The tegument protein U14 of human herpesvirus 6B (HHV-6B) constitutes the viral virion structure and is essential for viral growth. To define the characteristics and functions of U14, we determined the crystal structure of the N-terminal domain of HHV-6B U14 (U14-NTD) at 1.85 Å resolution. U14-NTD forms an elongated helix-rich fold with a protruding β hairpin. U14-NTD exists as a dimer exhibiting broad electrostatic interactions and a network of hydrogen bonds. This is first report of the crystal structure and dimerization of HHV-6B U14. The surface of the U14-NTD dimer reveals multiple clusters of negatively- and positively-charged residues that coincide with potential functional sites of U14. Three successive residues, L424, E425 and V426, which relate to viral growth, reside on the β hairpin close to the dimer's two-fold axis. The hydrophobic side-chains of L424 and V426 that constitute a part of a hydrophobic patch are solvent-exposed, indicating the possibility that the β hairpin region is a key functional site of HHV-6 U14. Structure-based sequence comparison suggests that U14-NTD corresponds to the core fold conserved among U14 homologs, human herpesvirus 7 U14, and human cytomegalovirus UL25 and UL35, although dimerization appears to be a specific feature of the U14 group.

Recently, we identified a novel receptor, CD134, which interacts with the human herpesvirus 6B (HHV-6B) glycoprotein (g)H/ gL/gQ1/gQ2 complex and plays a key role in the entry of HHV-6B into target cells. However, details of the interaction between the HHV-6B gH/gL/gQ1/gQ2 complex and CD134 were unknown. In this study, we identified a cysteine-rich domain (CRD), CDR2, of CD134 that is critical for binding to the HHV-6B glycoprotein complex and HHV-6B infection. Furthermore, we found that the expression of HHV-6B gQ1 and gQ2 subunits was sufficient for CD134 binding, which is different from the binding of human herpesvirus 6A (HHV-6A) to its receptor, CD46. Finally, we identified a region in gQ1 critical for HHV-6B gQ1 function. These results contribute much to our understanding of the interaction between this ligand and receptor. IMPORTANCEWe identified the domain in HHV-6B entry receptor CD134 and the components in the HHV-6B gH/gL/gQ1/gQ2 complex required for ligand-receptor binding during HHV-6B infection. Furthermore, we identified domains in gQ1 proteins of HHV-6A and -6B and a key amino acid residue in HHV-6B gQ1 required for its function. These data should be the basis for further investigation of ligand-receptor interaction in the study of HHV-6A and -6B. Shortly after its discovery, human herpesvirus 6 (HHV-6) was recognized as a single virus species comprised of two variants, HHV-6A and HHV-6B (1-4). Recently, HHV-6 variants have been reclassified as two virus species based on the different biocharacteristics of these two viruses with respect to the fields of biology, immunology, and epidemiology, and so on (5).One striking difference between HHV-6A and HHV-6B is their cell tropism. HHV-6A infects a wider range of cells than HHV-6B (5-7). Although determinants of viral cell tropism lie in each step of the virus life cycle in target cells, the differences in receptor preference of HHV-6A and -6B may contribute much to their cell tropism. Human CD46 was identified as the cell receptor for HHV-6 (8) and binds the HHV-6A gH/gL/gQ1/gQ2 complex (9, 10). Interestingly, we found that a similar glycoprotein complex exists in HHV-6B but does not bind to CD46 (11, 12), even though it shares a relatively high sequence identity with the glycoprotein in HHV-6A (especially the gH and gL components). Recently, we found that human CD134 is a specific cellular receptor for HHV-6B and binds to the HHV-6B gH/gL/gQ1/gQ2 complex (13). While CD46 is expressed on all nucleated cells (14, 15), CD134 is expressed mainly on activated T cells (16). The different expression profiles of these two cellular receptors may be one of the key determinants of the differences in cell tropism of HHV-6A and -6B. However, it is still unknown how these two similar viral ligands bind to different cellular receptors.Analysis of the receptor-ligand binding process would contribute not only to our understanding of the viral life cycle itself but also to the development of treatment methods to block the virus life cycle at the first step. M...

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All herpesviruses contain a tegument layer comprising a protein matrix; these proteins play key roles during viral assembly and egress. Here, liquid chromatography and tandem mass spectrometry analysis (LC-MS/MS) of proteins from human herpesvirus 6 (HHV-6)-infected cells revealed a possible association between two major tegument proteins, U14 and U11. This association was verified by immunoprecipitation experiments. Moreover, U11 protein was expressed during the late phase of infection and incorporated into virions. Finally, in contrast to its revertant, a U11 deletion mutant could not be reconstituted. Taken together, these results suggest that HHV-6 U11 is an essential gene for virus growth and propagation.