Lysenin is apore-forming toxin (PFT) present in thecoelomic fluid of theearthwormEisenia fetida. Pore-forming toxins are a group ofproteins that act as virulence factors of severalpathogenicbacteria. Lysenin proteins are chiefly involved in the defense against cellular pathogens.[1] Following the general mechanism of action of PFTs lysenin is segregated as a soluble monomer that binds specifically to amembrane receptor,sphingomyelin in the case of lysenin. After attaching to the membrane, the oligomerization begins, resulting in a nonamer on top of membrane, known as a prepore. After a conformational change, which could be triggered by a decrease ofpH, the oligomer is inserted into the membrane in the so-called pore state.
Lysenin water-soluble monomeric X-ray structure (PDB:3ZXD). Receptor binding domain on right in grey. Pore Forming Module (PFM) on left with regionpreviously thought to be responsible for β-barrel formation in green. Additional regionnow known to be important in β-barrel formation in yellow (from X-ray data),
Lysenin is aprotein produced in thecoelomocyte-leucocytes of the earthwormEisenia fetida.[2] This protein was first isolated from the coelomic fluid in 1996 and named lysenin (from lysis andEisenia).[3] Lysenin is a relatively small water-soluble molecule with a molecular weight of 33 kDa. UsingX-ray crystallography, lysenin was classified as a member of theAerolysin protein family by structure and function.[4] Structurally, each lysenin monomer consists of a receptor binding domain (grey globular part on right of Figure 1) and a Pore Forming Module (PFM); domains shared throughout the aerolysin family.[4] The lysenin receptor binding domain shows threesphingomyelin binding motifs. The Pore Forming Module contains the regions that undergo large conformational changes to become the β-barrel in the pore.[5]
The naturalmembrane target of lysenin is an animal plasma membranelipid calledsphingomyelin located mainly in its outer leaflet, involving at least three of itsphosphatidylcholines (PC) groups.[6] Sphingomyelin is usually found associated withcholesterol inlipid rafts.[7] Cholesterol, which enhancesoligomerization, provides a stable platform with high lateral mobility where monomer-monomer encounters are more probable.[6] PFTs have shown to be able to remodel the membrane structure,[8] sometimes even mixing lipid phases.[9]
The region of the lysenin pore β-barrel expected to be immersed in the hydrophobic region of the membrane is the 'detergent belt', the 3.2 nm high region occupied by detergent inCryogenic Electron Microscopy (Cryo-EM) studies of the pore.[10] On the other hand, sphingomyelin/Cholesterol bilayers are about 4.5 nm height.[11] This difference in height between the detergent belt and the sphingomyelin/cholesterol bilayer implies a bend of the membrane in the region surrounding the pore, called negative mismatch.[12] This bending results in a net attraction between pores that induce pores aggregation.
Lysenin mechanism of action Scheme.a) Lysenin monomers are segregated as soluble proteins that bind specifically to sphingomyelin by its receptor binding domain. After binding, and reach a certain density, the oligomerization starts.b) After a complete oligomerization, the prepore is formed. The prepore model shown here was assembled from the monomer structure and aligned with the pore structure (PDB:5GAQ) by their receptor-binding domains (residues 160 to 297). The height of the prepore was set to agree with the Atomic Force Microscopy measurements.c) Membrane inserted Lysenin assembly (PDB:5GAQ). The height of the pore was measured from the detergent belt to the last residue, assuming that the detergent belt corresponds with the part of the pore surrounded by the membrane. The membrane was placed in the β-barrel of the pore to match with the detergent belt, that englobe all the hydrophobic residues of the β-barrel. The hydrophobic surface colour scale is according to the hydrophobicity scale of Kyte and Doolittle.
Membrane binding is a requisite to initiate PFT oligomerization. Lysenin monomers bind specifically to sphingomyelin via the receptor binding domain.[13] The final lysenin oligomer is constituted by nine monomers without quantified deviations.[14] When lysenin monomers bind to sphingomyelin-enriched membrane regions, they provide a stable platform with a high lateral mobility, hence favouring the oligomerization.[15] As with most PFTs, lysenin oligomerization occurs in a two-step process, as was recently imaged.
The process begins with monomers being adsorbed into the membrane by specific interactions, resulting in an increased concentration of monomers. This increase is promoted by the small area where the membrane receptor accumulates since the majority of PFT membrane receptors are associated with lipid rafts.[16] Another side effect, aside from the increase of monomer concentration, is the monomer-monomer interaction. This interaction increases lysenin oligomerization. After a critical threshold concentration is reached, several oligomers are formed simultaneously, although sometimes these are incomplete.[17] In contrast to PFTs of thecholesterol-dependent cytolysin family,[18] the transition from incomplete lysenin oligomers to complete oligomers has not been observed.
