# Comprehensive Insights into the Molecular Basis of HIV Glycoprotein
## 2 Proteins Engaged in HIV Entry
### gp41
The difference between the SOSIP.650 and SOSIP.664 maps corresponds to a short helical segment (red) at the end of HR2 that projects toward the adjacent protomer. Reprinted (adapted) with permission from [123] Copyright Science (2024). Unlike other retroviruses, lentiviral envelope (Env) glycoproteins feature an unusually long CT in their transmembrane (TM) subunit [124]. Typically, the CT of gp41 in lentivirus viruses is approximately 150 amino acids in length, whereas the equine infectious anemia virus (EIAV) has even longer CTs, about 200 amino acids [125]. In contrast, other retroviruses such as Rous sarcoma virus (RSV), Mason-Pfizer monkey virus, and murine leukemia virus (MLV) have TM CTs that are only 20-40 amino acids long. In animal models, truncation of the lentiviral CT has been shown to inhibit virus replication. This observation supports the hypothesis that the long CT in lentiviral TM Env glycoproteins plays a role in suppressing viral transmission [126]. The CT of HIV-1 gp41 influences various viral properties, including Env incorporation, shedding of gp120, viral infectivity, cell surface expression of Env, and Env-induced membrane fusion [31]. Mutations in the CT of HIV-1 and SIV gp41 also affect the conformation of gp120 and the ectodomain, impacting antibody recognition and neutralization. Some HIV-1 isolates that have evolved CD4 independence exhibit truncated gp41 CTs, suggesting that this domain is crucial for regulating gp120 conformation [127; 128]. The gp41 CT contains several functional and structural domains that modulate its activity (Figures 2 and 5b-f). The adaptor protein complex 2 (AP-2) mediates clathrin-dependent endocytosis of the Env protein from the plasma membrane via a membrane-proximal tyrosine-based sorting signal, YaxL [129; 130]. This motif facilitates the release of viral particles from the basolateral surface of polarized epithelial cells and supports cell-to-cell transmission in T cells [131]. Mutation of the analogous signal in simian immunodeficiency virus (SIV) significantly attenuates viral replication in vivo [132]. Additionally, the clathrin adaptor protein complex 1 (AP-1) interacts with a dileucine motif in the gp41 CT to regulate the intracellular distribution of Env [133]. However, the YaxL motif is not involved in the endocytosis of Env [113; 114]. As shown in Figure 5a, The central and C-terminal regions of the gp41 CT contain three conserved amphipathic \(\alpha\)-helical segments known as LLP-1, LLP-2, and LLP-3 [31]. The \(\alpha\)-helices of LLP-1 and LLP-2 possess arginine residues on one face, conferring a high positive charge. LLP-3 is situated between these segments. These LLP domains are highly conserved among lentiviruses and are linked to various functions, including Env fusogenicity, multimerization, cell-surface expression, protein stability, and incorporation into virions. Additionally, LLP fragments can bind to and perturb membranes, causing cytolysis, hence the designation \"lytic peptide\" [31]. Substituting arginine (Arg) residues in LLP-1 with glutamic acid (Glu) preserves this segment's secondary structure and hydrophilicity. Nonetheless, it abolishes its cell lysis activity, indicating that these residues are crucial for LLP function [134]. The CT of gp41 also contains two cysteine (Cys) residues that undergo palmitoylation, a post-translational modification posited to target Env to lipid rafts, although this remains somewhat controversial [135]. Furthermore, the LLP-1 segment appears to contain determinants for lipid raft association, as truncation of LLP-1 diminishes Env localization to these membrane microdomains [136]. These findings highlight the diverse biological functions of the gp41 CT, many of which are not fully understood. Elucidating the role of this region of Env necessitates investigating its biological properties in relevant primary cell types, assessing its role in cell-cell communication, and identifying its cellular partners. At the base of the trimer, gp41 forms a pedestal structure (Figure 5b,c).