SV40 T antigen and the exocytotic pathway.

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Abstract
A chimeric gene consisting of DNA coding for the 15‐amino acid signal peptide of influenza virus hemagglutinin and the C‐terminal 694 amino acids of SV40 large T antigen was inserted into a bovine papilloma virus (BPV) expression vector and introduced into NIH‐3T3 cells. Cell lines were obtained that express high levels (approximately 5 X 10(6) molecules/cell) of the chimeric protein (HA‐T antigen). The biochemical properties and intracellular localization of HA‐T antigens were compared with those of wild‐type T antigen. Wild‐type T antigen. Wild‐type T antigen is located chiefly in the cell nucleus, although a small fraction is detected on the cell surface. By contrast, HA‐T antigen is found exclusively in the endoplasmic reticulum (ER). During biosynthesis, HA‐T antigen is co‐translationally translocated across the membrane of the ER, the signal peptide is cleaved and a mannose‐rich oligosaccharide is attached to the polypeptide (T antigen contains one potential N‐linked glycosylation site at Asn154). HA‐T antigen does not become terminally glycosylated or acylated and little or none reaches the cell surface. These results suggest that T antigen is incapable of being transported along the exocytotic pathway. To explain the presence of wild‐type T antigen on the surface of SV40‐transformed cells, an alternative route is proposed involving transport of T antigen from the nucleus to the cell surface.