Alpha-toxin, also known as alpha-hemolysin (Hla), is the major cytotoxic agent released by bacterium Staphylococcus aureus

Staphylococcus aureus

Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...

and the first identified member of the pore forming beta-barrel toxin family. This toxin consists mostly of beta-sheets (68%) with only about 10% alpha-helices. The hla gene on the S. aureus chromosome encodes the 293 residue protein monomer, which forms heptameric units on the cellular membrane to form a complete beta-barrel pore. This structure allows the toxin to perform its major function, development of pores in the cellular membrane, eventually causing cell death.

Function

Alpha-toxin has been shown to play a role in pathogenesis of disease, as hla knockout strains show reductions in invasiveness and virulence. Interestingly, the dosage of toxin can result in two different modes of activity. Low concentrations of toxin bind to specific, but unidentified, cell surface receptors and form the heptameric pores. This pore allows the exchange of monovalent ions, resulting in DNA fragmentation and eventually apoptosis

Apoptosis

Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

. Higher concentrations result in the toxin absorbing nonspecifically to the lipid bilayer and forming large, Ca²⁺ permissive pores. This in turn results in massive necrosis and other secondary cellular reactions triggered by the uncontrolled Ca²⁺ influx.

Structure

The structure of the protein has been solved by X-ray crystallography

X-ray crystallography

X-ray crystallography is a method of determining the arrangement of atoms within a crystal, in which a beam of X-rays strikes a crystal and causes the beam of light to spread into many specific directions. From the angles and intensities of these diffracted beams, a crystallographer can produce a...

and is deposited in the PDB

Protein Data Bank

The Protein Data Bank is a repository for the 3-D structural data of large biological molecules, such as proteins and nucleic acids....

 as id code 7ahl. Seven monomers each contribute a long beta-hairpin to a fourteen stranded beta barrel that forms a pore in the cell membrane. This pore is 14 Ångström

Ångström

The angstrom or ångström, is a unit of length equal to 1/10,000,000,000 of a meter . Its symbol is the Swedish letter Å....

 wide at its narrowest point.

Role in Apoptosis

Recently, studies have shown that alpha-toxin plays a role in inducing apoptosis in certain human immune cells. Incubation of T-cells, monocytes, and peripheral blood lymphocytes with either purified alpha-toxin or S. aureus cell lysate resulted in the induction of apoptosis via the intrinsic death pathway. This activity was inhibited when two different anti-alpha-toxin antibodies were introduced. In the same study, alpha toxin was shown to activate caspase 8

Caspase 8

Caspase 8 is a caspase protein, encoded by the CASP8 gene. It most likely acts upon caspase 3.CASP8 orthologs have been identified in numerous mammals for which complete genome data are available...

 and caspase 9, which in turn activate caspase 3

Caspase 3

Caspase 3 is a caspase protein that interacts with caspase 8 and caspase 9. It is encoded by the CASP3 gene. CASP3 orthologs have been identified in numerous mammals for which complete genome data are available...

, which causes massive DNA degradation and apoptosis. This activity was shown to be independent of the death receptor pathway.

Vaccine Development

Alpha-toxin is also one of the key virulence factors in S. aureus pneumonia. The level of alpha-toxin expressed by a particular strain of S. aureus directly correlates with the virulence of the strain. Recent research has shown that immunization with a mutant form of alpha-toxin that is no longer able to form pores protects against S. aureus pneumonia in mice. Also, introduction of alpha-toxin specific antibodies into an unimmunized animal protects against subsequent infection. Cultures of human lung epithelial cells incubated with anti-alpha-toxin and infected with S. aureus showed marked reductions in cellular damage when compared to control cells. As many strains of S. aureus are proving to be resistant to most available antibiotics, specific targeting of virulence factors with antibodies may be the next step to treating this pathogen.