Synthesis, proteolytic processing and complex formation of N-terminally nested precursor proteins of the Rift Valley fever virus glycoproteins

The genomic M RNA segment of Rift Valley fever virus is transcribed to produce a single mRNA with multiple translation initiation sites. The products of translation are an N-terminal nested series of polyproteins. These polyproteins enter the secretory system of the host cell and are proteolytically processed to yield the mature virion glycoproteins, Gn and Gc, and two non-structural glycoproteins. By means of pulse-chase immune precipitation experiments we identify the Gn and Gc precursor molecules and also show that signal peptidase cleavage is required for mature Gn and Gc production. We also demonstrate that a hydrophobic domain at the N-terminus of Gn acts as a signal peptide only in the context of the polyprotein precursors that initiate at the second, fourth or fifth AUGs. In addition, we document that formation of Gn/Gc heteromeric complexes occur rapidly (< 5 min) and can occur prior to signal peptidase processing of Gn, suggesting that this complex forms in the endoplasmic reticulum. Interestingly, Gc can form a complex with a glycoprotein that has been considered nonstructural, a discovery that has implications for both the topology and potential packaging of this glycoprotein.