Isolation of multiple forms of DNA polymerase delta: evidence of proteolytic modification during isolation

The subunit structures of a number of human placenta DNA polymerase delta preparations were investigated by Western blotting with polyclonal antisera and by activity staining following polyacrylamide gel electrophoresis. When immunoblots and activity stains were performed on different enzyme preparations, putative catalytic subunits of (a) 170, (b) 120, or (c) 50-70 kilodaltons (kDa) were observed. It was also observed that the lower molecular weight forms could be generated upon storage of the preparations. Western blotting of human placental tissue extracts showed that the major immunoreactive polypeptide was 160-170 kDa. Treatment of the extracts with trypsin or Staphylococcus aureus V8 protease led to the generation of immunoreactive polypeptides of lower molecular weights. These studies suggest that the 120-kDa and lower forms of the enzyme are generated via uncontrolled proteolysis and provide a rationale for the observation of different apparent subunit structures previously reported for DNA polymerase delta. In addition, these findings suggest that DNA polymerase delta has a catalytic domain which resides in a protease-resistant domain.