How a single Sindbis virus mRNA directs the synthesis of one soluble protein and two integral membrane glycoproteins.

Previous work has shown that the 26S RNA found in Sindbis-infected chicken embryo fibroblasts encodes the three viral structural proteins, one internal protein, core, and two membrane glycoproteins, E1 and E2. This mRNA has one initiation site; core, E1, and E2 are derived by proteolytic cleavage. Here we show that during infection, the 26S RNA is found mainly in membrane-bound polysomes which synthesize all three virion structural proteins. These polysomes are released from the membrane upon treatment with puromycin and high salt. Newly synthesized core protein is localized on the cytoplasmic side of endoplasmic reticulum membranes, while newly synthesized envelope proteins are sequestered by the lipid bilayer. These results suggest that the nascent glycoproteins, presumably their amino termini, are of major importance in directing the binding of polysomes containing 26S mRNA to endoplasmic reticulum membranes and the subsequent transfer of glycoproteins into the bilayer.