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IntroductionIntroduction

spinqualitylabelsall models Staphylococcal protein A Show:Asymmetric Unit Biological Assembly Drag the structure with the mouse to rotate   Export Animated Image

Staphylococcal protein A is originally a component of the cell wall in over 90% of Staphylococcus aureus strains.This protein, which is also called immunoglobulin G binding protein A is covalently linked to the peptidoglycan of the cell wall. Nowadays the protein is often used for biochemical analysis due to its structural and biochemical properties. The expression of the chromosomal spa gene, which encodes the protein A, is regulated by the agr system and Rot. The level of expression is up-regulated by Rot and down-regulated by agr. Furthermore protein A serves as a virulence factor to Staphylococcus aureus . It is essential for the colonization and infections mediated by this kind of bacteria.

Structure of the proteinStructure of the protein

The structural details of protein A were solved by the nuclear magnetic resonance method. The length of the amino acid chain of protein A contains 508 residues. The amino acids cystein and tryptophan do not occur in the amino acid sequence. The molecular weight of the described protein is 55439 Dalton and it consists of only one protein chain. The 3D structure is build up of three α-helixes and it consists of five extracellular domains, which are designated as E, D, A, B and C. Furthermore the protein contains cell-wall spanning regions, called Xr and Xc, and a hydrophobic membrane spanning domain, which is distal to LPXTG and consists of 18-20 residues ( Hartleib et al 2000 Protein A is the vWF binding protein on S. aureus). Protein A exists in a secreted and a cell wall anchored form. If it is bound to the cell wall of Staphylococcus aureus it is covalently linked to the peptidoglycan via its C-terminal domain.

Function associated to this proteinFunction associated to this protein

Protein A has been identified as a cell surface protein of Staphylococcus aureus which contributes to the staphylococcal virulence. The virulence is mediated by the ability to interact with plasma protein. Protein A is able to bind to the Fc (constant region of IgG which is involved in effector functions) and the Fab fragment (also a part of Ig which is responsible for antigen recognition). The interactions of protein A with plasma proteins are mediated by five homologous domains, which are called E, D, A, B and C. Each of the homologous repeat domains comprises 56-61 residues, which are followed by a polymorphic variable repeat region, which is called Xr, and a conserved region Xc, which includes a cell wall attachment sequence. Each domain is able to bind one IgG molecule through its Fcγ binding site . The binding of protein A to the Fc fragment plays a major role in the virulence of Staphylococcus aureus as it competes with phagocytic cells for available IgG-Fc sites. This results in a reduction of IgG- mediated opsonization. The domains D and E of protein A are able to bind to the Fab fragments of immunoglobulins through variable (V) regions. The Fv-binding sites enable protein A to cross-link membrane IgM on B cells and therefore mediate the activation of these cells, which confers a superantigen function to protein A. Several features of the interactions of protein A with host B lymphocytes are similar to those of superantigens for T lymphocytes, which can cause various inflammatory diseases, such as toxic shock syndrome and food poisoning. Furthermore it was shown that the staphylococcal protein A is able to activate the classical complement pathway. This activation depends on the binding of a VH3+ IgM molecule to protein A, which results in the generation of an inflammatory reaction. The protein A induced complement activation is another factor, which contributes to the staphylococcal virulence. Moreover, invasive Staphylococcus aureus disease are often associated with the complication of endovascular infections. In order to cause this kind of complication staphylococci must first adhere to endovascular foci and colonize these tissues. One of the factors released by endothelial cells and by platelets is the von Willbrrand factor (vWF). This factor mediates the adhesion of platelets at damaged endothelial sites. It was shown that vWF binds to and also promotes the adhesion of staphylococcal cells to vWF-absorbed surfaces. It could also be demonstrated that the recognition of vWF is mediated by the staphylococcal protein A (Hartleib et al. 2000 Protein A is the von Willebrand factor binding protein on Staphylococcus aureus). Protein A defective mutants have shown reduced virulence in murine models. These observations can be explained most likely by the antiphagocytic effect of protein A binding IgG Fc fragments.