LLO pathway




[Hamon et al., Trends in Microbiology 2012]

Structure and pore formation mechanism of listeriolysin O (LLO). (a) Schematic representation of the Listeria hly gene encoding LLO. Important amino acids and their positions are indicated above, and the undecapeptide sequence is below. The two transmembrane b-hairpins (TMH) are indicated in red. (b) Structural model of one LLO monomer (without the signal peptide), generated with HHMpred, based on perfringolysin O (PFO), intermedilysin, and suilysin crystal structures (Protein DataBank references PDB1PFO, PDB1S3R and PDB3HVN, respectively). The a-carbon backbone of the protein is shown as a ribbon and relevant amino acids are highlighted by space filled models of their side chains. (c, d) Mechanism of pore formation. A schematic representation of the LLO monomer is shown with each domain color coded similarly to the structural model. Following secretion, toxin monomers bind to the cholesterol of host membranes (c) and diffuse laterally to oligomerize in a ‘prepore complex’. A conformational change in the structure of two a-helical bundles in domain 3 of each monomer to two extended amphipathic TMHs allows the switch from the prepore complex to an effective pore (d). A vertical collapse in the monomer height during this conformational transition allows the TMHs to insert into the membrane and to form a large transmembrane b-barrel pore, as exemplified by the case of PFO [100]. (e) Schematic representation of Listeria intracellular life cycle. After inducing its own uptake by receptor-mediated phagocytosis, Listeria is entrapped in a phagosome (1), which it destabilizes by expressing LLO and two broad-range phospholipases (PLC), PC-PLC and PI-PLC, allowing bacterial escape (2). Intracellular LLO seems to be rapidly degraded to avoid inside-out damaging of host cell membranes. Cytosolic bacteria replicate (3) and express ActA, inducing host cell actin polymerization to propel the bacterium across the cytosol allowing spread to neighboring cells (5). There, Listeria is entrapped in a double-membrane vacuole, employing LLO and PLC expression and possibly additional molecular mechanisms to disrupt it (6) and start the intracellular replication cycle again (7).




[Hamon et al., Trends in Microbiology 2012]

New roles of intracellular listeriolysin O (LLO) during infection. Upon internalization of Listeria monocytogenes in the vacuole and pore-forming activity of LLO, bacteria are released in the cytoplasm of the host cell. Membrane remnants recruit ubiquitin (Ub), the autophagy markers p62 and LC3, and galectins (Gal) 3, 8, and 9. Additionally, LC3 is recruited to bacteria that have escaped the vacuole. Another intracellular role of LLO is to inhibit the NADPH oxidase (NOX2 NADPH Ox) by preventing its localization to phagosomes. This allows bacteria to avoid being killed by reactive oxygen species (ROS) during the respiratory burst.




[Hamon et al., Trends in Microbiology 2012]

Host cell responses to extracellular listeriolysin O (LLO). The involved mechanisms and downstream effects are indicated as far as they have been characterized. Although all of these effects require pore formation, ion flux through the pore has been shown to play a role only in the induction of specific histone modifications, mitochondrial fragmentation, and inflammasome activation. Interestingly, calcium influx, but not potassium efflux, mediates mitochondrial fragmentation, whereas the opposite is true for signaling leading to histone modifications and to inflammasome activation. Abbreviations: AMPK, AMP-activated protein kinase; PKR, protein kinase receptor; SUMO, small ubiquitin-like modifier.




[Hamon et al., Nature Reviews Microbiology 2006]

Listeriolysin O (LLO) pore-forming mechanism. At low (acidic) pH, the soluble LLO monomer interacts with the host-cell plasma membrane, presumably by binding cholesterol. On contact with the membrane, structural rearrangements in one monomer expose residues that can form hydrogen bonds with other monomers, thereby allowing oligomerization into a pre-pore complex. Following oligomerization, two α-helical bundles from each monomer extend to form transmembrane β-hairpins (red) that punch through the membrane. Pores formed by LLO and other cholesterol-binding proteins can be 250–300 Å in diameter. At neutral pH, domain 3 (D3) of the monomer prematurely unfolds, rendering the protein unable to form pores.




[Pizarro-Cerda et al., Journal of Pathology 2006]

Listeriolysin O (LLO) activity during bacterial entry. Active LLO is secreted during the invasion phase and potentiates the entry of L. monocytogenes. Once the bacteria are within a phagosome, the LLO becomes more active because it is adapted to the acidic pH of the intra-phagosomal environment, favouring

the lysis of this compartment. Once the phagosome is broken, the bacteria are freed into the cytoplasm, where the LLO is neutralized by the neutral pH of the cytosol; the host cell also recognizes a PEST sequence in the LLO, targeting this molecule to the cell degradation machinery.

Updated on 13/05/2014



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