# Towards Understanding the Function of Aegerolysins
## 2 Aegerolysins
### Bacterial Aegerolysins from Firmicutes  
#### 2.5.1 Aegerolysins from _Bacillus thuringiensis_  
_Bacillus thuringiensis_ Berliner 1915 is a rod-shaped, Gram-positive, spore-forming, aerobic bacterium usually found in soils, grain dusts, dead insects, and water around the world. It has adapted its lifestyle as an efficient pathogen, synthesizing a large number of protein toxins that are effective against a variety of organisms in nature, including not only a broad range of insect orders but also nematodes, human pathogenic protozoans, animal, and human parasites. A molluscicidal activity against the snail pest has also been demonstrated [38,39].  
_Bacillus thuringiensis_ owes its insecticidal potential to several factors, and the main factor is the parasporal crystal containing a series of entomotoxins. The main _B. thuringiensis_ entomotoxins, Cry (crystals), and/or Cyt (cytolytic) are pore-forming proteins, which are dissolved in the gastrointestinal tract of an insect after ingestion. A variety of different insecticidal proteins produced by _Bacillus_ species is very rich. Cry toxins exert their membrane-permeabilizing activity on insect epithelial midgut cells after binding to a variety of protein receptors [129]. Among more than 300 members of the Cry toxins, a 13.6 kDa **Cry34 Ab1** from _B. thuringiensis_ (Bt strain PS149B1), is the only with the aegerolysin fold. It is composed of two \(\beta\)-sheets in a \(\beta\)-sandwich structure [130]. Similarly to aegypti/MACPF protein pairs described in oyster mushrooms, **Cry34 Ab1** exerts its membrane activity and insecticidal action towards western corn rootworm in the presence of a 43.8 kDa partnering protein, **Cry35 Ab1**. The latter protein has a toxin 10 (PF05431) C-terminal domain. **Cry34 Ab1/Cry35 Ab1** binary toxins have been transformed into maize plants under the commercialized events. After ingestion, they bind to the brush border membranes in the epithelial lining of western corn rootworm midgut, where they form pores. This leads to necrosis and ends with the insect's death. The **Cry35 Ab1** protein does not convey activity in the absence of **Cry34 Ab1**, indicating that the smaller **Cry34 Ab1** protein is critical for membrane binding and recruitment of **Cry35 Ab1** to induce insecticidal effect [39; 131; 132; 133; 134; 135]. Although until now the exact nature of **Cry34 Ab1** protein receptor(s) in insect larva midgut membranes has not been determined [132; 135], it appears that these receptors are unique and unrelated to other Cry toxins [136]. Further, although the **Cry34 Ab1/Cry35 Ab1** mixture was demonstrated to be slightly lytic to artificial lipid membranes in absence of proteins [133]; the lipid specificity seems to be low and the SM/Chol or CPE/Chol complexes cannot be regarded as receptor molecules, as in the case of oyster mushroom-derived aegypti/MACPF cytolytic insecticidal complexes [79]. Cristal structure of **Cry34 Ab1/Cry35 Ab1** binary toxin has been solved (PDB ID: 4 JOX and 4 JP0, respectively) [130].  
Another characteristic feature of _B. thuringiensis_ strains is the size of their extrachromosomal pool, with strains containing more than ten different plasmid molecules [137]. The size of these extrachromosomal molecules varies from two to more than 500 kb, and the number of encoded molecules can be as high as 17. For example, the complete sequence and organization of the transmitted 128 kb megaplasmid pHotaxis is known and has been compared to the large plasmid pXO1 of _B. antracis_; however, pBotaxis does not encode aegerolysin [138]. The main focus placed on these elements relates to the entomogenesis of their host strains and the plasmid-based genetic determinants of entomoxins [139].