The natural signal sequence of HIV-1 gp120 contains an unusually long hydrophobic domain and five positively charged amino acids. When the gp120 gene was cloned into a baculovirus expression vector under the control of the baculovirus polyhedrin gene promoter, it exhibited an extremely low level of secretion. However, deletion of the signal sequence resulted in the production of large quantities of a nonglycosylated form of gp120 and fusion of honeybee melittin or murine interleukin 3 signal sequences, which contain only one or no positively charged residues, respectively, resulted in a high level of expression as well as glycosylation and secretion. Four charge-altered signal mutants were generated by oligonucleotide-directed mutagenesis. Positively charged amino acids in the natural signal sequence were substituted with neutral amino acids. The results of these experiments showed that the expression and secretion of gp120 was progressively increased by decreasing the positive charge in a stepwise fashion from + 5 to + 3, + 2, and + 1. However, elimination of all five positive charges (leaving a net negative charge of -1 at the NH 2 terminus) caused accumulation of large amounts of a nonglycosylated form of gp120 but decreased the amounts of glycosylated forms of gp120. These signal peptide mutants clearly demonstrate that the positively charged amino acids in the natural signal sequence of HIV-1 gp120 are key factors determining its poor expression and secretion in insect cells. Analysis of intracellular transport and folding of gp120 further indicates that the highly charged uncleaved signal peptide rather than disulfide bond formation is an important factor limiting transport of gp120 from the rough endoplasmic reticulum (RER) to the Golgi apparatus; its presence affects gp120 folding and slows its rate of transport to the cell surface. The requirement for carbohydrate on HIV gp120 in CD4 binding has been the subject of much debate. There have been conflicting reports regarding the role of gp120 glycans in binding to CD4. An important question is whether the carbohydrate itself plays an important role in this interaction. Nonglycosylated and glycosylated forms of gp120 from HIV-1 and HIV-2 were produced using the baculovirus expression system and their CD4 binding properties were determined. The nonglycosylated forms of gp120 generated by either deletion of the signal sequence or synthesized in the presence of tunicamycin failed to bind to CD4. In contrast, highly mannosylated recombinant gp120 bound well to soluble CD4. Enzymatic removal of carbohydrate chains from glycosylated gp120 by endoglycosidase H (endo H) or by a mixture of endoglycosidase F and N-glycanase (endo FNG) in the presence or absence of SDS had little or no effect on the ability of gp120 to bind CD4. The data indicate that carbohydrate chains per se do not play a significant role in interaction between gp120 and CD4 molecules but that N-linked glycosylation is required for correct protein folding that provides the proper conformation for CD4 binding. Analysis of intracellular folding of gp120, using its ability to bind CD4 as a functional assay for overall conformation, further supports the hypothesis that N-linked glycosylation of HIV gp120 plays an essential role in promoting either the correct folding of the protein or in its stabilization.
| Identifer | oai:union.ndltd.org:uottawa.ca/oai:ruor.uottawa.ca:10393/10962 |
| Date | January 1992 |
| Creators | Li, Yan. |
| Publisher | University of Ottawa (Canada) |
| Source Sets | Université d’Ottawa |
| Detected Language | English |
| Type | Thesis |
| Format | 174 p. |
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