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J. Cell Biol.,
Volume 145, Number 1, April 5, 1999 57-68
Institut National de la Santé et de la Recherche Medicale U332, Institut Cochin de Génétique Moléculaire, 75014 Paris, France
As for most integral membrane proteins, the
intracellular transport of retroviral envelope glycoproteins depends on proper folding and oligomeric assembly in the ER. In this study, we considered the hypothesis that a panel of 22 transport-defective mutants of the
human T cell leukemia virus type 1 envelope glycoprotein might be defective in ER assembly. Upon cell
cotransfection with wild-type envelope, however, the
vast majority of these transport-defective mutants (21 of 22) exerted a specific trans-dominant negative effect.
This effect was due to random dimerization of the mutated and wild-type glycoproteins that prevented the intracellular transport of the latter. This unexpected result suggests that association of glycoprotein monomers
precedes the completion of folding. The only mutation
that impaired this early assembly was located at the
NH2 terminus of the protein. COOH-terminally truncated, soluble forms of the glycoprotein were also
trans-dominant negative provided that their NH2 terminus was intact. The leucine zipper-like domain, although involved in oligomerization of the envelope glycoproteins at the cell surface, did not contribute to their
intracellular assembly. We propose that, at a step subsequent to translation, but preceding complete folding
of the monomers, glycoproteins assemble via their
NH2-terminal domains, which, in turn, permits their cooperative folding.
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