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

The sis-E1 fusion incorporates a 25-amino acid segment of the avian coronavirus E1 protein, containing the entire first transmembrane domain of E1 and a short cytoplasmic tail. This region has been shown to confer localization of E1 to the cis-Golgi cisternae, allowing for assembly of the coronavirus to occur at intracellular membranes (Swift and Machamer, 1991; Machamer et al., 1990) . When incorporated into heterologous proteins, this E1 transmembrane domain can function as a cis-Golgi localization signal, as shown by incorporation of this transmembrane domain into fusion proteins with VSV-G and a m (a human chorionic gonadotropin/VSV-G fusion protein) (Swift and Machamer, 1991) . The sis-El(QI) construct incorporates a mutation that changes Gin 37 to fie, and this mutation abolishes correct localization (Swift and Machamer, 1991) . The sis-El(ins) construct contains an insertion of two lie residues in the transmembrane domain, which similarly abolishes cis-Golgi localization. The sis-E1-G constructs are quite similar to the above, but they have an extended COOH-terminal domain provided by a portion of the G protein from VSV-G (Rose and Gallione, 1981) . The addition of the G tail allows for doublelabel immunofluorescence experiments to be performed, and does not significantly alter the localization efficiency or function of the fusion proteins, as described below.