Tightly associated lipids may anchor SV40 large T antigen in plasma membrane.

Simian virus 40 (SV40) large T antigen, a multifunctional protein necessary for lytic growth and cell transformation, is located mainly in the nucleus and in small amounts on the cell surface (surface T). Surface T may have a passive role in SV40 tumour rejection by cytotoxic T cells as a component of SV40-TSTA (tumour-specific transplantation antigen). The unusual induction of this immune response by immunizing mice with soluble T antigen led us to investigate the in vitro binding of T antigen to the surface of living cells in more detail. Our results show that native surface T and a minor subset of large T antigen having a high cell surface binding affinity in vitro, behave like integral membrane proteins. Several viral proteins including SV40 T antigen and cellular proteins seem to be linked to fatty acids (acylation). To analyse whether this mechanism is involved in the stable attachment of in vitro-bound T antigen to the plasma membrane of living target cells, we determined the degree of labelling of this molecule by using target cells prelabelled with 3H-fatty acid. Here we report that T antigen extracted from unlabelled SV40-transformed cells (SV80) becomes 3H-labelled after in vitro binding to the cell surface of 3H-palmitate-prelabelled HeLa cells. These results suggest that T antigen attached externally to living cells, may be anchored by tightly linked lipids.