Rift Valley fever virus M segment: cellular localization of M segment-encoded proteins

The Phlebovirus Rift Valley fever virus (RVFV), like other members of the Bunyaviridae family, matures intracellularly at the smooth-surfaced vesicles in the Golgi region of infected cells. Here we show that in cultured cells the RVFV glycoproteins G2 and G1 accumulate and are retained at this site. To investigate the parameters governing this subcellular localization, we have engineered portions of the cloned RVFV M segment (which encodes a 14- and a 78-kDa protein, in addition to glycoproteins G2 and G1) into vaccinia virus. Immunofluorescent analysis of cells infected with a vaccinia virus recombinant containing the entire open reading frame of the RVFV M segment revealed Golgi localization for glycoproteins G2, G1, the 78-kDa protein, and Golgi as well as some reticular distribution for the 14-kDa protein. These distributions paralleled those seen in authentic RVFV-infected cells. RVFV-vaccinia virus recombinants possessing progressive deletions within the 152 amino acid preglycoprotein sequence of the M segment were analyzed for possible effects on the cellular distribution of G2 and G1. Removal of the first 130 amino acids of the open reading frame amino-terminal to the mature glycoprotein coding sequences, while abolishing production of the 78- and 14-kDa proteins, did not alter the Golgi location of G2 and G1. The data suggest that Golgi-specific signals reside within the G2 and/or G1 glycoprotein sequences. The use of vaccinia virus recombinants provides a genetically manipulable expression system with which to further investigate the sequences involved in the intracellular localization of these Phlebovirus proteins.