Proteolytic activation of pro-spatzle is required for the induced transcription of antimicrobial peptide genes in lepidopteran insects

Microbial infection leads to proteolytic activation of Drosophila spatzle, which binds to the toll receptor and induces the synthesis of immune proteins. To test whether or not this mechanism exists in lepidopteran insects, we cloned the cDNA of Bombyx mori spatzle-1 and overexpressed the full-length and truncated BmSpz1 cDNA in Escherichia coli. The insoluble fusion proteins were affinity-purified under denaturing condition. After the silkworm larvae were injected with renatured BmSpz1, mRNA levels of antimicrobial peptide genes greatly increased. Similar transcriptional up-regulation was also found in Manduca sexta. Injection of pro-BmSpz1 had no such effect. When pro-BmSpz1 and Micrococcus luteus were incubated with the plasma from M. sexta larvae, we detected proteolytic processing of pro-BmSpz1. These results suggest that active spatzle is required for the induced production of antimicrobial peptides in B. mori and M. sexta. (C) 2007 Elsevier Ltd. All rights reserved.