A new determinant of endoplasmic reticulum localization is contained in the juxtamembrane region of the ectodomain of hepatitis C virus glycoprotein E1

Hepatitis C virus glycoproteins E1 and E2 do not reach the plasma membrane of the cell but accumulate intracellularly, mostly in the endoplasmic retic ulum, Previous studies based on transient expression assays have shown that the transmembrane domains of both glycoproteins are sufficient to localize reporter proteins in the endoplasmic reticulum and that other localization signals may be contained in the ectodomain of El protein. To identify such signals we generated chimeric proteins between El and two reporter protein s, the human CD8 glycoprotein and the human alkaline phosphatase, and analy zed their subcellular localization in stable as well as transient transfect ants. Our results showed that (i) an independent localization determinant f or the endoplasmic reticulum is present in the juxtamembrane region of the ectodomain of El protein and (ii) the localization dictated by this determi nant is either due to direct retention or to a recycling mechanism from the intermediate compartment/cis-Golgi complex region, which is clearly differ ent from those previously described for other retrieval signals. These resu lts show for the first time in mammalian cells that the localization in the endoplasmic reticulum of transmembrane protein can be determined by specif ic targeting signals acting in the lumen of the compartment.