# Comprehensive Insights into the Molecular Basis of HIV Glycoprotein
## 2 Proteins Engaged in HIV Entry
### Env/gp160
#### 2.1.1 Synthesis and Trafficking of Env/gp160  
The HIV-1 Env glycoprotein is initially synthesized as a 160-kDa precursor protein known as gp160 within the rough endoplasmic reticulum (RER), derived from a single vpu/env bicistronic mRNA [31]. At the N-terminus of the unprocessed gp160, an endoplasmic reticulum (ER) signal sequence is present, guiding the Env protein to the RER membrane. During synthesis and for approximately 15-30 min afterward, a transient signal anchor attaches the N-terminus of HIV-1 gp160 to the ER membrane (Figure 1). Cysteine residues in the signal peptide facilitate disulfide isomerization, promoting conformational flexibility, mainly via the 54-74 disulfide bond. This process aids in completing the inner-domain \(\beta\) sandwich and folding the gp120 molecule, enabling the assembly of gp120 C and N termini (Figure 2) [34]. During translation within the ER, cellular signal peptidases enzymatically remove the signal peptide from gp160, cleaving it at the C28 residue. This cleavage halts further disulfide isomerization and stabilizes the N-terminus of gp120. Intramolecular quality-control processes are influenced by single charge reversals in the gp120 inner domain, which can impair or restore this process [34]. Additionally, a hydrophobic stop-transfer signal within the transmembrane domain (TMD) of gp41 prevents gp160 full release into the ER lumen [31; 35]. Consequently, portions of the Env extracellular domain extend into the ER lumen, while the gp41 cytoplasmic tail (CT) first extends into the cytoplasm and later into the virion lumen [35]. During translation, gp160 undergoes glycosylation, where N-linked (and some O-linked) oligosaccharideside chains are added. gp160 monomers typically assemble into trimers within the ER, although dimers and tetramers have also been observed [36]. N-linked oligosaccharideside modification begins in the ER, where one out of nine Man residues and all three glucose (Glc) residues are enzymatically removed from each sugar block. Correctly folded and assembled glycoproteins in the ER form oligomers, after which N-linked oligosaccharides undergo extensive and heterogeneous processing in the Golgi complex. Three Man residues are removed, N-acetylglglucosamine (GlcNAc) residue is added, and another two Man residues are removed. Subsequently, additional sugars such as fucose (Fuc), galactose (Gal), N- GlcNAc, and sialic acid are removed in the trans-Golgi compartment, further contributing to glycoprotein heterogeneity [37]. The oligomerization process facilitates the transport of gp160 to the Golgi. Modifications in the trans-Golgi network (TGN) primarily affect high-Man oligosaccharideside chains as gp160 traverses the secretory pathway. Through ubiquitin-mediated proteasomal degradation, the HIV-1 accessory protein Vpu downregulates its expression upon binding with CD4 and inhibits premature interaction between oligomerized Env and CD4 within the secretory pathway [31]. N-linked glycosylation involves the attachment of high-Man chains to asparagine (Asn) residues at either Asn-X-Thr or Asn-X-Ser glycosylation sites [38]. Pentasaccharides comprising Man3 GlcNAc2, with the Asn residue linked to their side chains' amide nitrogen, are termed Asn-linked oligosaccharides. The term \"trimannosidic core\" refers to the three-Man residues at the ends of the core pentasaccharide. N-linked carbohydrates sharing a pentasaccharide core can be categorized as high-Man, hybrid, or complex, all presentin the HIV-1 envelope gp120 protein. Structural analysis of oligosaccharide moieties has revealed their composition: approximately 33% high-Man, 4% hybrid, and 63% complex. Among the complex oligosaccharides, around 90% are fucosylated, and 94% are sialylated. Furthermore, these structures include approximately 4% monoantennary, 61% bientennary, 19% triantennary, and 16% tetraantennary arrangements.