# Towards Understanding the Function of Aegerolysins
## 4 Aegerolysin Binary Partner Proteins

Table 2: _Cont._virulence. Little is known about the function of MACPF-like domain proteins in filamentous fungi and their taxonomic distribution. The number of putative MACPF proteins in a single fungal species ranges from zero to ten or more [155]. The identification and annotation of putative MACPF-like proteins is generally more error-prone because these genes have a higher number of introns [155]. The sequences can be divided into two groups. The proteins in the first group, such as **PlyB** or **EryB**, were assigned to the MACPF domain PF01823, which was confirmed by the 13-amino acid signature Y/F-G-X\({}_{2}\)-F/Y-X\({}_{6}\)-G-G typical of this domain, and they are grouped with human perforin. The second group of sequences is not recognized by the Pfam tool but still contains the typical MACPF/CDC signature, such as **BlyB**[155]. Fungal MACPF proteins probably contribute to various specific processes. While some of them were found in secretomes (without a typical signal sequence being recognized), others are intracellular, and some of them might be involved in pathogenesis, although probably not all [155]. The large number of introns, the sporadic taxonomic distribution, and the different number of MACPF proteins per fungal species might indicate the involvement of horizontal gene transfer mechanisms in specific ecological niches [155]. Putative MACPF-like domains were identified in the genomes of fungal species with different lifestyles; some of them are pathogenic, such as plant pathogen _M. perniciousa_, the caterpillar fungus _C. militaris_, or the nematode trap fungus _Arthrobotrys oligospora_. Some of the species are also saprophytic, such as the white rot fungus _P. ostreatus_, grassland-litter decomposers _P. enrygii_, saprophytic and food-producing _A. oryzae_, or saprophytic soil fungus _A. nidulans_[155]. However, the binary protein partner **Cry35 Ab1** does not have a MACPF domain associated with it. Instead, it contains the C-terminal domain toxin 10 (PF05431), which is typical of a family of insecticidal crystal toxins of _Bacillus_, named after the insecticidal crystal toxin P42. **Cry35 Ab1** also has an additional N-terminal ricin-type \(\beta\)-trefoil lectin domain (PF00652) (classified as insignificant only) [52; 130]. **Cry16 Aa** and **Cry17 Aa** are delta endotoxins composed of three distinct structural domains, endotoxin N, M, and C, respectively. An N-terminal helical bundle domain is involved in membrane insertion and pore formation (PF03945), a central \(\beta\)-sheet domain is involved in receptor binding (PF00555), and the C-terminal \(\beta\)-sandwich domain interacts with the N-terminal domain to form a channel (PF03944). During sporulation, the bacteria produce crystals of delta endotoxins. When an insect ingests these proteins, they are activated by proteolytic cleavage. For all such proteins, the N-terminus is cleaved, and for some members, a C-terminal extension is also cleaved. After activation, the endotoxin binds to the intestinal epithelium and, by the forming cation-selective channels, causes cell lysis, which leads to death [137; 156]. Surprisingly, it was observed in the genome of _P. ostreatus_ that the two genes encoding the pair of pore-forming proteins also form a bidirectional pair with 5\({}^{\prime}\)-5\({}^{\prime}\) orientation of **plyA (priA)** and **plyB**[91; 92]. Similar gene pairs encoding putative bicomponent toxins have been observed previously, such as **Asp-HS** with **Asp-HSB**, **NigA2** with **NigB1**, and **BlyA** with **BlyB** from _A. fumigatus_, _A. niger_, and _B. bassiana_, respectively [52; 92]. Moreover, such gene pairs have been also identified in other entomopathogenic fungi besides in _B. bassiana_, in the genomes of _Cordiceps militaris_, _Metrizium acridum_, _M. anisopliae_, _M. robertsii_, and _Ophicocordiceps sinensis_, but the involvement of these protein pairs in pore formation remains to be confirmed [52]. Here, we identified two additional putative pairs: **PlyA2** with **EryB** from _P.