Herpesvirus assembly: a tale of two membranes

Herpes virions are amongst the most complex virus particles: they comprise in excess of thirty virally encoded proteins, and also contain cellular components. Capsid formation and the cleavage and encapsidation of replicated viral DNA occur in the nucleus and resemble similar processes in tailed dsDNA (double-stranded DNA) bacteriophages, which indicates they might have common ancestry. In contrast, final virion maturation takes place in the cytoplasm. Nucleocapsids gain access to this compartment by envelopment at the inner nuclear membrane, which involves the interaction between viral and cellular proteins in order to locally alter nuclear architecture. Fusion of the primary viral envelope with the outer nuclear membrane results in translocation of the nucleocapsid to the cytoplasm. Here, the majority of the tegument - a structure, composed of a multitude of different proteins, that links the capsid and the envelope - is added to nucleocapsids, which obtain their final envelope by budding into glycoprotein-containing Golgi-derived vesicles. Thus, herpesvirus morphogenesis proceeds in two different cellular compartments, involving different viral and cellular proteins.