Essential ionic triggers for enveloped virus entry

点击翻译按钮获取中文摘要

In order to cause disease, all viruses must gain entry to their target cells. A common way viruses do this is by hijacking a cellular system called the endocytic network, which is normally used by cells to take up nutrients from the external environment. This network consists of tiny compartments called endosomes and many viruses have evolved the ability to trick cells that they are useful cargos in order to enter the network, and thus gain entry to the cell. However, once a virus enters an endosome, it is faced with a problem; it is effectively trapped, and so the virus must escape from the endosome in order to start an infection. The aim of this proposal is to understand in detail how viruses are able to escape the endosomes, and this work is based on exciting new research findings from our laboratory. Viruses escape endosomes using specialised spike proteins that cover the virion exterior and cause a process called fusion. To cause fusion, these spikes interact with specific chemicals within endosomes and dramatically change shape. After changing shape, the fusion spikes interact with the endosomal membrane from the inside, and force this membrane to mix with the viral envelope. When these two membranes merge together, fusion has occurred and the viral genome is released into the cytoplasm to continue the viral replication cycle. In simple terms, the ability of these viral spikes to change shape is critical in order for a virus to cause fusion, to escape the endosome and thus to continue the infection process. Despite this critical role, the mechanism by which spikes change shape and then cause fusion is poorly understood. Highlighting this poor understanding, we recently showed for the first time that some viruses within an important class of viruses known as bunyaviruses (which include many haemorrhagic fever viruses and viruses that are transmitted by mosquitoes - an increasing risk in the UK due to global warming) require potassium ions (K+) to cause the spikes to change shape and produce fusion. This finding represents a critical and previously overlooked requirement of the fusion mechanism.This proposal describes a set of experiments that will first reveal whether the requirement for K+ is a general characteristic of other viruses within the broader bunyavirus group. Next, we will investigate whether any other ions within endosomes have the same effect as K+ during virus entry, and then use state-of-the-art cryo-electron microscopy and X-ray crystallography techniques to reveal the high-resolution structure of the spikes in the inactive (pre-fusion) and activated shapes. Finally, we will use a variety of genetic techniques to identify parts of the spikes that are critical for responding to the biochemical signals, and also for mediating the shape changes themselves. Taken together, these experiments will provide a major advance in the understanding of the fusion mechanism, revealing in high detail how spikes are able to respond to chemical signals within endosomes, to change shape and cause fusion. This information is required to provide an essential foundation on which to design strategies to block spike fusogenesis; drugs that can do this would prevent infection and disease.