The HIV-1 Env Protein Signal Sequence Retards Its Cleavage and Down-regulates the Glycoprotein Folding

Abstract

Secretory proteins and most membrane proteins are synthesized with a signal sequence that is usually cleaved from the nascent polypeptide chain, during its transport, into the lumen of the endoplasmic reticulum (ER). We have analyzed the kinetics of the cleavage of the HIV-1 Env protein signal sequence from gp160 and gp120 in HeLa, BHK, and Jurkat cells. Furthermore, we have determined the effects of this cleavage on the association of the gp160 and gp120 glycoproteins with the ER protein calnexin and the effects of the signal sequence cleavage on protein folding. The cleavage of the HIV-1 Env protein signal sequence on both gp160 and gp120 occurred very slowly in all three cell lines with a t1/2 of 45–60 min. The core glycosylated and signal-sequence-retained forms of gp160 and gp120 associated with calnexin while the signal-sequence-cleaved forms of gp160 and gp120 had disassociated from calnexin and correctly folded as determined by their ability to associate with the CD4 cellular receptor. Further analysis of the folding state of gp160 and gp120 in nonreducing SDS–PAGE revealed that the signal-sequence-retained and calnexin-associated forms of gp160 and gp120 migrated as broad, diffuse bands, whereas the signal-sequence-cleaved or CD4-associated forms of gp160 and gp120 migrated as single sharper bands. The cause of this retardation in the rate of folding and intracellular transport of HIV-1 glycoproteins was localized to their signal sequences by fusing the vesicular stomatitis virus G protein with the HIV-1 Env protein signal sequence and expressing this chimeric protein in mammalian cells. The HIV-1 Env protein signal sequence on the VSV-G protein also confers a reduced rate of cleavage and slow intracellular transport and folding of the chimeric G protein. These results provide direct evidence that in vivo the HIV-1 glycoprotein signal sequence inhibits the folding of HIV-1 Env protein. Our data also suggest a direct correlation between the rate of the signal sequence cleavage and protein folding.