exploit altered expression of APNs to overcome the deleterious effects of Cry toxicity, which might facilitate toxin tolerance in the long run.