A-hemolysin

Staphylococcal a-hemolysin is another widely studied pore-forming toxin. It is used by infectious bacteria to perforate host animal cells by a mechanism that is distinct from that of gramicidin. Several aspects of the stmcture and function of this heptameric protein complex have been smdied. 

Aeromonas hydrophila is a bacterium that causes diarrheal diseases and deep wound infections. These complications arise due to pore formation in sensitive cells by the protein toxin aerolysin. Proteolytic processing of the 52-kD precursor proaerolysin (Figure 10.34) produces the toxic form of the protein, aerolysin. Like a-hemolysin, aerolysin monomers associate to form a heptameric transmembrane pore. Michael Parker and coworkers have proposed... 

In reconstitution experiments, the self-assembly of the pore-forming protein a-hemolysin of Staphylococcus aureus (aHL) [181-183] was examined in plain and S-layer-supported lipid bilayers. Staphylococcal aHL formed lytic pores when added to the lipid-exposed side of the DPhPC bilayer with or without an attached S-layer from B coagulans E38/vl. The assembly of aHL pores was slower at S-layer-supported compared to unsupported folded membranes. No assembly could be detected upon adding aHL monomers to the S-layer face of the composite membrane. Therefore, the intrinsic molecular sieving properties of the S-layer lattice did not allow passage of aHL monomers through the S-layer pores to the lipid bilayer [142]. 

The prototype of a small pore-forming toxin is the S. aureus a-toxin, also called ct-hemolysin, that has been extensively investigated hy Bhakdi and coworkers. Monomers of ct-hemolysin (33 kDa) hind to the surface of erythrocytes, and after lateral diffusion within the lipid hilayer, seven monomers oligomerize to form pores in the cell membrane. The ct-hemolysin forms mushroom-shaped pores with an outer diameter of lOnm and an inner diameter of approximately 2.5 nm. Small molecules can pass through the pore and diffuse into/out of the cytosol, along with water. As a consequence of such movement, cell homeostasis is greatly disturbed and pushed into an unhealthy state. In animals, the a-hemolysin represents a major virulence factor of S. aureus which causes hemolysis as well as tissue destruction. ... 

The expression of the a-hemolysin pore protein from S. aureus inside the vesicle solved the energy and material limitations the reactor could sustain expression for up to four days. 

Squalene, squalane Phosvitin Phytic acid Phospholipids Spermidine Hyaluronic acid Sphingomyelin Biopolymer Fibronectin Spermine-bile acids Deacylated saponin a-Hemolysin... 

Whereas the previous example relies on a series of channels etched into the membrane, typical single-channel stochastic sensors can be created in a closely related way.91 For such sensors, usually one protein (e.g., a-hemolysin) is introduced into a lipid bilayer. However, the problem here is that lipid bilayers are rather fragile objects. As an alternative, Martin and coworkers embedded a single gold nanotube... 

A completely different principle has been followed by the heptameric a-hemolysines (Song et al., 1996 Olson et al., 1999). These proteins associate with their extramembrane domains. Subsequently, each subunit donates a /3-hairpin to form a common 14-stranded /3-barrel through the membrane. In a similar manner TolC is assembled from three a-helical subunits (Koronakis et al., 2000). The subunits form a long, wide channel that spans the periplasm in their a-helical part and that is prolonged through the outer membrane by the /3-barrel. The channel is used for the export of xenobiotics. 

Bacterial pore-forming proteins, such as a-hemolysin, secreted by Staphylococcus aureus can be modified so that pore formation is activated by chemical, biochemical, or physical triggers. Such hemolysins, when targeted to tumors, could increase tumor permeability, and hence susceptibility to various cytotoxic drugs (78). 

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