Photorhabdus luminescens PirAB-fusion protein exhibits both cytotoxicity and insecticidal activity

The binary toxin 'Photorhabdus insect-related' proteins (PirAB) produced by Photorhabdus luminescens have been reported to possess both injectable and oral activities against a range of insects. Here, PirAB-fusion protein was constructed by linking pirA and pirB genes with the flexible linker (Gly4 Ser)3 DNA encoding sequence and then efficiently expressed in Escherichia coli. To better understand the role of PirAB toxin played in the process of invasion, its cytotoxicity against insect midgut CF-203 cells was investigated. Application of purified PirAB-fusion protein as well as PirA/PirB mixture caused loss of viability of CF-203 cells after 24 h incubation. CF-203 cells treated by PirAB-fusion protein displayed morphological changes typical of apoptosis, such as cell shrinkage, cell membrane blebbing, nuclear condensation and DNA fragmentation. Moreover, PirAB-fusion protein also exhibited injectable insecticidal activity against Spodoptera exigua larvae. The bodies of S. exigua fourth-instar larvae injected with PirAB-fusion protein turned completely black. Thus, we concluded that PirAB-fusion protein possessed similar biological activity (cytotoxicity and insecticidal activity) to PirA/PirB mixture, which would enable it to be used as an efficient agent for pest control.