A subclass of simian virus 40 T antigen with a high cell surface binding affinity.

SV40 virus-infected and -transformed cells express large T antigen on the cell surface (surface T). In the present study the cell surface binding properties of T antigen extracted from SV40-transformed cells were investigated. Only small amounts of T antigen with tight cell surface binding properties were efficiently removable by absorption on living cells from the majority of T antigen detectable in cell extracts. As shown in immunofluorescence microscopy both native surface T and experimentally in vitro cell surface bound T antigen were stained in similar microcluster patterns. Comparative SDS-polyacrylamide gel electrophoretic analysis indicated that T antigen extracted from SV40-transformed cells and in vitro cell surface bound T antigen had the same apparent molecular weight of approximately 90,000 da. A quantitative 125I-protein A binding assay using antisera directed against purified T antigen demonstrated that a metal-ion chelating agent (EDTA) or hypertonic salt solutions were unable to remove surface T or in vitro cell surface bound T antigen from living cells. In contrast, both antigens could be solubilized by detergents. Moreover, both types of cell surface associated T antigens seemed to be metabolically stable. Altogether, one can postulate a minor subclass of T antigen with a tight binding affinity to the cell surface of living cells. According to these properties this experimentally membrane bound subclass, as well as native surface T, seem to belong to the class of integral rather than peripheral membrane proteins.