Toxic shock syndrome toxin-1 (TSST-1) is a superantigen with a size of 22 kDa produced by 5 to 25% of Staphylococcus aureus isolates. It causes toxic shock syndrome (TSS) by stimulating the release of large amounts of interleukin-1, interleukin-2 and tumor necrosis factor. In general, the toxin is not produced by bacteria growing in the blood; rather, it is produced at the local site of an infection, and then enters the blood stream.
Characteristics Toxic shock syndrome toxin-1 (TSST-1), a prototype superantigen secreted by a Staphylococcus aureus bacterium strain in susceptible hosts, acts on the vascular system by causing inflammation, fever, and shock. The bacterium strain that produces the TSST-1 can be found in any area of the body, but lives mostly in the vagina of infected women. TSST-1 is a bacterial exotoxin found in patients who have developed toxic shock syndrome (TSS), which can be found in menstruating women or any man or child for that matter. One-third of all TSS cases have been found in men. This statistic could possibly be due to surgical wounds or any skin wound. TSST-1 is the cause of half of non-menstrual TSS cases, and the sole cause for menstrual TSS cases.
Structure In the nucleotide sequence of TSST-1, there is a 708 base-pair open-reading frame and a Shine-Dalgarno sequence which is seven base pairs downstream from the start site. In the entire nucleotide sequence, only 40 amino acids make up the signal peptide. A single signal peptide consists of a 1 to 3 basic amino acid terminus, a hydrophobic region of 15 residues, a proline (Pro) or glycine (Gly) in the hydrophobic core region, a serine (Ser) or threonine (Thr) amino acid near the carboxyl terminal end of the hydrophobic core, and an alanine (Ala) or glycine (Gly) at the cleavage site. A mature TSST-1 protein has a coding sequence of 585 base pairs. The entire nucleotide sequence was determined by Blomster-Hautamaazg, et al., as well as by other researchers with other experiments. Consisting of a single polypeptide chain, the structure of holotoxin TSST-1 is three-dimensional and consists of an alpha (α) and beta (β) domain. This three-dimensional structure of the TSST-1 protein was determined by purifying the crystals of the protein. The two domains are adjacent from each other and possess unique qualities. Domain A, the larger of the two domains, contains residues 1-17 and 90–194 in TSST-1 and consists of a long alpha (α) helix with residues 125-140 surrounded by a 5-strand beta (β) sheet. Domain B is unique because it contains residues 18–89 in TSST-1 and consists of a (β) barrel made up of 5 β-strands. Crystallography methods show that the internal β-barrel of domain B contains several hydrophobic amino acids and hydrophilic residues on the surface of the domain, which allows TSST-1 to cross mucous surfaces of epithelial cells. Even though TSST-1 consists of several hydrophobic amino acids, this protein is highly soluble in water. TSST-1 is resistant to heat and proteolysis. It has been shown that TSST-1 can be boiled for more than an hour without any presence of denaturation or direct effect on its function.
Production TSST-1 is a protein encoded by the tst gene, which is part of the mobile genetic element staphylococcal pathogenicity island 1. The toxin is produced in the greatest volumes during the post-exponential phase of growth, which is similar among pyrogenic toxin superantigens, also known as PTSAgs. Oxygen is required in order to produce TSST-1, in addition to the presence of animal protein, low levels of glucose, and temperatures between 37–40 °C (99–104 °F). Production is optimal at pH's close to neutral and when magnesium levels are low, and is further amplified by high concentrations of S. aureus, which indicates its importance in establishing infection. TSST-1 differs from other PTSAgs in that its genetic sequence does not have a homolog with other superantigen sequences. TSST-1 does not have a cysteine loop, which is an important structure in other PTSAgs. TSST-1 is also different from other PTSAgs in its ability to cross mucous membranes, which is why it is an important factor in menstrual TSS. When the protein is translated, it is in a pro-protein form, and can only leave the cell once the signal sequence has been cleaved off. The agr (accessory gene regulator) locus is one of the key sites of positive regulation for many of the S. aureus genes, including TSST-1. Additionally, alterations in the expression of the genes ssrB and srrAB affect the transcription of TSST-1. Further, high levels of glucose inhibit transcription, since glucose acts as a catabolite repressor.
Mutations Based on studies of various mutations of the protein it appears that the superantigenic and lethal portions of the protein are separate. One variant in particular, TSST-ovine or TSST-O, was important in determining the regions of biological importance in TSST-1. TSST-O does not cause TSS, and is non-mitogenic, and differs in sequence from TSST-1 in 14 nucleotides, which corresponds to 9 amino acids. Two of these are cleaved off as part of the signal sequence, and are therefore not important in the difference in function observed. From the studies observing the differences in these two proteins, it was discovered that residue 135 is critical in both lethality and mitogenicity, while mutations in residues 132 and 136 caused the protein to lose its ability to cause TSS, however there were still signs of superantigenicity. If the lysine at residue 132 in TSST-O is changed to a glutamate, the mutant regains little superantigenicity, but becomes lethal, meaning that the ability to cause TSS results from the glutamate at residue 132. The loss of activity from these mutations is not due to changes in the protein's conformation, but instead these residues appear to be critical in the interactions with T-cell receptors.
Isolation Samples of TSST-1 can be purified from bacterial cultures to use in in vitro testing environments, however this is not ideal due to the large number of factors that contribute to pathenogenesis in an in vivo environment. Additionally, culturing bacteria in vitro provides an environment which is rich in nutrients, in contrast to the reality of an in vivo environment, in which nutrients tend to be more scarce. TSST-1 can be purified by preparative isoelectric focusing for use in vitro or for animal models using a mini-osmotic pump.
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