Distinct conformational states of HSV-2 gH/gL captured by cryoEM reveal structural principles governing fusion activation

Orthoherpesviridae infect most humans for life and cause myriad diseases. These viruses distribute cell-entry functions among receptor-binding proteins such as gD, the conserved membrane fusogen gB, and the gH/gL heterodimer, which couples receptor engagement to gB activation. Here, we report cryoEM structures of HSV-2 gH/gL, including complexes with four antibody fragments, in distinct conformational states. Structural analysis suggests that these structures represent pre-activation and intermediate states rather than a post-activation state. Antibody binding appears to shift the gH/gL conformational equilibrium of gH/gL, either stabilizing the pre-activation state or promoting refolding into intermediate or post-activation-like states. Two regions – the N terminus and a domain H2 switch – adopt distinct conformations and may transmit the activating signal from gD to gB. We propose that receptor-bound gD displaces the gH N terminus, initiating allosteric rearrangements that culminate in switch refolding and may help release prefusion gB from an autoinhibited gH/gL-gB complex. Switch conservation suggests that this activation mechanism may be shared across Orthoherpesviridae. These findings uncover structural principles governing gH/gL-mediated activation of membrane fusion.