Simian virus 40 small-t protein is required for loss of actin cable networks in rat cells

The ability of the two early simian virus 40 (SV40) coded proteins, the large and small T-antigens, to abortively induce the disappearance of cytoplasmic actin-containing networks in cultured cells has been studied in rat embryo fibroblasts after microinjection of intact SV40 DNA, DNA fragments from the early region of SV40, and a purified SV40 large T-antigen related protein (the D2 hybrid protein) isolated from cells infected with the adenovirus-SV40 hybrid virus Ad2+D2. Injection of either the 107,000-dalton D2 hybrid protein or SV40 DNA from the deletion mutant dl 884 SV40, which lacks part of the region (0.54 to 0.59) encoding small t-antigen, failed to cause any detectable change in the structure of actin cables in recipient cells over a period of 72 h. By contrast, injection of wild-type SV40 DNA or a DNA fragment containing the entire region coding for a small-t antigen leads to the disruption of actin cable networks within 24 h of injection. It appears likely that the SV40 small-t protein is necessary for the abortive loss of actin cables in injected cells. Epidermal growth factor also causes loss of actin cables in rat embryo fibroblasts or Rat 1 cells (an established rat embryo line), but only after exposure of the cells to epidermal growth factor in the culture medium and not after injection of epidermal growth factor into the cells.