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 polyclonal anti-SAA antibodies. Tryptophan fluorescence shows smooth denaturation curves for the fusion protein in guanidinium HCl or potassium thiocyanate. Fluorescence also indicates that only a single tryptophan residue (of the four present) is accessible to iodide quenching and, presumably, is exposed on the surface of the fusion protein. Circular dichroism (CD) shows a significant signal indicating alpha-helix, which can be attributed to the SAA portion of the molecule; these are the first CD spectral data available for SAA. pH titration shows persistence of helix domains for the fusion protein at pH 3.0, in contrast to the denaturation of SN under the same conditions. (The entire fusion protein shows 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 further physical and physiologic studies of SAA.