Requirements for the insertion of the Sindbis envelope glycoproteins into the endoplasmic reticulum membrane.

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Abstract
Sindbis structural proteins, core, the internal protein, and PE2 andE1, the integral membrane glycoproteins, are synthesized as a polyprotein from a 26S mRNA; core PE2 and E1 are derived by proteolytic cleavage of a nascent chain. Newly synthesized core protein remains on the cytoplasmic side of the endoplasmic reticulum while newly synthesized PE2 and E1 are inserted into the lipid bilayer, presumably via their amino-termini. PE2 and E1 are glycosylated as nascent chains. A temperature-sensitive mutant of Sindbis virus was examined; it fails to cleave the structural proteins, resulting in the production of a polyprotein of 130,000 MW in which the amino-termini of PE2 and E1 are internal to the protein. Although the envelope sequences are present in this protein, it is not inserted into the endoplasmic reticulum bilayer but remains on the cytoplasmic side as does the core protein in cells infected with wild-type Sindbis virus. In cells treated with tunicamycin, an inhibitor of glycosylation, unglycosylated PE2 and E1 are inserted normally into the lipid bilayer as are the glycosylated proteins. Apparently a specific amino-terminal sequence is required for the proper insertion of membrane proteins into the endoplasmic reticulum bilayer but glycosylation is not required for this insertion.