# Comprehensive Insights into the Molecular Basis of HIV Glycoprotein
## 2 Proteins Engaged in HIV Entry
### Env/gp160
#### 2.1.2 Structural Characteristics of Env/gp160
This experimental approach, illustrated in Figure 3, has unveiled the intricate relationship between the CT, TMD, and Env ectodomain, offering insights into how their interplay regulates the structural dynamics of the protein. Such findings are pivotal for understanding viral entry mechanisms and designing targeted interventions against HIV-1 [59]. TMDs are anticipated to assemble into an organized trimeric structure, shielding three conserved arginine residues (Rs), including R696, positioned centrally within the transmembrane helix. This arrangement establishes distinct polar and hydrophobic cores within the membrane. Due to the presence of the polar core, R696 can maintain hydration despite its location within the lipid bilayer [59; 72; 73]. Mutations that disturb the trimeric structure of the TMD lead to changes in antibody sensitivity within the ectodomain. This observation underscores the TMD's critical role in maintaining the Env protein's stability and antigenicity. Consequently, recombinant soluble Env preparations lacking the TMD may display antigenic properties distinct from native proteins [64; 69; 74]. Additionally, employing the bicelle system, NMR analysis has elucidated the structure of both the TMD and MPER (Figure 3d) [75]. Early structural studies of monomeric MPER peptides revealed that the MPER region is buried within the viral membrane [76; 77; 78]. Contrarily, findings demonstrate that MPER forms a highly structured and exposed trimeric assembly in conjunction with the lipid bilayer and TMD, closely resembling the native Env prefusion conformation. Moreover, mutations within the MPER region have been shown to impact the antigenic properties of the Env ectodomain. These observations support the hypothesis that MPER acts as a regulatory relay, modulating the antigenic structure of the Env trimer [22; 75]. A novel structure was discovered featuring the TMD and CT in conjunction with the lentiviral lytic peptide (LLP)-2 region. In this arrangement, the trimerized CT region encircles the TMD, supporting the baseplate within the membrane. The two-dimensional baseplate consists of amphipathic helices in the lipids' head-group region. As a result of this architectural configuration, lipids are excluded from the inner leaflet of the membrane, leading to significant effects on membrane reshaping [61]. However, substantial portions of the CT remain relatively unexplored, particularly, residues 789-856 (LLP1 and LLP3). There is a belief that deleting the last 30 residues of the CT could open up the Env trimer, thereby altering the antibody binding profile [68]. Concurrently, genetic studies have suggested a direct interaction between residues 802-806 of the Env CT and the matrix domain of the Gag polyprotein during viral assembly [79]. This remaining fragment could extend the baseplate further, impacting lipid partitioning and inducing deformation of the lipid bilayer [28]. Despite the extensive knowledge regarding the Env structure, a comprehensive comprehension of viral entry mechanisms remains an ongoing pursuit. Prefusion Env structuresplay a pivotal role in this endeavor. Through single-molecule fluorescence resonance energy transfer (smFRET) studies, researchers have delineated three distinct transition conformations of HIV-1 Env. State 1: Low-FRET, representing the prefusion state; state 2: High-FRET, indicative of a default intermediate conformation; and state 3: Intermediate-FRET, encompassing the three CD4-bound conformations. These findings provide valuable insights into the dynamic conformational changes underlying viral entry, offering potential targets for therapeutic intervention [22; 80; 81]. Most broadly neutralizing antibodies target the untriggered prefusion state of the HIV-1 Env glycoprotein and demonstrate a preference for binding to the CD4-triggered Env conformation [82]. A study determined the full-length CT NMR structure utilizing protein fragments containing CT and TMD within a lipid bilayer mimic called bicelles.