The HIV Gp41 Conserved Pocket Binding Domain is Bifunctional, Alternatively Mediating Both Immunosuppression and Membrane Fusion

The human immunodeficiency virus (HIV) utilizes its envelope glycoprotein gp160, specifically the gp41 subunit, to enter its host cell. During this process, a gp41-central coiled coil is formed from three N- and three C-terminal heptad repeats (NHR and CHR, respectively). This structure is termed the six-helix bundle (SHB) and it drives membrane fusion. Situated at the CHR N-terminus is the conserved pocket binding domain (PBD) that stabilizes SHB formation. A Recent study reported that a region housing the PBD acts as an immune suppressor by binding the T-cell receptor (TCR) in the membrane. Since the PBD is highly conserved, we hypothesized that it might mediate gp41 based immune suppression. We used independent cell culture models along with biophysical and biochemical techniques such as fluorescence resonance energy transfer and circular dichroism to investigate the PBD functions. We revealed that the PBD is bifunctional, alternating between fusion facilitation and immunosuppression. Since SHB formation is proposed to occur in the virological synapse, the PBD would need to alternate between being buried in the SHB to binding the TCR in the membrane for the immune suppression activity. Interestingly, we found that the PBD stabilizes a TCR binding conformation in the membrane resulting in signaling inhibition. Moreover, successful membrane fusion doesn’t entirely rely on the PBD. Our data suggests that immunosuppression may be a possible driving force for the high degree of PBD conservation. Through this multidisciplinary approach we shed light on the mechanisms behind many gp41 functions and detail its complex relationship with the membrane and the proteins residing within. While structural data at atomic resolution detailing membrane proteins is limited, our approach may be implicative to other membrane proteins in various biological systems.