1.C.43 The Earthworm Lysenin Toxin (Lysenin) Family
Lysenin (also called eiseniapore) is a 297 aa protein that specifically binds to sphingomyelin and cholesterol-containing membranes of mammalian cells, including red blood cells and tissue cells, and induces lysis. It is derived from the coelomic fluid of the earthworm, Eisenia foetida. The protein binds to both sphingomyelin and galactosyl ceramide but not to ceramide or galactosyl sphingosine. It probably causes lysis by producing oligomeric protein pores in the target membrane. Lysenin has been reported to have antibacterial activity. It shows weak motif similarity to crystal protein, CryET33.the crystal structure of the lysenin pore and provide insights into its assembly mechanism. The lysenin pore is assembled from nine monomers via dramatic reorganization of almost half of the monomeric subunit structure leading to a β-barrel pore ∼10 nm long and 1.6-2.5 nm wide. The lysenin pore is devoid of additional luminal compartments as commonly found in other toxin pores. Mutagenic analysis and atomic force microscopy imaging, together with these structural insights, suggest a mechanism for pore assembly for lysenin. These insights are relevant to the understanding of pore formation by other aerolysin-like pore-forming toxins, which often represent crucial virulence factors in bacteria. The mechanism of the voltage dependence of lysenin has been studied revealing that the movement of the voltage domain sensor into a physically different environment that precludes electrostatically bound ions may be an integral part of the gating mechanism (Bryant et al. 2018).
Lysenin forms stable oligomers upon membrane binding and causes cell lysis. To get insight into the mechanism of the transition of lysenin from a soluble to a membrane-bound form, the binding of lysenin to lipid membranes was studied by Hereć et al. (2008). The total content of alpha-helices, turns and loops, and beta-structures did not change when it became membrane bound. The alpha-helical component was oriented at 41 degrees to the normal to the membrane, indicating that this protein segment could be anchored in the hydrophobic core of the membrane (Hereć et al., 2008). Non-permeant dyes could be incorporated into liposomes via lysenin channels by controlling their conducting state with multivalent metal cations (Shrestha et al. 2020).
The crystal structure of the lysenin pore has been reported (Podobnik et al. 2016). The pore is assembled from nine monomers via dramatic reorganization of almost half of the monomeric subunit structure leading to a β-barrel pore ∼10 nm long and 1.6-2.5 nm wide. Mutagenic analysis and atomic force microscopy imaging suggested a mechanism for pore assembly (Podobnik et al. 2016). The voltage sensor domain strongly affects the voltage regulation only during inactivation (channel closing). In contrast, channel reactivation (reopening) presents a more stable, almost invariant voltage dependency. In the presence of ATP, which binds at a different site in the channel's structure and occludes the conducting pathway, both inactivation and reactivation pathways are significantly affected. Therefore, the movement of the voltage domain sensor into a physically different environment that precludes electrostatically bound ions may be an integral part of the gating mechanism (Bryant et al. 2018).
The generalized reaction catalyzed by members of the Lysenin family is:
Ions (in) 
ions (out)