Only a minor fraction of plasma membrane-associated large T antigen in simian virus 40-transformed mouse tumor cells (mKSA) is exposed on the cell surface.
The bulk of simian virus 40 (SV40) large T antigen in SV40-infected and -transformed cells localizes within the cell nucleus, while a minor fraction specifically associates with the plasma membrane (PM) and is exposed on the cell surface. PM-associated large T seems to span the lipid bilayer but, on the other hand, does not display typical features of a transmembrane protein. To further characterize the postulated transmembrane orientation of large T, we asked whether all large T molecules associated with the plasma membrane indeed are exposed on the cell surface. We compared the amount of cell surface-exposed large T, determined on living cells by a sensitive 3H-protein A-binding assay and by external immunoprecipitation, with that of total PM-associated large T extracted from isolated PM. We demonstrate that in mKSA cells (SV40-transformed BALB/c mouse fibroblasts), total PM-associated large T accounted for a substantial portion (ca. 2%) of total cellular large T. However, only 0.1 to 0.2% of it could be detected on the cell surface. Thus, only a minor fraction of PM-associated large T (less than 10%) is exposed on the surface of these cells. Interior PM-associated large T is stably associated with the plasma membrane, while the small fraction of surface-exposed large T is rapidly released from the cell surface.
Heinrich-Pette-Institut für Experimentelle Virologie und Immunologie, Universität Hamburg, Federal Republic of Germany.
A
A
Walser
Y
Y
Rinke
W
W
Deppert
D000256:Adenoviridae; D000818:Animals; D000952:Antigens, Polyomavirus Transforming / Q000032:analysis* / Q000276:immunology; D000954:Antigens, Surface / Q000032:analysis*; D002461:Cell Line, Transformed; D002462:Cell Membrane / Q000032:analysis; D051379:Mice; D011233:Precipitin Tests; D013205:Staphylococcal Protein A / Q000378:metabolism
D000952:Antigens, Polyomavirus Transforming; D000954:Antigens, Surface; D013205:Staphylococcal Protein A