ANALYSIS OF ENDOPROTEOLYTIC CLEAVAGE AND INTRACELLULAR-TRANSPORT OF HUMAN-IMMUNODEFICIENCY-VIRUS TYPE-1 ENVELOPE GLYCOPROTEINS USING MUTANTCD4 MOLECULES BEARING THE TRANSMEMBRANE ENDOPLASMIC-RETICULUM RETENTION SIGNAL

We investigated endoproteolytic processing of the human immunodeficien cy virus (HIV) envelope glycoprotein precursor, gp160, as well as enve lope-mediated membrane fusion in the presence of CD4 molecules that we re either partially or fully retained in the endoplasmic reticulum (ER ). Pulse-chase analyses revealed that gp160 formed complexes with CD4 molecules, and gp160 in the complex was endoproteolytically cleaved to gp120 and gp41 in the secretory pathway. The gp120/gp41 complex thus generated was properly targeted to the plasma membrane in cells expres sing gp160 and wild-type CD4 or mutant CD4 molecules that were partial ly retained in the ER. Additionally, membrane fusion (syncytium) assay s were performed to monitor the presence or absence of gp120/gp41 comp lexes at the cell surface of cotransfected cells and demonstrated that the HIV-1 envelope glycoprotein-mediated membrane fusion was apprecia bly reduced in the presence of wild-type CD4 or either one of the muta nt CD4 molecules. Reduction in the formation of syncytia appears to be due predominantly to saturation of the CD4 binding site on the gp120/ gp41 complex at the cell surface of cotransfected cells, but partial r etention of the complex in the ER could also partly account for the re duction. However, the intracellular gp120/gp41 complex generated in ce lls expressing gp]60 and CD4 mutant having the transmembrane ER retent ion signal (KKTC underbar) was completely retained in the ER and hence could not participate in membrane fusion events at the plasma membran e. Taken together. these data suggest that the endoproteolytic cleavag e of gp160 occurs in the ER or cis-Golgi network, and ER retention str ategies can potentially be used in preventing the spread of HIV-1 infe ction in permissive cells.