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

Localization of v-sis to the TGN by means of a retention signal derived from TGN38 resulted in decreased levels of transforming ability. However, immunofluorescence data indicate that a small portion of this sis-TGN38 fusion protein was able to reach the cell surface, a finding that is consistent with reports by Reaves et al. (1993) that TGN38 recycles be: tween the TGN and the cell surface. Suramin treatment of cells expressing sis-TGN38 leads to reversion of the transformed phenotype, further implicating a cell surface pool of sis-TGN38 in the transformation of these cells. However, we are unable to conclusively determine from these experiments that v-sis targeted to the TGN is not transforming. Currently, we are undertaking studies which should further clarify interactions within the late Golgi compartments. These new studies utilize a similar approach of constructing fusion proteins, this time using the transmembrane domains and cytoplasmic tails of well-characterized glycosyltransferases, which are resident Golgi enzymes. These membrane anchors should give tighter retention in the later Golgi compartments than seen with the TGN38-derived retention signal, and will hopefully provide a conclusive indication of whether v-sis is able to engage in productive autocrine interactions within the late Golgi complex.