Purification and characterization of two major DNA-binding proteins in human serum

The two major DNA-binding proteins (designated DNA-binding protein 1 and DNA-binding protein 2) in human serum have been purified and physically characterized. The two proteins co-purify through an ion exchange chromatographic step and DNA-cellulose affinity chromatography. Subsequently, DNA-binding protein 1 can be precipitated by 40% saturated ammonium sulfate; DNA-binding protein 2 precipitates in the 55% to saturation fraction. From these fractions, the two proteins are isolated by different protocols. Both purified proteins are homogeneous by the criteria of sodium dodecyl sulfate slab gel electrophoresis after reduction and denaturation and by sedimentation equilibrium centrifugation. DNA-binding protein 1 has a minimum molecular weight of 126,000; DNA-binding protein 2, 86,000. Amino acid analyses of the two proteins indicate that both are relatively rich in proline and cysteine and contain little methionine. Both proteins contain carbohydrate. Gel electrofocusing confirms the acidic nature of these proteins. DNA-binding protein 1 exhibits a single band upon isoelectric focusing, but DNA-binding protein 2 appears to be polymorphic, exhibiting three bands. NH2-terminal end group analysis of DNA-binding protein 2 yields two major amino acids. DNA-binding protein 1 is an alpha2-globulin as determined by immunoelectrophoresis; DNA-binding protein 2 is only weakly immunogenic. Neither of the proteins appears to be identical to any previously characterized serum protein.