Essential ionic triggers for enveloped virus entry

To cause disease, viruses must first enter cells and many viruses do this by hijacking the endocytic network by mimicking cellular cargos. Once inside an endosome, a virus is effectively trapped, and must escape to continue its lifecycle. Here, we propose experiments that will provide new understanding of the endosome escape mechanism, based on our exciting new research findings. Viruses escape from endosomes using protein spikes that cover their exterior and mediate the merging of the viral and endosomal membranes (fusion). The spikes respond to chemical signals within endosomes causing structural changes, switching spikes from a pre-fusion to a fusion-active conformation. This allows spikes to interact with the endosomal membrane from the inside, and soon after, fusion occurs allowing the viral genome to enter the cell. How spikes respond to chemical signals is poorly understood, and highlighting this, we recently showed that an important class of viruses known as bunyaviruses require potassium ions (K+) to escape from endosomes. We showed K+ is required to switch spikes from pre-fusion to fusion-active states, which represents a paradigm shift in the understanding of virus entry. In this project, we will examine whether the K+ requirement is a characteristic of the wider bunyavirus group and whether other ions within endosomes mediate similar spike changes. Next, we will use cryo-EM and X-ray crystallography to reveal high-resolution structures of spikes in their pre-fusion and fusion-active conformations. Finally, we will use genetic techniques to identify spike residues that mediate these structural changes. Together these experiments will characterise endosomal fusion triggers and reveal the molecular details of how spikes become fusogenic, which is critical for virus entry. Improved understanding of endosome escape is required to aid the rational design of strategies to block spike fusogenesis; drugs that can do this would prevent infection and disease.