To infect a host cell, viruses usually recognize particular structures on the cell surface. Following binding to these ‘viral receptors’, the virons are taken up into the cell along different entry routes. Most of these pathways converge in vesicular structures, the endosomes. Depending on whether the virus is covered with a lipid membrane (enveloped) or lacking such a membrane (naked), its genome is then being released into the cytoplasm by different mechanisms. Enveloped viruses usually fuse with cellular membranes, which results in the nucleocapsid arriving in the cytosol. Non-enveloped viruses either disrupt the endosomal membrane, with subviral particles being transferred into the cytoplasm, or the genomic nucleic acids are threaded through a pore and the remaining empty capsids are further shuttled to lysosomes for degradation. For some viruses there is experimental support for RNA transfer between endosomal and cytoplasmic compartments which, however, has so far not been demonstrated explicitly and the putative membrane pore has not been visualized.  

Working with human rhinoviruses (HRVs) that lack a lipid membrane and are the predominant cause of common colds, we aim at identifying so far unknown viral receptors, the different mechanisms underlying viral uptake, the process of genome release and the structural basis of the transfer of the viral genome through lipid membranes. We address these questions by using biochemical, molecular biological, biophysical, and structural biology techniques, such as selection and characterization of viral mutants, expression library screening, fluorescence correlation spectroscopy, capillary electrophoresis, and cryo-electron microscopy.

In the last few years we have identified heparan sulphate as an additional receptor for some rhinovirus types and characterized the uptake pathway by this proteoglycan and by the intercellular adhesion molecule 1, the receptor of about 90 different HRV types. We developed a liposomal in vitro system mimicking the transfer of the viral RNA through the endosomal membrane that is currently being used for structural analysis. Within the frame of several international collaborations, solving the 3D structure of subviral particles is underway.