Like all other secretory proteins, the HIV-1 envelope glycoprotein gp160 is targeted to the endoplasmic reticulum (ER) by its signal peptide during synthesis. Proper gp160 folding in the ER requires core glycosylation, disulfide-bond formation and proline isomerization. Signal-peptide cleavage occurs only late after gp160 chain termination and is dependent on folding of the soluble subunit gp120 to a near-native conformation. We here detail the mechanism by which co-translational signal-peptide cleavage is prevented. Conserved residues from the signal peptide and residues downstream of the canonical cleavage site form an extended alpha-helix in the ER membrane, which covers the cleavage site, thus preventing cleavage. A point mutation in the signal peptide breaks the alpha helix allowing co-translational cleavage. We demonstrate that postponed cleavage of gp160 enhances functional folding of the molecule. The change to early cleavage results in decreased viral fitness compared to wild-type HIV.

Cellular Protein Chemistry Bijvoet Center for Biomolecular Research Faculty of Science Utrecht University Utrecht Netherlands

Department of Biochemistry and Biophysics Stockholm University Stockholm Sweden

Department of Medical Microbiology Laboratory of Experimental Virology Center for Infection and Immunity Amsterdam Academic Medical Center Amsterdam Netherlands

Janelia Research Campus Howard Hughes Medical Institute Ashburn United States

National Institute of Public Health National Reference Laboratory for Viral Hepatitis Prague Czech Republic

Science for Life Laboratory Stockholm University Solna Sweden

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