The v-sis oncoprotein loses transforming activity when targeted to the early Golgi complex

To further examine the mechanism of transformation occurring in our cells, we treated NIH3T3 cells expressing the transforming constructs with suramin to see if the transformed phenotype would revert in its presence. Transformed cells expressing sis-El(Ql), sis-El(ins), sis-q~3N38, sis-TGN38A, and v-sis as a positive control, were examined in the absence of suramin (Fig. 7 , A, C, E, G, and I), or in the presence of suramin (Fig. 7, B, D, F, H, and J) . In all cases, suramin did indeed revert the phenotype. It has been shown that E5, an oncoprotein derived from the bovine papillomavirus, can interact with immature intracellular forms of PDGF receptors, and may stimulate their autophosphorylation activity (Goldstein et al., 1992; Petti and DiMaio, 1992; Cohen et al., 1993) . It has also been shown by Xu et al. (1993) that BPV-E5-transformed cells do not revert in the presence of suramin. Thus, cells transformed by E5 were included as a negative control in the suramin reversion assay. As expected, the presence of suramin did not affect the transformed phenotype of NIH3T3 cells expressing E5 (Fig. 7 , K and L). These results indicate that the productive transforming interactions between PDGF receptors and the v-sis fusion proteins described here are occurring in a suraminsensitive site, most likely the cell surface.