Structural changes in HIV-1 Env that promote viral membrane fusion
Infection of host cells by the virus HIV-1 is mediated by the trimeric envelope glycoprotein (Env). During the process, Env transitions from its native state on the surface of the virus into an intermediate state bridging the viral and target cell membranes. Afterwards, Env conforms into a trimer-of-hairpins structure which brings the membranes together and allows them to fuse. The focus of this work was to further our understanding of these two major transitions of Env, both in the context of developing and refining treatment strategies to prevent HIV-1 entry and in the context of understanding viral membrane fusion in greater detail. In the first portion of these studies, fusion inhibitors, which target the gp41 subunits of Env in the pre-hairpin intermediate state, were used to explore how Env exposure evolves from the native state into the intermediate state. We found that the transition is asymmetric, meaning that the individual subunits of gp41 are not exposed simultaneously after the process of Env-mediated membrane fusion begins. We developed a functional complementation strategy to determine that this asymmetric transition is attributable primarily to initial binding of CD4 to the gp120 subunit of Env inducing local exposure of gp41, followed by exposure of the adjacent gp41 subunits. The next portion focused on the transition from the pre-hairpin intermediate state into the trimer-of-hairpins state. To do so, we determined the stoichiometry of fusion inhibition of Env and related this finding to the folding of gp41 into the trimer-of-hairpins, again using fusion inhibitors and a functional complementation strategy. We found that, in contrast to entry into the pre-hairpin state, exit occurs symmetrically. Additionally, the process appears to be cooperative, suggesting a simultaneous transition from the pre-hairpin state into the trimer-of-hairpins. These studies represent a comprehensive view of the transitions a viral fusion protein undergoes from its native state into its final fusogenic form.
Halkidis, Konstantine, "Structural changes in HIV-1 Env that promote viral membrane fusion" (2013). ETD Collection for Thomas Jefferson University. AAI3608836.