Novel aminopeptidase N (APN) isoform cDNAs, BmAPN3 and PxAPN3, from the midguts of Bombyx mori and Plutella xylostella, respectively, were cloned, and a total of eight APN isoforms cloned from B. mori and P. xylostella were classified into four classes. Bacillus thuringiensis Cry1Aa and Cry1Ab toxins were found to bind to specific APN isoforms from the midguts of B. mori and P. xylostella, and binding occurred with fragments that corresponded to the BmAPN1 Cry1Aa toxin-binding region of each APN isoform. The results suggest that APN isoforms have a common toxin-binding region, and that the apparent specificity of Cry1Aa toxin binding to each intact APN isoform seen in SDS–PAGE is determined by factors such as expression level in conjunction with differences in binding affinity.
