Detection and use of recombinant staphylococcal protein A fusion proteins to localize nucleic-acid-binding domains of proteins

We developed a rapid method to visualize recombinant staphylococcal protein A fused to segments of another protein and detect DNA-binding proteins simultaneously. Fusion genes were expressed in Escherichia coli to produce fusion proteins of protein A and segments of c-myc oncoprotein. Polypeptides from total E. coli lysates were separated in sodium dodecyl sulfate-polyacrylamide gels and transferred onto nitrocellulose membranes. Without the use of an antigen-specific antibody, protein A/c-myc fusion proteins were visualized by incubation of nitrocellulose membranes with horseradish-peroxidase (HRP)-conjugated immunoglobulin, the Fc region of which directly binds protein A. The sensitivity of detection of the protein A/c-myc fusion proteins was greatly enhanced by incubation of nitrocellulose membranes with rabbit immunoglobulin before incubation with HRP-conjugated goat anti-rabbit antibody. Prior to incubation of nitrocellulose membranes with immunoglobulin, the membranes were incubated with radiolabeled DNA to visualize DNA-binding proteins by autoradiography. Purified staphylococcal protein A does not bind DNA, whereas a segment of c-myc oncoprotein fused to protein A binds DNA in vitro. These methods may be applied to identify nucleic-acid-binding domains of other proteins without prior purification of the fusion proteins.