The HIV-1 envelope protein is a conformationally dynamic protein that binds to CD4 (the cellular receptor for HIV-1) and that mediates HIV-1 entry. Previous work using single-molecule fluorescence resonance energy transfer (smFRET) has shown that the HIV-1 envelope protein transitions through three conformations: state 1 (pre-triggered unbound conformation), state 2 (intermediate) and state 3 (receptor-bound). Walther Mothes and colleagues now use smFRET to show that the soluble form of the HIV-1 envelope protein trimer that has been presumed to represent the pre-triggered conformation in structural studies instead seems predominantly to occupy downstream conformations that represent states 2 and 3. Therefore, the structure of the state 1 conformation may still remain to be determined.
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