Spodoptera frugiperda (J. E. Smith) Aminopeptidase N1 Is a Functional Receptor of the Bacillus thuringiensis Cry1Ca Toxin

Abstract

<i>Bacillus thuringiensis</i> Cry1Ca is toxic to different <i>Spodoptera</i> species. The aims of this work were to identify the Cry1Ca-binding proteins in <i>S. frugiperda</i>, to provide evidence on their participation in toxicity, and to identify the Cry1Ca amino acid residues involved in receptor binding. Pulldown assays using <i>Spodoptera frugiperda</i> brush border membrane vesicles (BBMV) identified aminopeptidase N (APN), APN1, and APN2 isoforms as Cry1Ca-binding proteins. Cry1Ca alanine substitutions in all residues of domain III β16 were characterized. Two β16 nontoxic mutants (V505A and S506A) showed a correlative defect on binding to the recombinant <i>S. frugiperda</i> APN1 (SfAPN1). Finally, silencing the expression of <i>APN1</i> transcript, by double-stranded RNA (dsRNA) feeding, showed that silenced larvae are more tolerant of the Cry1Ca toxin, which induced less than 40% mortality in silenced larvae whereas nonsilenced larvae had 100% mortality. Overall, our results show that Cry1Ca relies on APN1 binding through domain III β16 to impart toxicity to <i>S. frugiperda</i><b>IMPORTANCE</b><i>Bacillus thuringiensis</i> Cry toxins rely on receptor binding to exert toxicity. Cry1Ca is toxic to different populations of <i>S. frugiperda</i>, a major corn pest in America. Nevertheless, the <i>S. frugiperda</i> midgut proteins that are involved in Cry1Ca toxicity have not been identified. Here we identified aminopeptidase N1 (APN1) as a functional receptor of Cry1Ca. Moreover, we showed that Cry1Ca domain III β16 is involved in APN1 binding. These results give insights on potential target sites for improving Cry1Ca toxicity to <i>S. frugiperda</i>.