Aegerolysins: History
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Aegerolysins are remarkable proteins. They are distributed over the tree of life, being relatively widespread in fungi and bacteria, but also present in some insects, plants, protozoa, and viruses. Their function, in particular, is intriguing. Aegerolysin proteins are involved in various interactions by recognizing a molecular receptor in the target organism. Despite their abundance in cells of certain developmental stages and their presence in secretomes, only a few aegerolysins have been studied in detail. Formation of pores with various larger non-aegerolysin-like protein partners is one of the possible responses of the aegerolysin-producing organism in competitive exclusion of other organisms from the ecological niche.

  • aegerolysins
  • bacteria
  • fungi
  • insecticidal
  • lipid binding
  • lifestyle
  • membrane-attack complex/perforin domain (MACPF)
  • pore forming proteins

1. Introduction

The aegerolysin family (Pfam 06355) is a lesser-known protein family that has received increasing attention in recent years. The aegerolysin family consists of proteins that are biochemically characterized as β-structured proteins and share some common features: similar small molecular weights (15–20 kDa), low isoelectric points, and stability in a wide pH range [1]. Because they are non-core proteins, without a member of this protein family in each of the sequenced fungi, their distribution among fungal species is inconsistent, and different numbers of homologs have been reported for species within the same genus [2,3,4]. They are not only relatively widespread in fungi and bacteria, but also identified in few plants, protozoa, viruses, and insects [1,2].
In recent years, several reviews of this protein family have been published, but none of them included data on the ecology of the organisms producing them. In particular, their function is enigmatic, although some authors suggest a role in the development of the organism [1,2,3]. However, some of them function as two-component cytolysins that exhibit membrane permeabilization activity together with another non-aegerolysin-like protein [2,5]; these act together to perforate natural and artificial lipid membranes [1,2,5]. The aegerolysin-like proteins provide membrane lipid selectivity and recruit partner protein molecules to form a pore complex inserted into the membrane [2,5].
Despite the limited scientific knowledge about the function of aegerolysins, several potential applications are already emerging. Most commonly, some fungal aegerolysins serve as probes for the detection, labeling, and imaging of specific membrane lipids, lipid rafts, cancer cells, invertebrates, or parasites [4,6,7,8,9,10,11]. In high concentrations, they can induce both artificial lipid vesicles as well as live cells, such as blood cells or neuroblastoma cells, to bend and bud [10]. A role of some aegerolysins in combating obesity and related metabolic disorders has been recognized [10]. Their genes and expression may serve as markers for the progression of fruiting body differentiation during mushrooms cultivation [10] or as biomarkers to detect fungal exposure and progression of infectious disease [3,4]. In addition, antibodies produced against aegerolysins can serve as immuno-diagnostic tools [4]. Due to their variable sequence, aegerolysins serve as tools to identify of fungal phytopathogen isolates compared to some closely related species where the internal transcribed spacer barcoding method has failed [4]. Strong promoters regulating aegerolysin genes can promote the secretion of heterologous proteins from fungi in concomitant multi-gene expression [4]. Certain aegerolysins that combine with larger protein partners to form pore-forming complexes can be used to selectively eliminate insect pests [4,10] or to treat certain types of cancer cells [4,10].

