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

Focus assays were also performed by transfecting NIH3T3 cells with the sis-TGN38 and sis-TGN38A constructs. Table I shows that, when fused to a portion of TGN38 containing the TGN-localization signal, v-sis can interact with the PDGF receptor to result in autocrine transformation with an efficiency of 32% of the sis-G control. The mutant derivafive, sis-TGN38A, which lacks the tyrosine-containing TGN localization signal, was consistently more active in transformarion assays, exhibiting 55 % as many foci as the sis-G control. Recently, it was demonstrated that TGN38 recycles from the TGN to the cell surface (Reaves et al., 1993) . This observation complicates our results, in that we are at this point unable to determine if the transforming potential of the sis-TGN38 fusion protein is due to a subpopulation of protein molecules present on the cell surface at any given time, or whether it is truly mediated by ligand/receptor interactions occurring within the TGN. Further experiments will be needed to clarify this.