Here, we applied a combination of structural biology and biochemistry to better define the role of the essential accessory protein ORF68 in KSHV packaging. Phages lack an identifiable homolog of U元2 or ORF68, suggesting that an additional level of complexity exists in herpesvirus packaging. In contrast, despite observations that U元2 and its homologs in HCMV (UL52) and KSHV (ORF68) are essential for production of packaged virions, their function in packaging remains unknown ( Albright et al., 2015 Gardner and Glaunsinger, 2018 Lamberti and Weller, 1998 Borst et al., 2008). UL17 encodes a capsid vertex-specific protein important for stabilizing the capsid ( Gong et al., 2019 Grzesik et al., 2017 Liu et al., 2019). Three of these proteins (UL15/UL28/U元3) form the terminase motor, and the portal protein is composed of a dodecamer of UL6 ( Newcomb et al., 2001 Patel et al., 1996). Packaging minimally requires recognition of the viral genome by the terminase, docking of the terminase-bound genome at the portal of a nascent capsid, translocation of the genome into the capsid by the terminase, and cleavage to release the remaining unpackaged concatemeric genome.Ĭleavage and packaging in the herpesviruses, best studied in HSV-1, requires six conserved proteins in addition to the nascent capsid and concatemeric genome ( Heming et al., 2017): HSV-1 UL6, UL15, UL17, UL28, U元2, and U元3. The large subunit of the terminase is the most conserved gene across the herpesviruses and possesses sequence and structural similarity to phage terminases, supporting the hypothesis that packaging occurs through an evolutionarily ancient mechanism ( Rixon and Schmid, 2014 Nadal et al., 2010 Selvarajan Sigamani et al., 2013). Furthermore, both depend on a ‘terminase’ motor responsible for packaging and cleavage of the genome. Despite infecting hosts in different kingdoms, both groups of viruses use an icosahedral capsid and an architecturally similar portal protein through which DNA is packaged ( Rixon and Schmid, 2014 Dedeo et al., 2019). DNA packaging in tailed bacteriophages is thought to be mechanistically similar to that of herpesviruses ( Rixon and Schmid, 2014). Cleavage to produce a unit-length genome is intimately tied to packaging and occurs only after that genome is successfully transferred into a capsid. Near the end of the lytic cycle, herpesviruses replicate their genome as a head-to-tail concatemer of linked genomes separated by terminal repeats. The human gamma-herpesviruses Kaposi’s sarcoma-associated virus (KSHV) and Epstein–Barr virus (EBV) are oncogenic viruses, causing cancers such as primary effusion lymphoma and Kaposi’s sarcoma (in the case of KSHV).ĭespite 400 million years of evolution separating the human herpesviruses, several core pathways in replication are conserved ( McGeoch et al., 2006). Human cytomegalovirus (HCMV) is a betaherpesvirus that can cause mononucleosis and congenital birth defects. Herpes simplex virus type 1 (HSV-1) is an alphaherpesvirus that causes cold sores and genital sores. The ability of herpesviruses to efficiently evade the immune system and establish latency, coupled with few available treatments and vaccines, means that nearly all adults in the world harbor at least one of the nine human herpesviruses. Comparison of these structures will be valuable for improving structure-function studies of ligand-gated ion channel receptors, including signal transduction, homology modeling, and drug design.ĭivision of Molecular Carcinogenesis, Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX Amsterdam, The Netherlands.Herpesviruses are large double-stranded DNA viruses that cause a variety of diseases in humans. Structural, pharmacological and mutational analysis of both AChBPs shows how 3 amino acid changes in the binding site contribute to a 5-10-fold difference in affinity for nicotinic ligands. These include rigid body movements of loops that are close to the transmembrane interface in the receptors and changes in the intermonomer contacts, which alter the pentamer stability drastically. Here we present the crystal structure of a remote homolog, AChBP from Bulinus truncatus, which reveals both the conserved structural scaffold and the sites of variation in this receptor family. The crystal structure of acetylcholine-binding protein (AChBP) from the mollusk Lymnaea stagnalis is the established model for the ligand binding domains of the ligand-gated ion channel family, which includes nicotinic acetylcholine, 5-hydroxytryptamine (5-HT3), gamma-aminobutyric acid (GABA), types A and C, and glycine receptors. Diversity, Equity, Inclusion, and Access.Citation, Usage, Privacy Policies, Logo.Biologically Interesting Molecule Reference Dictionary (BIRD).
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