Glycosylation is necessary for the correct folding of human immunodeficiency virus gp120 in CD4 binding

Abstract

Conflicting results have been reported regarding the role of carbohydrate on human immunodeficiency virus (HIV) envelope glycoprotein gpl20 in CD4 receptor binding. Glycosylated, deglycosylated, and nonglycosylated forms of HIV type 1 (HIV-1) and HIV-2 gpl20s were used to examine CD4 receptor-binding activity. Nonglycosylated forms of gpl20 generated either by deletion of the signal sequence of HIV-1 gpl20 or by synthesis in the presence of tunicamycin failed to bind to CD4. In contrast, highly mannosylated gpl20 bound to soluble CD4 molecules well. Enzymatic removal of carbohydrate chains from glycosylated gpl20 by endoglycosidase H or an endoglycosidase F/N glycanase mixture had no effect on the ability of gpl20 to bind CD4. An experiment which measured the ability of gpl20 to bind to CD4 as an assay of the proper conformation of gpl20 showed that carbohydrate chains on gpl20 are not required for the interaction between gpl20 and CD4 but that N-linked glycosylation is essential for generation of the proper conformation of gpl20 to provide a CD4-binding site.

Antigenic differences between European and American isolates of porcine reproductive and respiratory syndrome virus (PRRSV) are encoded by the carboxyterminal portion of viral open reading frame 3

“…Like the homologous predicted LDV and EAV ORF 3 proteins, the PRRSV ORF 3 protein contains the greatest number of sites for potential N-glycosylation of all the proteins encoded by ORFs 2 through 6 (den Boon et al, 1991;Conzelmann et al, 1993;Godeny et al, 1993). N-linked glycosidic moieties have frequently been found important in mediating viral infectivity, in directing soluble and cell-associated host immune responses, in protecting critical viral protein epitopes from immune attack, and in interacting with viral polypeptide components to ensure correct viral glycoprotein conformational structure (Rademacher et al, 1988;Li et al, 1993;Grigera et al, 1991). Attempts to produce observably deglycosylated and nonglycosylated forms of BPO3-P using both tunicamycin inhibition and enzymatic deglycosylation approaches indicated that the recombinant protein did not contain N-linked glycosidic modifications.…”

Folding of HIV‐1 Envelope glycoprotein involves extensive isomerization of disulfide bonds and conformation‐dependent leader peptide cleavage

“…Heavily glycosylated proteins require their glycans for proper folding. Gp120 and gp160 are no exception: unglycosylated gp120 does not fold into a biologically active conformation (38). Even the deletion of only three conserved glycosylation sites within the gp41 domain (Asn-621, -630, and -642) prevented gp160 from leaving the ER (39).…”

Hyperactivation of mammalian target of rapamycin complex 1 by HIV‐1 is necessary for virion production and latent viral reactivation

“…Of interest, the pyrimidine pathway also generates uridine, which can be conjugated to sugars to form UDP-activated intermediates for protein glycosylation (57). It has been shown that HIV-1 envelope is heavily glycosylated, which assists in its proper folding (58) and enables antibody evasion (59). Hence, it will be worth exploring whether the mTORC1 pathway also regulates the glycosylation of envelope protein.…”