A signal for Golgi retention in the bunyavirus G1 glycoprotein

The G1 and G2 glycoproteins of Punta Toro virus, a member of the bunyaviruses, are targeted to the Golgi complex, where viral budding occurs. We found that the G1 protein, when expressed in the absence of G2, is also targeted to the Golgi complex. A series of G1 proteins truncated at the carboxyl-terminal region was constructed, and the localization of the expressed proteins was examined. It was found that the proteins expressed from constructs with partial deletions in the cytoplasmic domain were transported to the Golgi complex at a significantly slower rate than G1. Although a major fraction of these proteins was eventually transported to the Golgi complex, they did not exhibit as clearly defined a pattern of accumulation as G1, but rather appeared to be distributed throughout the endoplasmic reticulum as well as the Golgi complex. The proteins expressed from constructs lacking most of the cytoplasmic domain and, in some cases, part of the transmembrane domain sequences as well were transported to the cell surface. We have also constructed chimeric proteins with the envelope protein of a murine leukemia virus (MCFenv), which is efficiently transported to the plasma membrane. A MCF-G1 chimera that contained the G1 transmembrane and cytoplasmic domains was found to be efficiently retained in the Golgi complex, and a construct that contained only the G1 transmembrane domain was also partially retained in the Golgi complex. Thus, the transmembrane domain as well as a portion of the cytoplasmic domain adjacent to the transmembrane domain are apparently crucial for Golgi retention of the G1 protein.