Polyomavirus small t antigen: overproduction in bacteria, purification, and utilization for monoclonal and polyclonal antibody production

Abstract

Polyomavirus small t antigen was purified from genetically engineered Escherichia coli and used as the immunogen for the production of polyclonal and monoclonal antibodies. A new series of plasmids for increased expression of polyomavirus T antigens or a T antigen-beta-galactosidase fusion protein was constructed by replacing sequences coding for the ribosome-binding site of previously published plasmids with a chemically synthesized sequence that has a higher degree of complementarity to the 3' end of the 16S rRNA. Cells expressing the fusion protein from the plasmid with the synthetic sequence contained 5- to 10-fold more fusion protein after a 3-h induction than did control cells. Pulse-labeling of cells bearing the new plasmids revealed that the T antigens were synthesized at high levels after induction: 10% of total synthesis for small t; 15% for Py-1387T middle T, a truncated mutant of middle T; and probably 1 to 5% for middle T. Small t and Py-1387T middle T, but not wild-type middle T, were seen as minor bands in total cell protein analyzed on sodium dodecyl sulfate-polyacrylamide gels stained with Coomassie blue. A simple, rapid procedure for purification of bacterial small t from the pellet of sonicated bacteria yielded 1 to 2 mg of small t per liter of bacterial culture at 80 to 90% homogeneity. High-titer polyclonal rabbit antisera raised against purified small t recognized all three T antigens and were suitable for immunoaffinity purification of middle T. Mouse monoclonal antibodies raised against bacterial small t were of four classes, immunoprecipitating either all three polyomavirus T antigens, small t and middle T only, primarily small t, or middle T and large T in preference to small t. One of the latter monoclonal antibodies also immunoprecipitated large T but not small t of simian virus 40, suggesting that the site recognized by this antibody may be functionally important. None of the monoclonal antibodies yielded an immunoprecipitate active in phosphorylating middle T in vitro.