Subclasses of simian-virus-40 large tumor antigen. Partial purification and DNA-binding properties of two subclasses of tumor antigen from productively infected cells.
Two major subclasses of simian virus 40 tumor antigen were prepared from productively infected monkey cells. These subclasses can be distinguished by their sedimentation properties: one tumor antigen form sediments at 5-6S and the other at 14-16S. The DNA-binding properties of these subclasses were investigated by two different experimental procedures. In the first procedure, the DNA binding of subclasses of crude tumor antigen, separated by zone velocity sedimentation, were assayed by immunoprecipitation of the DNA-protein complexes. In the second procedure, the two tumor antigen forms were partially purified by column chromatography and DNA binding was tested in a filter binding assay. Both procedures gave comparable results. (a) The 5-6-S and the 14-16-S tumor antigen bound specifically to a DNA restriction fragment containing the viral genome control regions. (b) At low salt concentrations, both subclasses bound to specific and to nonspecific DNA sequences; competition experiments in the presence of nonspecific DNA showed, however, that the affinity of both tumor antigen forms for the viral genome control region was at least 10-fold higher than their affinity for nonspecific DNA sequences. (c) The binding of the 5-6-S subclass to viral control region DNA was optimal at 60-80 mM NaCl while specific DNA binding of the 14-16-S form was optimal at 150-200 mM NaCl; however, binding of the 14-16-S form to nonspecific DNA sequences was also more resistant to high salt concentrations than that of the 5-6S form. (d) Both tumor antigen forms bound well to specific and to nonspecific DNA at pH 6-6.5; with increasing pH values, binding to nonspecific DNA decreased while binding to specific DNA reached an optimum at pH 7-7.5. Binding of the 14-16-S form to viral origin DNA was more resistant to pH values above 7.5 than binding of the 5-6-S form.