Structure of the herpes-simplex virus portal-vertex

Herpesviruses include many important human pathogens such as herpes simplex virus, cytomegalovirus, varicella-zoster virus and the oncogenic Epstein-Barr virus and Kaposi-sarcoma associated herpesvirus. Herpes virions contains large icosahedral capsids that have a portal at a unique five-fold vertex, similar to that seen in the tailed bacteriophages. The portal is a molecular motor through which the viral genome enters the capsid during virion morphogenesis. The genome also exits the capsid through the portal-vertex when it is injected through the nuclear-pore into the nucleus of a new host cell to initiate infection. Structural investigations of the herpesvirus portal-vertex have proven challenging, owing to the small size of the tail-like portal-vertex associated tegument (PVAT), and the dense tegument layer that lays between the nucleocapsid and the viral envelope, obscuring the view of the portal-vertex. Here we show the structure of the herpes simplex virus portal-vertex at sub-nanometer resolution, solved by electron cryomicroscopy (cryoEM) and single-particle 3D reconstruction. This led to a number of new discoveries including the presence of two previously unknown portal associated structures that occupy the sites normally taken by the penton and the Ta triplex. Our data revealed that the PVAT is composed of ten copies of the C-terminal domain of pUL25, which are uniquely arranged as two tiers of star-shaped density. Our 3D reconstruction of the portal-vertex also shows that one end of the viral genome extends outside the portal in the manner described for some bacteriophage but not previously seen in any eukaryote viruses. Finally, we show that the viral genome is consistently packed in a highly-ordered left-handed spool to form concentric shells of DNA. Our data provide new insights into the structure of a molecular machine critical to the biology of an important class of human pathogens.