Protein A from Staphylococcus aureus. The effect of iodination on the reactivity with immunoglobulin G

Dynamic light scattering as a powerful tool for gold nanoparticle bioconjugation and biomolecular binding studies

Dynamic light scattering (DLS) is an analytical tool used routinely for measuring the hydrodynamic size of nanoparticles and colloids in a liquid environment. Gold nanoparticles (GNPs) are extraordinary light scatterers at or near their surface plasmon resonance wavelength. In this study, we demonstrate that DLS can be used as a very convenient and powerful tool for gold nanoparticle bioconjugation and biomolecular binding studies. The conjugation process between protein A and gold nanoparticles under different experimental conditions and the quality as well as the stability of the prepared conjugates were monitored and characterized systematically by DLS. Furthermore, the specific interactions between protein A-conjugated gold nanoparticles and a target protein, human IgG, can be detected and monitored in situ by measuring the average particle size change of the assay solution. For the first time, we demonstrate that DLS is able to directly and quantitatively measure the binding stoichiometry between a protein-conjugated GNP probe and a target analyte protein in solution.

Desorption of immunoglobulins from Protein A-Sepharose CL-4B under mild conditions

Conditions which permit the dissociation of IgG-staphylococcal protein A interactions without resort to low pH buffers or the use of chaotropic ions are discussed in relation to: (a) the mechanism of the binding reaction; (b) the use of immobilised protein A for purification of cells or removal of immune complexes from serum. Glycyl-tyrosine, glycyl-histidine, glycyl-phenylalanine and tryptophan, representing the class of competing ligand desorbents, and ethylene glycol, an agent which disrupts hydrophobic interactions, were found to be the best desorbents.

The use of protein A in solid-phase binding assays: a comparison of four radioiodination techniques

Preparations of protein A radioiodinated by 4 different methods have been compared in indirect radioimmunoassays. The oxidative methods (chloramine-T and iodogen) for direct iodination of tyrosyl and histidyl residues were applied with high efficiency and gave a suitable product, provided the substitution ratio was kept low (1 iodine atom/molecule of protein A). Higher levels of modification tended to perturb the Fc-binding characteristics of the protein, especially with the use of iodogen. Introduction of the isotope via substitution of lysyl residues (Bolton-Hunter and Wood reagents) was also examined. The Bolton-Hunter modification of protein A gave an unsuitably low labeling efficiency; in contrast, the Wood reagent gave efficiencies approaching 50%. Protein A could be extensively substituted with the latter reagent (greater than 5 diiodinated benzimidate molecules per protein molecule). Thus, the use of the Wood-labeled protein A could raise the sensitivity of the binding assay at least an order of magnitude compared to using protein A iodinated by the oxidative methods. The effects on the biological activity of protein A exerted by the different labeling procedures are rationalized on the basis of the amino acid composition and tertiary structure of the protein.

Protein A from Staphylococcus aureus. Spectropolarimetric and Spectrophotometric studies

Protein A, a cell-wall protein from Staphylococcus aureus, has been studied by spectrophotometry and spectropolarimetry. All the four tyrosines are similarily titrated with pK-a equals 10.25. Circular dichroism (CD) spectra show that the conformation of protein A is very stable over a large pH interval (0.99-11.8). The conformation is partly intact even in 6 M guanidine hydrochloride and at 80 degrees C. Protein A contains about 50% alpha-helical structure and 10-20% beta-structures. CD band maxima at 261 and 268 nm are ascribed to transitions in the phenylalanine residues and ellipticities between 275-285 nm to the tyrosine residues. Of the four tyrosines, 3.5 are perturbed by 20% polyethylene glycol, while all of them are perturbed by 20% dimethylsulfoxide. The role of the tyrosines in the reaction with the Fc-region of immunoglobulins is discussed.