2. Aegerolysins

We have collected (experimental) published data on 23 different aegerolysins and their variants. In total, they were characterized from 18 different species belonging to different kingdoms of tree of life. Twelve of these aegerolysins belong to fungi, four to bacteria, and one to insects and viruses. In fungi, they were characterized from four mushrooms (Agaricomycotina) from the order AgaricalesPleurotus ostreatusP. eryngiiAgrocybe aegerita, and Moniliophthora pernicious, as well as in the ordo PolyporalesLignosus rhinocerotis (Table 1). The origin of these aegerolysins were also four filamentous Eurotimycetes from the ordo EurotialesAspergillus fumigatusA. nigerA. terreus, and A. oryzae (Table 1). Two species belonged to the Sordariomycetes, ordo HypocrealesBeauveria bassiana, and Trichoderma atroviride, and another to the Dothideomycetes, ordo PleosporalesAlternaria geisen (Table 1). There were four bacterial species, two belonging to FirmicutesBacillus thuringiensis and Clostridium bifermentans, and another two to ProteobacteriaPseudomonas aeruginosa and Alcaligenes faecalis (Table 1). Another species belongs to Insecta—LepidopteraNoctuidaePseudoplusia includes, and another to VaridnaviriaAscoviridaeTrichoplusia ni ascovirus 2c (Table 1).
Table 1. Organisms that contain aegerolysins according to taxonomy and lifestyles.
Organism Name Other Names Taxonomy Lifestyle/Niche Reference
    Fungi    
Pleurotus ostreatus Oyster mushroom
Hiratake
Agaricomycotina
Agaricales
Saprotroph
White rot
Nematocidal
[15]
Pleurotus eryngii King oyster or trumpet or brown mushroom
Boletus of the steppes
French horn mushroom
Aliʻi oyster
Agaricomycotina
Agaricales
Saprotroph
Grassland-litter decomposer
Facultatively biotrophic
Nematocidal
[16]
Cyclocybe aegerita Agrocybe aegerita
Poplar mushroom
Tea tree mushroom
Cha shu gu
Yanagi-matsutake
Sword-belt mushroom
Velvet pioppini
Agaricomycotina
Agaricales
Saprotroph
Weak white rot on hardwoods
Facultatively pathogenic
[17]
Moniliophthora perniciosa Crinipellis perniciosa
Witches’ broom disease
Agaricomycotina
Agaricales
Hemibiotrophic plant pathogen
Broad range of host
[18]
Lignosus rhinocerotis Tiger milk mushroom Agaricomycotina
Polyporales
Saprotroph
White rot
[19]
Neosartorya fumigata Aspergillus fumigatus Eurotimycetes
Eurotiales
Saprotroph
Ubiquitous in soil and compost
Human (opportunistic) pathogen
[20,21,22,23]
Aspergillus niger   Eurotimycetes
Eurotiales
Saprotroph
Ubiquitous in soil and compost
Human opportunistic pathogen
[20,21,24,25,26,27]
Aspergillus terreus   Eurotimycetes
Eurotiales
Saprotroph
Human opportunistic pathogen
[20,21,28]
Aspergillus oryzae   Eurotimycetes
Eurotiales
Saprotroph [20,21,29]
Beauveria bassiana   Sordariomycetes
Hypocreales
Entomopathogen
Endophyte
Soil and insects
[30,31]
Hypocrea atroviridis Trichoderma atroviride Sordariomycetes
Hypocreales
Mycoparasitic (including oomycetes)
Cosmopolitan, soil
[32,33,34,35]
Alternaria geisen Black spot of Japanese pear Dothideomycetes
Pleosporales
Plant pathogen [36,37]
    Bacteria    
Bacillus thuringiensis   Firmicutes Ubiquitous opportunistic pathogen on vertebrates, plants, insects, nematodes, mollusks, protozoan, animal, and human parasites.
Soils, grain dusts, dead insects, water
Aerobic and spore-forming
[38,39]
Paraclostridium bifermentans
subsp. malaysia
Clostridium bifermentans
subsp. malaysia
Firmicutes Anaerobic, forming endospores
Mosquito larvicidal
[40]
Alcaligenes faecalis   Proteobacteria Soil, water, environments associated with humans
Human opportunistic pathogen
Nematocidal
[41]
Pseudomonas aeruginosa   Proteobacteria Ubiquitous opportunistic pathogen on:
humans, vertebrates, plants, and insects
[42,43,44,45,46]
    Insecta    
Chrysodeixis includens Pseudoplusia includes Lepidoptera
Noctuidae
Plant pest (defoliator)
Larvae feed on a wide range of plants
[47,48]
    Viria    
Trichoplusia ni ascovirus 6a1 Trichoplusia ni ascovirus 2c Varidnaviria
Ascoviridae
Obligate pathogen
Pseudoplusia includens moth larvae
[47,49,50,51]

