A RETENTION SIGNAL NECESSARY AND SUFFICIENT FOR ENDOPLASMIC-RETICULUMLOCALIZATION MAPS TO THE TRANSMEMBRANE DOMAIN OF HEPATITIS-C VIRUS GLYCOPROTEIN E2

The hepatitis C virus (HCV) genome encodes two envelope glycoproteins (El and E2). These glycoproteins interact to form a noncovalent hetero dimeric complex which is retained in the endoplasmic reticulum (ER), T o identify whether El and/or E2 contains an ER-targeting signal potent ially involved in ER retention of the E1-E2 complex, these proteins we re expressed alone and their intracellular localization was studied. D ue to misfolding of El in the absence of E2, ho conclusion on the loca lization of its native form could be drawn from the expression of El a lone, E2 expressed in the absence of El was shown to be retained in th e ER similarly to E1-E2 complex. Chimeric proteins in which E2 domains were exchanged with corresponding domains of a protein normally trans ported to the plasma membrane (CD4) were constructed to identify the s equence responsible for its ER retention, The transmembrane domain (TM D) of E2 (C-terminal 29 amino acids) was shown to be sufficient for re tention of the ectodomain of CD4 in the ER compartment, Replacement of the E2 TMD by the anchor signal of CD4 or a glycosyl phosphatidylinos itol (GPI) moiety led to its expression on the cell surface, In additi on, replacement of the E2 TR?ID by the anchor signal of CD4 or a GPI m oiety abolished the formation of E1-E2 complexes. Together, these resu lts suggest that, besides having a role as a membrane anchor, the TMD of E2 is involved in both complex formation and intracellular localiza tion.