Viral Genome Release

Viral infection starts with specific attachment of the pathogen to host cell receptors. Subsequently, the virus releases its genome into the cytosol. This occurs either directly from the plasma membrane or after entry into endosomes, the endoplasmic reticulum or the Golgi via a number of possible pathways. For enveloped viruses, the process of viral penetration and the relocation of the genome into the cytoplasm are relatively well understood. However, in the case of naked viruses, many questions are still open. What pathways are exploited to gain access to the cell? What receptors are involved? How and where does the genome leave the virus and penetrate into the cytosol? Is there any directionality of this process (i.e. which end of the nucleic acid leaves the capsid first)? What are the driving force and the speed of this process? What is the role of the receptors in genome release and penetration? Does the nucleic acid pass through a pore or is the endosomal membrane disrupted altogether? We propose to address these questions using human rhinoviruses, the main causative agents of the common cold, as a model system. These small icosahedral, naked viruses circulate as more than 100 different genotypes, which give rise to recurrent infections. Despite being structurally and functionally closely related, they bind different receptors. These shuttle them into distinct entry pathways and cause the genome to be released from diverse compartments. A better understanding of these initial events in viral infection might pave the way to novel strategies for intervention.

Common cold viruses are usually harmless but can become life threatening if paired with asthma, chronic obstructive pulmonary disease, or cystic fibrosis. They cause enormous economic loss because of sickness absences and (mostly inefficient) medication. The first step towards infection is the attachment of the virus to a receptor in the plasma membrane of the host cell and subsequent uptake into endosomes. In these membrane-bounded cellular organelles a conformational change takes place and the RNA genome is transported through the endosomal membrane into the cytosol where replication ensues.We studied these early steps of infection by using methods of structural biology. First, we established an in vitro system allowing to demonstrate RNA transport from the viral interior through a lipid membrane into liposomes. We solved the 3-dimensional (3D) structure of the subviral particles occurring at the intersection between native virus and empty capsid and demonstrated important rearrangements of the RNA genome in the process. These conformational changes appear necessary for the coordinated exit of the RNA from the virion. Furthermore, solving the 3D structure of membrane-bound virions revealed how the highly unstable RNA travels into the cytosol for replication and at the same time remains protected from the harsh environment of the late endosome. Our work describes the dynamic structural changes of the virion to the subviral particle and further to the empty shell and explains how these early events lead the way to infection.