3. Aegerolysin Binary Partner Proteins

Binary and quaternary cytolytic complexes of bacterial origin in which aegerolysin-like proteins are combined with larger, non-aegerolysin-like protein partner(s), described to date, include: Cry16Aa/Cry17Aa/Cbm17.1/Cbm17.2 from C. bifermentas subsp. malaysia [40]; Cry34Ab1/Cry35Ab1 from B. thuringiensis [130,134]; and AflP-1A/AflP-1B from A. faecalis [143]. In fungus P. ostreatusPlyA forms a pore embedded in the membrane together with PlyB (Figure 1) [60]. Similar cytolytic effects were observed when PlyB was combined with other Pleurotus-derived aegerolysins, e.g., OlyA6PlyA2 and EryA [78]. These heteromeric aegerolysin-based cytolytic complexes have been exploited as potent biopesticides for specific pests, with Cry16Aa/Cry17Aa/Cbm17.1/Cbm17.2 acting against Aedes mosquitoes, and Cry34Ab1/Cry35Ab1, AflP-1A/AflP-1B, OlyA6/PlyBPlyA/PlyB, PlyA2/PlyB, or EryA/PlyB acting against Coleoptera species, especially the western corn rootworm.
Unexpectedly, partner proteins can be classified into five groups (Table 2Figure 2): (1) PlyB and EryB have a similar MACPF fold; (2) the remaining models, including BlyB, showed a reasonably good superposition; BlyB has been shown to best align the structure of bacterial GNIP1Aa, another MACPF domain-containing protein [52,121]; (3) the Cry35Ab1 structure and (4) the AfIP-1B model do not superimpose with the PlyB structure or with each other; for AfIP-1B, the MACPF domain was found to be insignificant [52]; (5) Cry16Aa and Cry17Aa only superimpose with each other.
Table 2. List of published aegerolysins.
Proteins containing a membrane-attack complex/perforin (MACPF) domain are transmembrane pore-forming proteins important for both human immunity and pathogen 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-X2-F/Y-X6-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. perniciosa, 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. eryngii, saprophytic and food-producing A. oryzae, or saprophytic soil fungus A. nidulans [155].
However, the binary protein partner Cry35Ab1 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. Cry35Ab1 also has an additional N-terminal ricin-type β-trefoil lectin domain (PF00652) (classified as insignificant only) [52,130].
Cry16Aa and Cry17Aa 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 β-sheet domain is involved in receptor binding (PF00555), and the C-terminal β-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′–5′ 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-HSBNigA2 with NigB1, and BlyA with BlyB from A. fumigatusA. 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 militarisMetarizium acridumM. anisopliaeM. robertsii, and Ophiocordiceps 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. eryngii and L152 with L152B from A. geisen (Figure 3). The gene locations of the aegerolysin and putative MACPF-like genes in the genomes of some fungi and bacteria are schematically shown in Figure 3. A total of six fungal (putative) bicomponent toxins form a bidirectional gene pair with 5′–5′ orientation of the adjacent gene, and distances between the two genes vary (Figure 3). Clusters of genes encoding fungal secondary metabolites may also contain some reverse-oriented genes that may nevertheless show coordinate expression. In contrast to the fungal gene pairs, the AfIP-1A/AfIP-1B gene pair encoded on the bacterial chromosome has the same sense orientation (Figure 3). The bicomponent toxin gene pair Cry34Ab1/Cry34Ab1 is known to be plasmid-encoded, but the variation of the plasmid in the bacterium B. thuringiensis is exceptionally high to see if these two genes are adjacent. The Cry operon in plasmid pCryO of C. bifermentans subsp. malaysia contains four genes downstream of the promoter pCyt: Cry16Aa is located 91 base pairs (bp) downstream, followed by Cry17Aa, which is located 426 bp downstream, followed by Cbm17.1, and 1022 bp downstream, followed by Cbm17.2 [40]. Their placement at the same locus or under the control of the same promoter does not necessarily lead to their joint action, but rather indicates coordinated expression.
Figure 3. Aegerolysin gene loci in fungal and bacterial genomes. Grey arrows, aegerolysin genes; Cian arrows, membrane-attack-complex/perforin (MACPF)-like genes; plyA (priA) aegerolysin with plyB, pleurotolysin B gene from Pleurotus ostreatus PC9 whole genome sequence; [53]; PlyA2 with EryB from P. eryngii ATCC 907,970 [157]; Asp-HS with Asp-HSB from Neosartorya fumigata (Aspergilus fumigatus) Af293 [22]; nigA2 with nigB1, nigerolysin A2, and B1 gene A. niger CBS 513.88 [27]; blyA and blyB, beauveriolysin A and B gene from Beauveria bassiana ARSEF 2860 [158], L152 with L152B from Alternaria geisen BMP2338 [159], and AfIP-1A with AfIP-1B from Alcaligenes faecalis GCA_003521065; numbers, size of genes, and distance among genes in base pairs. Gene sizes and distances are scaled.
Some of the aegerolysins included in this review are also encoded in genomes that do not have an adjacent (MACPF domain-containing) partner protein; such aegerolysins are the fungal EryAAa-Pri1Asp-HS-likeTerAoHlyA, and NigA1, bacterial RahU, insects P23 and the ascoviral TnAV2c gp029 (Table 2). For some of them, insufficient data are available, such as for the MpPRIA1MpPRIA2, and MpPRIB, because the sequenced countings are too short, or no genomic data were found for GME7309 (Table 2). However, a combined action with MACPF domain-containing proteins encoded elsewhere in the genome cannot be excluded, as some aegerolysins, such as PlyA2 and EryA from P. eryngii, have been shown to exhibit cytolytic activity when combined with their non-native partner with component B, PlyB, from P. osteratus [78].

This entry is adapted from the peer-reviewed paper 10.3390/toxins14090629

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