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

Attempts to retain v-sis protein in a more distal Golgi region by attachment of a TGN retention signal yielded ambiguous results. While most of the sis-TGN38 fusion protein was retained intracellulady, some of the protein was able to reach the cell surface, as shown by immunofluorescence. Indeed, it has been demonstrated recently that TGN38 actually recycles from the cell surface and back to the TGN (Reaves et al., 1993) . It is likely that this population of molecules that reached the cell surface was responsible for the transformation seen in the focus assays in cells expressing this fusion protein, since treatment with suramin reverted the transformed phenotype. However, we cannot conclusively rule out the possibility that functional autocrine interactions can occur in the TGN. Significantly, however, when the COOHterminus of the TGN38-derived domain was truncated, the transforming efficiency of the derivative sis-TGN38A nearly doubled (Table I) . This data certainly provides a correlation between transformation and increased cell surface localization, compared with TGN localization.