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

Like most other secreted proteins, v-sis undergoes a variety of posttranslational modifications as it passes through the secretory pathway. One of these processing events is cleavage at a dibasic site to release the propeptide region of the protein. This is thought to occur late in the secretory pathway, between the trans-cistemae of the Golgi complex and the plasma membrane (Robbins et al., 1985; Lokeshwar et al., 1990) . Thus, sis-E1 and sis-El-G, if retained in the early Golgi complex, should not undergo this processing step. The mutant versions of these two fusion proteins, however, should reach the cell surface and thus should exhibit processing of this propeptide. Similarly, the sis-TGN38 and sis-TGN38A fusion proteins both should exhibit processing, since these should be either retained in a compartment that is past the site of this modification, or be present on the cell surface.