Characterization of middle T antigen expressed by using an adenovirus expression system

The adenovirus Ad5(pymT) has been used to express middle T antigen at very high levels in 293 cells. The middle T antigen produced was localized to membranes and was modified in the same way as that expressed in polyoma virus-infected mouse cells. It was phosphorylated in vivo on serine residues and in vitro on tyrosine residues. The in vivo phosphorylations occurred between residues 223 and 275. The middle T antigen encoded by A d5(pymT) was phosphorylated in vitro in a complex with human pp60c-src. Interestingly, the extreme overexpression of middle T antigen did not cause a parallel increase in the amount of complex; most of the pp60c-src remained unassociated. Immunoaffinity purification resulted in approximately 100 micrograms of middle T antigen from a 100-mm tissue culture dish. Several cell proteins copurified with the Ad5(pymT)-derived middle T antigen. Two of these, the 74- and 63-kilodalton species, are of particular interest because they were also purified from mouse tumors expressing middle T antigen.