Protein A from Staphylococcus aureus

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.