# 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
Complex oligosaccharides exhibit variations in the types and numbers of residues attached to their outer branches compared to high-Man oligosaccharides, which typically have 2-6 Man residues linked to the core [37; 39]. The inner N-acetylglucosamine residue can attach to a Fuc residue with bi-, tri-, or tetra-antennary chains, typically forming a sequence known as sialyllactosamine. Hybrid oligosaccharides contain complex carbohydrate and high-Man structures in 1\(\rightarrow\)6 or 1\(\rightarrow\)3 branches, respectively. To produce Man8 GlcNAc2 oligosaccharides, carbohydrate moieties are transferred to Asn residues in the RER. The Golgi apparatus can further process these hybrid oligosaccharides into complex carbohydrates, incorporating sugars such as Man, Gal, GlcNAc, N-acetylgalactosamine (GalNAc), L-Fuc, and sialic acids. Complex oligosaccharides can exhibit di-, tri-, tetra-, or penta-antennary structures. Notable differences exist between amino acid side chains and N-linked glycans. N-linked glycans possess an average molecular weight of more than 20 times greater than that of amino acid side chains. They occupy a larger volume, exhibit higher structural complexity, and have greater bulk. Pentasaccharides generally have surface areas comparable to antibody footprints [37; 39]. In a study, Thr499 showed no O-linked carbohydrates, as it is exposed on the surface of virions. This result suggests caution when interpreting post-translational analyses utilizing recombinant forms of the envelope protein [40]. In the Golgi complex, furin and furin-like proteases cleave the gp160 glycoprotein at a highly conserved K/R-X-K/R-R motif, yielding mature gp120 and transmembrane gp41 [41; 42; 43]. In the HIV-1 HXB2 strain, the primary furin cleavage site is located at position 511, which is C-terminal to arginine. Carboxypeptidase removes two amino acids from the gp120 C-terminus, corresponding to HIV-1 HXB2 R511. A secondary dibasic cleavage site at HIV-1 HXB2 R504 results in the cleavage of approximately 10% of the HIV-1 gp160 precursor protein [40; 44]. The proteolytic processing of gp160 activates the fusogenic activity of Env, which is essential for viral infectivity. Following cleavage, gp120 and gp41 remain noncovalently associated and assemble into heterotrimic HIV-1 glycoprotein spikes. Upon arrival at the plasma membrane, cellular clathrin adaptor complex interactions facilitate rapid endocytic recycling of Env [45; 46]. Env internalization and shedding of gp120 from the cell surface are crucial for the relatively low levels of Env incorporation into virus particles (\(\sim\)10 spikes/virion), attributed to the weak and noncovalent interaction between gp120 and gp41 [47]. Maintaining low levels of gp120 and gp41 on both the cell and virion surfaces enables HIV-1 to reduce virus-induced cytopathicity and evade host immune responses. Virological or infectious synapses refer to cell-cell contacts that efficiently propagate HIV-1 from one cell to another. This process involves the accumulation of Gag, Env, CD4, actin, adhesion molecules, tetraspanins, and coreceptors at virological synapses [31; 48; 49; 50]. The induction of virological synapses occurs through Env-receptor contacts by HIV-1 in T cells and murine leukemia virus (MLV) in fibroblasts [51; 52]. Env and Gag are concentrated at virological synapses in T cells and macrophages, although Gag localization to cell-cell contacts appears relatively independent of Env [53; 54]. Conversely, in the MLV system, the CT of Env seems to dictate the direction of Gag assembly toward virological synapses [55]. A fundamental question remains about how essential HIV-1 transfer across the virological synapses directed by the Env to the synapse in vivo. An elemental inquiry remains regarding how crucial HIV-1 transfer across virological synapses, guided by Env, is orchestrated in vivo. Figure 1: HIV-1 Env trafficking. A detailed mechanism. The HIV-1 envelope glycoprotein (Env) undergoes a complex trafficking pathway.