The C-terminal tail of the gp41 transmembrane envelope glycoprotein of HIV-1 clades A, B, C, and D may exist in two conformations: an analysis of sequence, structure, and function

Abstract

In addition to the major ectodomain, the gp41 transmembrane glycoprotein of HIV-1 is now known to have a minor ectodomain that is part of the long C-terminal tail. Both ectodomains are highly antigenic, carry neutralizing and non-neutralizing epitopes, and are involved in virus-mediated fusion activity. However, data have so far been biologically based, and derived solely from T cell line-adapted (TCLA), B clade viruses. Here we have carried out sequence and theoretically based structural analyses of 357 gp41 C-terminal sequences of mainly primary isolates of HIV-1 clades A, B, C, and D. Data show that all these viruses have the potential to form a tail loop structure (the minor ectodomain) supported by three, beta-sheet, membrane-spanning domains (MSDs). This means that the first (N-terminal) tyrosine-based sorting signal of the gp41 tail is situated outside the cell membrane and is non-functional, and that gp41 that reaches the cell surface may be recycled back into the cytoplasm through the activity of the second tyrosine-sorting signal. However, we suggest that only a minority of cell-associated gp41 molecules - those destined for incorporation into virions - has 3 MSDs and the minor ectodomain. Most intracellular gp41 has the conventional single MSD, no minor ectodomain, a functional first tyrosine-based sorting signal, and in line with current thinking is degraded intracellularly. The gp41 structural diversity suggested here can be viewed as an evolutionary strategy to minimize HIV-1 envelope glycoprotein expression on the cell surface, and hence possible cytotoxicity and immune attack on the infected cell.

Evidence against Extracellular Exposure of a Highly Immunogenic Region in the C-Terminal Domain of the Simian Immunodeficiency Virus gp41 Transmembrane Protein

“…Importantly, this hydrophilic region leads into a stretch of moderate but distinct hydrophobicity (aa Y768 to L802). These results mirror the outcome of a similar analysis on HIV-1 Env reported by Hollier and Dimmock (26), again despite a lack of high sequence similarity. The second hydrophobic stretch (aa Y768 to L802) is long enough to form an additional membrane-spanning domain; however, it includes five charged residues and a palmitoylated cysteine (C787) (2, 62), making it an unlikely candidate for a membrane crossing.…”

“…The HIR within gp41CTD of SIVmac239 is highly similar in location, if not in sequence, to the immunogenic region of the HIV-1 gp41CTD, the so-called "Kennedy epitope" (13,30,58). It was this high immunogenicity in HIV-1 which led to the proposal of an alternative topology featuring an extracellular loop (14,17,26). In order to assess whether any part of the gp41CTD of SIVmac239 fulfills the biophysical requirements for such an extracellular loop, we performed an in silico analysis of its hydrophilicity distribution (Fig.…”

Enhanced Central Nervous System Transduction with Lentiviral Vectors Pseudotyped with RVG/HIV-1gp41 Chimeric Envelope Glycoproteins

“…Several studies document that gp41 CT is found in a dynamic status where major conformation changes which favor membrane fusion occur upon viral attachment (4,44,45). Indeed, the N terminus (or first tyrosine signal) ( 719 YSPL 722 ) of gp41 CT, together with the "Ken" epitope and upon interaction with adaptor protein 2 (AP-2), functions in endocytosis (46). The fusion process is then further facilitated via interaction with AP-1 and AP-3-directed degradation which allows release of the LV genome into the host cell (46).…”

HIV-1 Glycoprotein Immunogenicity

“…Whereas it have been revealed that there are Abs directed to an epitope in the CT and some of them present neutralization properties on HIV virions (Cleveland et al, 2003;McInerney et al, 1999;McLain et al, 2001;Reading et al, 2003). Then, since antibodies do not traverse the membrane and infectious virus are by definition intact, this suggests that part of the tail is exposed on the virion surface allowing antibody binding and neutralization, thus contrasting with the traditional intracytoplasmic location of the entire C-Term sequence of gp41 (Hollier & Dimmock, 2005). Studies have attempted to address this divergence between the old model of an exclusively intracytoplasmic tail and an alternative model with external segments of the CT, as the Kennedy peptide (aa731-752) (Kennedy et al, 1986) containing three patterns a conserved one 741 EEEGGE 746 and two others 747 QDRDRS 752 or 731 PRGPDRPGRI 740 .…”

“…In addition epitopes may be modulated by conformational changes that affect the ERDRD sequence directly. (Hollier & Dimmock, 2005).…”