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

Retention of the v-sis oncogene in the early Golgi complex by means of a transmembrane retention signal abolishes its transforming ability, as demonstrated by a dramatic decrease in focus forming activity. All fusion proteins constructed for these experiments dimerized properly, indicating that this critical post-translational modification of the v-sis portion of the fusions was not altered. The fusion proteins sis-E1 and sis-E1-G were efficiently retained intracellularly, as evidenced by the lack of proteolytic processing of the constructs and lack of downregulated cell surface PDGF receptors. Immunofluorescence data are consistent with Golgi localization of the sis-E1 and sis-E1-G constructs. Colocalization with Lens culinaris lectin and mAb 10E6 confirm targeting to the early Golgi complex of the sis-E1 and sis-E1-G constructs. Mutant derivatives of the these constructs, containing defects in the cis-Golgi localization signal, were not retained inside the cell and were transforming. These proteins were proteolytically processed as expected, and were detectable on the cell surface by immunofluorescence. Suramin reverted the transformation induced by these latter constructs, providing further evidence that functional interactions between v-sis and PDGF receptors occur primarily on the cell surface. The results obtained with the sis-E1 and sis-E1-G constructs indicate that the intracellular compartment of the early Golgi complex does not allow for autocrine activation of PDGF receptors.