Biochemical and Structural Characterization of Proteins of the Herpesvirus Inner Tegument

The closely related alphaherpesviruses Herpes Simplex Virus 1 (HSV-1) and Pseudorabiesvirus (PRV) are neurotropic viruses of humans and pigs respectively. The hallmark of HSV infection is the mucocutaneous lesions it causes during lytic infection. PRV, the causative agent of Aujeszky's disease in pigs and other livestock, often results in lethal infection. The mechanisms by which these viruses replicate and are released from host cells are not well defined. PRV and HSV-1 are two model viruses used to better characterize alphaherpesvirus replication.;In cells infected with herpesviruses, two capsid-associated or inner-tegument proteins, UL37 and UL36 control cytosolic trafficking of capsids by as yet poorly understood mechanisms. In this work, we set out to further define the roles of UL36 and UL37 in viral replication though the biochemical and structural characterization of these proteins and their complexes. For the first time, we have purified and started biochemical characterization of the PRV UL37 protein and regions of the PRV and HSV-1 UL36 proteins in complex with either UL37 or the auxiliary capsid protein UL25.;Notably, we have determined the crystal structure of the N-terminal half of UL37 from PRV. The structure, which is the first for any alphaherpesvirus inner tegument protein - reveals an elongated molecule of a complex architecture, rich in helical bundles. Through evolutionary trace analysis we were able to identify a novel functional region important for cell-cell spread. These results suggest a novel role for UL37 in intracellular trafficking that promotes spread of viral infection, expanding the repertoire of UL37 functions in intracellular virus trafficking. Unexpectedly, the UL37 N terminus shares a structural similarity with cellular multi-subunit tethering complexes (MTCs), which control vesicular trafficking in eukaryotic cells by tethering transport vesicles to their destination organelles. We attempted to determine if this structural similarity also resulted in a functional similarity by looking for host-protein interactions mediated by UL37N. Although we currently have been unable to validate these putative interactions, results from our structural and mutagenesis studies support the notion that UL37 traffics capsids to cytoplasmic budding destinations and potentially further on to cell junctions for spread to nearby cells.