Visualizing intermediate stages of viral membrane fusion by cryo-electron tomography.

Protein-mediated membrane fusion is the dynamic process where specialized protein machinery undergoes dramatic conformational changes that drive two membrane bilayers together, leading to lipid mixing and opening of a fusion pore between previously separate membrane-bound compartments. Membrane fusion is an essential stage of enveloped virus entry that results in viral genome delivery into host cells. Recent studies applying cryo-electron microscopy techniques in a time-resolved fashion provide unprecedented glimpses into the interaction of viral fusion proteins and membranes, revealing fusion intermediate states from the initiation of fusion to release of the viral genome. In combination with complementary structural, biophysical, and computation modeling approaches, these advances are shedding new light on the mechanics and dynamics of protein-mediated membrane fusion.

Structural dynamics reveal subtype-specific activation and inhibition of influenza virus hemagglutinin.

Influenza hemagglutinin (HA) is a prototypical class 1 viral entry glycoprotein, responsible for mediating receptor binding and membrane fusion. Structures of its prefusion and postfusion forms, embodying the beginning and endpoints of the fusion pathway, have been extensively characterized. Studies probing HA dynamics during fusion have begun to identify intermediate states along the pathway, enhancing our understanding of how HA becomes activated and traverses its conformational pathway to complete fusion. HA is also the most variable, rapidly evolving part of influenza virus, and it is not known whether mechanisms of its activation and fusion are conserved across divergent viral subtypes. Here, we apply hydrogen-deuterium exchange mass spectrometry to compare fusion activation in two subtypes of HA, H1 and H3. Our data reveal subtype-specific behavior in the regions of HA that undergo structural rearrangement during fusion, including the fusion peptide and HA1/HA2 interface. In the presence of an antibody that inhibits the conformational change (FI6v3), we observe that acid-induced dynamic changes near the epitope are dampened, but the degree of protection at the fusion peptide is different for the two subtypes investigated. These results thus provide new insights into variation in the mechanisms of influenza HA's dynamic activation and its inhibition.

Structural dynamics reveal isolate-specific differences at neutralization epitopes on HIV Env.

The envelope glycoprotein (Env) is the sole target for neutralizing antibodies against HIV and the most rapidly evolving, variable part of the virus. High-resolution structures of Env trimers captured in the pre-fusion, closed conformation have revealed a high degree of structural similarity across diverse isolates. Biophysical data, however, indicate that Env is highly dynamic, and the level of dynamics and conformational sampling is believed to vary dramatically between HIV isolates. Dynamic differences likely influence neutralization sensitivity, receptor activation, and overall trimer stability. Here, using hydrogen/deuterium-exchange mass spectrometry (HDX-MS), we have mapped local dynamics across native-like Env SOSIP trimers from diverse isolates. We show that significant differences in epitope order are observed across most sites targeted by broadly neutralizing antibodies. We also observe isolate-dependent conformational switching that occurs over a broad range of timescales. Lastly, we report that hyper-stabilizing mutations that dampen dynamics in some isolates have little effect on others.