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

The transforming ability of the various v-sis fusion proteins was assayed by transfection of NIH3T3 cells using MLVbased retroviral constructs (Bold and Donoghue, 1985) . The relative ability to transform cells was based on the number of foci formed, with both positive and negative (mock) controis for comparison. The same amount of DNA was transfected for each construct, and the number of foci formed was normalized to the activity of the positive control, sis-G. As shown in Table I , cells transfected with sis-E1 or sis-E1-G exhibited negligible transforming activity, comparable to the mock-transfected cells. The presence of the VSV-G cytoplasmic tail in the sis-E1-G construct seemed to have little or no effect on its transforming activity. Thus, addition of the retention signal of the E1 protein abrogated the transforming potential of v-sis, presumably by localizing virtually all of the fusion protein to an intracellular location incapable of autocrine stimulation.