A FUSION PROTEIN BETWEEN SERUM AMYLOID-A AND STAPHYLOCOCCAL NUCLEASE - SYNTHESIS, PURIFICATION, AND STRUCTURAL STUDIES

We developed a recombinant DNA system to overexpress a fusion protein between the small, minimally soluble acute phase serum protein, serum amyloid A (SAA), and the bacterial enzyme staphylococcal nuclease (SN) , This fusion protein is very soluble and is immunoreactive to polyclo nal anti-SAA antibodies, Tryptophan fluorescence shows smooth denatura tion curves for the fusion protein in guanidinium HCl or potassium thi ocyanate, Fluorescence also indicates that only a single tryptophan re sidue (of the four present) is accessible to iodide quenching and, pre sumably, is exposed on the surface of the fusion protein, Circular dic hroism (CD) shows a significant signal indicating a-helix, which can b e attributed to the SAA portion of the molecule; these are the first C D spectral data available for SAA. pH titration shows persistence of h elix domains for the fusion protein at pH 3.0, in contrast to the dena turation of SN under the same conditions, (The entire fusion protein s hows a random coil pattern below pH 3.0.) By exploiting the structural and solubility properties of SN, this fusion protein has provided the first structural data about SAA-the precursor of the amyloid deposits in secondary amyloidosis, This fusion protein should be useful for fu rther physical and physiologic studies of SAA. (C) 1998 Wiley-Liss, In c.