Date Published: May 6, 2016
Publisher: Public Library of Science
Author(s): Bochao Wang, Mitsuhiro Nishimura, Huamin Tang, Akiko Kawabata, Nora F. Mahmoud, Zahra Khanlari, Daizo Hamada, Hiroki Tsuruta, Yasuko Mori, Z. Hong Zhou.
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.
Human herpesvirus 6B (HHV-6B) and the closely-related virus HHV-6A are classified as Roseolovirus genus of beta herpesvirus subfamily    , which also includes human herpesvirus 7 (HHV-7) and human cytomegalovirus (HCMV). HHV-6B is a causative agent of exanthema subitum for children   by primary infection and for immunocompromised adults by reactivation of the latent virus. Diseases induced by HHV-6 primary or reactivated infection are sometimes severe, causing encephalitis  .
The HHV-6B U14-NTD structure, comprised of residues 2–458, reveals a helix-rich fold forming a compact homodimer. The broad and intricate interactions between each monomer (Figs 3 and 4), as well as the retention time of the size-exclusion column chromatography (Fig 1B), support the suggestion that a dimer is the natural form for U14-NTD. Multimerization of viral proteins has been frequently reported, particularly for structural proteins constituting the capsid and associated proteins. A number of tegument proteins have also been shown to form self-associated multimers, such as HSV-1 UL36  and VP22 , HCMV pp65  and pp28 , and murine gammaherpesvirus 68 ORF52 . Compared with these, the ~50 kDa U14-NTD is relatively large as a dimerization domain with a broad interface in which all four SDs are included. Although the viral matrix is considered to be an amorphous/disordered protein pool in general, multimerization of its constituents would impose local order to some extent as a corollary to the symmetries of their own and of their interaction sites for other partners. Such local order in the viral matrix has been suggested for matrix proteins of RNA virus; multimerization of matrix proteins relates to the formation of a protein lattice in the matrix and contributes to the membrane deformation required for the budding process , . Thus, the dimerization of U14 revealed in this research implies a role for this protein as a scaffold in the viral matrix. Analyzing the expression amount of U14 protein in virions would be required. As far as we know, the expression amount of HHV-6 U14 has not been investigated, hence it should be addressed in a future research. In the case of HHV-7, U14 is known as a major antigen pp85 , and U14 is thought to be relatively expressed abundantly. On the other hand, one of predominant major antigens of HHV-6 has been shown to be U11 , , which has been revealed to interact with U14 . It may be noteworthy to mention that the HCMV UL25 was expressed abundantly especially in the dense body .