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

Abstract

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 PMassociated 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.

Phosphorylation of Human Polyomavirus Large and Small T Antigens: An Ignored Research Field

“…SV40 LTAg, like all eukaryotic proteins, is synthesized in the cytoplasm, but it is mainly relocalized to the nucleus. A minor fraction associates with the plasma membrane and is exposed to the cell surface [88]. Nuclear import is regulated by a nuclear localization report (NLS) with the sequence PKKKRKV (residues 126-131) and the nuclear transport factor importin [89].…”

Insulin‐induced mitogenesis associated with transformation by the SV40 large T antigen

“…significant relationship, either directly or indirectly, between SV40 T antigen and the mitogenic responsiveness of cells t o insulin. It has been reco nized for several years that up to 2% of the total cellu ar T anti en in SV40-transformed both the carboxy and amino termini of the T antigen molecule are extracellularly exposed (Santos and Butel, 1984;Whittaker et al, 1985;Walser et al, 1989). Although the specific effect of plasma membrane T antigen is unknown, it has been suggested that surface T antigen may be involved in determining the growth characteristics in SV40-transformed cells (Santos and Butel, 1985).…”

Functional interaction of nuclear transport-defective simian virus 40 large T antigen with chromatin and nuclear matrix

“…The multitude of tasks performed by the simian virus 40 (SV40) large tumor antigen (large T) in viral replication and in cellular transformation requires its functional interaction with a variety of cellular targets. At the structural level, these targets are associated with different structural systems of the cell, leading to an unusual subcellular distribution of large T in SV40-infected and -transformed cells: approximately 2% of total large T in SV40-transformed cells is specifically associated with the plasma membrane, with about 10% of plasma membrane-associated large T specifically exposed on the cell surface (19,44). Some further 10% of total large T is associated with intracellular membranes (44), but the vast majority of large T is found in the cell nucleus.…”