A complete oligomerization results in the so-called prepore state, a structure on the membrane. Determining the prepore's structure by X-ray or Cryo-EM is a challenging process that so far has not produced any results. The only available information about the prepore structure was provided byAtomic Force Microscopy (AFM). The measured prepore height was 90 Å; and the width 118 Å, with an inner pore of 50 Å.[17] A model of the prepore was built aligning the monomer structure (PDB:3ZXD) with the pore structure (PDB:5GAQ) by their receptor-binding domains (residues 160 to 297). A recent study in aerolysin suggests that the currently accepted model for the lysenin prepore should be revisited, according to the new available data on the aerolysin insertion.[19]
Aconformational change transforms the PFM into the transmembraneβ-barrel, leading to the pore state.[20] The trigger mechanism for the prepore-to-pore transition in lysenin depends on three glutamic acid residues (E92, E94 and E97), and is activated by a decrease in pH,[21] from physiological conditions to the acidic conditions reached after endocytosis, or an increase in calcium extracellular concentration.[22] These three glutamic acids are located in an α-helix that forms part of the PFM, and glutamic acids are found in aerolysin family members in its PFMs. Such a conformational change produces a decrease in the oligomer height of 2.5 nm according to AFM measurements.[17] The main dimensions, using lysenin pore X-ray structure, are height 97 Å, width 115 Å and the inner pore of 30 Å.[20] However, complete oligomerization into the nonamer is not a requisite for the insertion, since incomplete oligomers in the pore state can be found.[17] The prepore to pore transition can be blocked in crowded conditions, a mechanism that could be general to all β-PFTs. The first hint of crowding effect on prepore to pore transition was given by congestion effects in electrophysiology experiments.[23]
The ultimate consequences of lysenin pore formation are not well documented; however, it is thought to induceapoptosis via three possible hypotheses:
Breaking the sphingomyelin asymmetry between the two leaflets of the lipid bilayer by punching holes in the membrane[24] and inducinglipid flip-flop (reorientation of a lipid from one leaflet of a membrane bilayer to the other).[25]
Increasing the calcium concentration in the cytoplasm.[26]
Decreasing the potassium concentration in the cytoplasm.[27]
The biological role of lysenin remains unknown. It has been suggested that lysenin may play a role as adefence mechanism against attackers such asbacteria,fungi or smallinvertebrates.[28] However, lysenin's activity is dependent upon binding to sphingomyelin, which is not present in the membranes of bacteria, fungi or most invertebrates. Rather, sphingomyelin is mainly present in the plasma membrane ofchordates.[29] Another hypothesis is that the earthworm, which is able to expel coelomic fluid under stress,[30][31] generates an avoidance behaviour to itsvertebrate predators (such as birds,hedgehogs ormoles).[32] If that is the case, the expelled lysenin might be more effective if the coelomic fluid reaches the eye, where the concentration of sphingomyelin is ten times higher than in other body organs.[33] A complementary hypothesis is that the pungent smell of the coelomic fluid - giving the earthworm its specific epithetfoetida - is ananti-predator adaptation. However, it remains unknown whether lysenin contributes to avoidance ofEisenia by predators.[34]
Lysenin's conductive properties have been studied for years.[35] Like most pore-forming toxins, lysenin forms a non-specific channel that is permeable to ions, small molecules, and small peptides.[36] There have also been over three decades of studies into finding suitable pores for converting intonanopore sequencing systems that can have their conductive properties tuned by point mutation.[37] Owing to its binding affinity for sphingomyelin, lysenin (or just the receptor binding domain) has been used as a fluorescence marker to detect the sphingomyelin domain in membranes.[38]
^abIshitsuka, R.; Kobayashi, T. (2007). "Cholesterol and Lipid/Protein Ratio Control the Oligomerization of a Sphingomyelin-Specific Toxin, Lysenin".Biochemistry.46 (6):1495–1502.doi:10.1021/bi061290k.PMID17243772.S2CID22016219.
^Simons, K.; Gerl, M. J. (2010). "Revitalizing membrane rafts: new tools and insights".Nature Reviews Molecular Cell Biology.11 (10):688–699.doi:10.1038/nrm2977.PMID20861879.S2CID1866391.
^Ros, U.; García-Sáez, A. J. (2015). "More Than a Pore: The Interplay of Pore-Forming Proteins and Lipid Membranes".The Journal of Membrane Biology.248 (3):545–561.doi:10.1007/s00232-015-9820-y.PMID26087906.S2CID16305100.
^Quinn, P. J. (2013). "Structure of Sphingomyelin Bilayers and Complexes with Cholesterol Forming Membrane Rafts".Langmuir.29 (30):9447–9456.doi:10.1021/la4018129.PMID23863113.
^Ishitsuka, R.; Kobayashi, T. (2007). "Cholesterol and Lipid/Protein Ratio Control the Oligomerization of a Sphingomyelin-Specific Toxin, Lysenin".Biochemistry.46 (6):1495–1502.doi:10.1021/bi061290k.PMID17243772.S2CID22016219.
^Mulvihill, E.; van Pee, K.; Mari, S. A.; Müller, D. J.; Yildiz, Ö. (2015). "Directly Observing the Lipid-Dependent Self-Assembly and Pore-Forming Mechanism of the Cytolytic Toxin Listeriolysin O".Nano Letters.15 (10):6965–6973.Bibcode:2015NanoL..15.6965M.doi:10.1021/acs.nanolett.5b02963.PMID26302195.
^Ros, U.; García-Sáez, A. J. (2015). "More Than a Pore: The Interplay of Pore-Forming Proteins and Lipid Membranes".The Journal of Membrane Biology.248 (3):545–561.doi:10.1007/s00232-015-9820-y.PMID26087906.S2CID16305100.
^Orrenius, S.; Zhivotovsky, B.; Nicotera, P. (2003). "Regulation of cell death: the calcium–apoptosis link".Nature Reviews Molecular Cell Biology.4 (7):552–565.doi:10.1038/nrm1150.PMID12838338.S2CID19079491.
This article was submitted toWikiJournal of Science for externalacademic peer review in 2019 (reviewer reports). The updated content was reintegrated into the Wikipedia page under aCC-BY-SA-3.0 license (2019). The version of record as reviewed is:Ignacio L. B. Munguira; et al. (17 August 2019)."Lysenin"(PDF).WikiJournal of Science.2 (1): 6.doi:10.15347/WJS/2019.006.ISSN 2470-6345.Wikidata Q76846397.